lable at ScienceDirect

Solid State Sciences 25 (2013) 93e102

Contents lists avai

Solid State Sciences

journal homepage: www.elsevier .com/locate/ssscie

Coexistence of several sillenite-like phases in pseudo-binaryand pseudo-ternary systems based on Bi2O3

Aleksandra Dap�cevi�c a,*, Dejan Poleti a, Ljiljana Karanovi�c b, Jelena Rogan a, Goran Dra�zi�c c

aDepartment of General and Inorganic Chemistry, Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbiab Laboratory of Crystallography, Faculty of Mining and Geology, University of Belgrade, Ðu�sina 7, 11000 Belgrade, SerbiacDepartment for Nanostructured Materials, Jo�zef �Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 21 June 2013Received in revised form16 August 2013Accepted 21 August 2013Available online 29 August 2013

Keywords:SillenitesBismuth(III) oxidesCoexistenceDopingSolid solutionsPhase equilibria

* Corresponding author. Tel.: þ381 11 3303 784; faE-mail address: hadzi-tonic@tmf.bg.ac.rs (A. Dap�c

1293-2558/$ e see front matter � 2013 Elsevier Mashttp://dx.doi.org/10.1016/j.solidstatesciences.2013.08.0

a b s t r a c t

A series of polycrystalline samples containing sillenite-like (doped g-Bi2O3) phases were prepared byhigh-temperature reactions from a-Bi2O3 powder and different oxides in six pseudo-binary Bi2O3eMxOy

(M ¼ Mn, Ti, V) and six pseudo-ternary Bi2O3eM1xOyeM2xOy (M1 ¼ Pb, Zn, Ti and M2 ¼ Zn, Ti, Si)systems. The products were characterized by XRD, SEM/EDX, HRTEM, SAED and DTA techniques. It isshown that for pseudo-binary systems, the phase composition of specimens depends on dopant content,while, for pseudo-ternary systems, depends on dopants radii, as well. In pseudo-binary systems, single-phase sillenites are obtained if the dopant content is in accordance with formula Bi12M4þO20, for M ¼Mnand Ti, and Bi12ðM5þ

0:8ÞO20, for M ¼ V. However, two coexisting sillenite-like phases, doped compound andnominally undoped solid solution, are found if a half of that dopant quantity is applied. In pseudo-ternary systems, the phase-pure double-doped sillenite specimens are identified if ionic radii of dop-ants differ less than 40%. Otherwise, two coexisting sillenites were obtained. The possibility to preparethe undoped g-Bi2O3 phase was also discussed.

� 2013 Elsevier Masson SAS. All rights reserved.

1. Introduction

Bismuth(III) oxide is an extraordinary pleomorphic materialpossessing five major structural modifications, one stable: mono-clinic a-, three metastable: tetragonal b-, body-centered cubic g-and orthorhombic 3-, and one high-temperature: face-centeredcubic d-Bi2O3 [1e9]. Two additional polymorphs are also found: u-Bi2O3, known as a thin film only [10] and one high-pressure phase,tentatively called h-Bi2O3 [11,12]. Bi2O3 also forms solid solutionswith many other oxides. In this way, the metastable and high-temperature polymorphs can be stabilized at room temperatureby doping with diverse cations [6,13e15]; the synthetic conditionsand dopant type determine which modification will be obtained.

Among mentioned seven polymorphs, doped g-Bi2O3 phaseswith different chemical composition have drawn our interestbecause of their outstanding properties, which are mainly theconsequence of their noncentrosymmetric crystal structure. Theyare the wide-band gap high-resistivity semi-insulators, photocon-ductive, photoluminescent, electronic, optoelectronic, acoustic andpiezoelectric materials and hence have real or potential application

x: þ381 11 3370 387.evi�c).

son SAS. All rights reserved.10

in many fields of electronics and electro-optics, as well as inceramic and glass industry [6,16e18].

The doped, body-centered cubic g-Bi2O3 modification adoptsthe Bi12SiO20 or sillenite structure type and crystallizes in I23 spacegroup. These phases are capable to accommodate cations (M) ofvery different radii by stretching or shrinking the unit cell. As aconsequence, the doped g-Bi2O3 phases are easily stabilized atroom temperature. There are two cationic sites in the sillenites: Bi1,situated at general 24f, andM (or Bi2), located at special 2a position(Fig. 1). In an ideal structure, the dopants are M4þ ions (e.g. Si4þ,Ge4þ, Ti4þ, Mn4þ) and the formula of matching doped g-Bi2O3 is:

Bi12MO20, i.e. 6Bi2O3$MO2. (1)

However, if the charge of dopant, n, is less than 4þ, it is morecomplicated to calculate the ideal composition and, in that case,two essential structural models have been suggested: CraigeSte-phenson (CS-model) and Radaev (R-model). The CS-model assumesa mixed occupancy of the tetrahedral M-site and the presence ofBi5þ ions to keep the charge balance [19]. According to the R-model,Mnþ ions could reside in tetrahedral 2a site, but also, together withBi23þ ions, could be slightly displaced to the 8c position with apyramidal coordination [1]. The space group symmetry is pre-served on account of random character of this disorder and

Fig. 1. Polyhedral presentation of an ideal sillenite structure showing connection of.Bi1-pyramids (gray) located at 24f, and M-tetrahedra (hatched) located at 2a position.The centering is omitted for the sake of clarity.

Fig. 2. Parts of the XRD patterns of the thermally treated pseudo-binary mixtures: (a)12Bi2O3$MnO2, 3 h at 750 �C (b) 12Bi2O3$TiO2, 3 h at 800 �C (c) 12Bi2O3$0.4V2O5, 3 h at800 �C.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e10294

Bi5þ ions do not exist. In our previous papers, it was shown thatR-model is more appropriate [20,21]. Therefore, for n � 4, thecomposition of corresponding doped sillenites can be calculatedfrom the general formula [17]:

Bi112�Bi23þð4�nÞ=ð5�nÞM

nþ1=ð5�nÞ

�O19þ1=ð5�nÞ; (2)

e.g. 6Bi2O3$0.32MO for n ¼ 2.But, if the charge of dopant is greater than 4þ, the formulas are:

Bi12�M5þ

0:8

�O20; i:e: 6Bi2O3$0:4M2O5 (3)

and

Bi12�M6þ

0:67

�O20; i:e: 6Bi2O3$0:67MO3; (4)

due to charge balance requirements [17].We have previously noticed an unexpected behavior of some

dopants, i.e. Mn4þ, Ti4þ and V5þ [20,22]. Namely, if their quantitywas smaller than predicted by the Formulas (1) and (3), two iso-structural phases, both of the sillenite-type, having different unitcell parameters and very likely containing different amounts ofdopants, appeared in the same sample. This phenomenon waspreviously observed several times [23e26], but, with one exception[26], it was not investigated in detail.

Rozaj-Brvar et al. [24] just mentioned that the intensive mixingof the molten Bi24CoO37 sample resulted in occurrence of twosillenite-like phases after cooling, one labeled as sillenite com-pound Bi24CoO37 and other as nominally undoped g-Bi2O3.Watanabe et al. [23] studied the phase equilibria in the Bi-richpart of the Bi2O3eP2O5 phase diagram and noticed the appear-ance of two sillenite-like phases if P2O5 amount is less than6.73 mol%. That sample was obtained by a solid-state reaction at850 �C and then quenched in an airstream to room temperature. Itis found that one of sillenite phases contains 6.73 mol% P2O5 andhas the unit cell parameter 10.1691(1) �A, which is expected for P-doped sillenite [27]. The second phase corresponded to the

unknown composition having larger a-parameter (the exact valuewas not published) and it was unstable under annealing. Duringthe attempts to prepare Bi12(B0.5P0.5)O20, Valant and Suvorov [25]observed a significant loss of phosphorus and/or boron whichresulted in the formation of two sillenite phases. The solid-statereaction at 700 �C caused the formation of sillenite Bi12(B0.5P0.5)-O20 compound with the unit cell parameter 10.1515(5) �A and a

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102 95

certain amount of phase with a larger lattice (the composition andunit cell parameter were not published). Through the investiga-tion of the phase equilibria in binary Bi2O3eZnO system, Guhaet al. [26] detected the coexistence of two sillenite phases at2.2 mol% ZnO. This mixture of two isostructural phases withdifferent unit cell parameters was obtained after the heat-treatment at 730 �C followed by quenching in air to room tem-perature. One phase was identified as a sillenite compound withcomposition Bi38ZnO58 and other as g-Bi2O3 solid solution con-taining 2.0 mol% ZnO. The unit cell parameter of g-Bi2O3 solidsolution was not published, but the shift of its diffraction maximatoward lower 2q values comparing to Bi38ZnO58 was noticed (ref.[26], Fig. 5). The coexistence of g-Bi2O3 solid solution and V-dopedsillenite between 5.3 and 7.7 mol% of V2O5 was also predicted inthe Bi2O3eV2O5 phase diagram published by Kargin and Voe-vodskii [28].

Fig. 3. SEM images of thermally treated pseudo-binary mixtures. Mn-rich (a) and Mn-poor sfound in 12Bi2O3$TiO2 (3 h at 800 �C) system. V-rich (e) and V-poor spots (f) found in 12Bi2O

The observed appearance of two coexisting sillenite-like phasesalso imposes the examination of the simultaneous application oftwo dopants. Many examples of the double-doped (co-doped) sil-lenites have been published in the literature [18], but most of them,as Bi12(Si4þ0.995Mn4þ

0.005)O20 [29], contained so small amount ofone dopant that practically no influence on the structural charac-teristics was found. These studies were usually focused on theoptical and spectral properties of such samples [18]. There areseveral well-documented crystallographic studies about specimenswith equal or similar content of dopants: Bi12ðB3þ

0:5P5þ0:5ÞO20 [25],

Bi12ðMg2þ0;33P5þ0;67ÞO20 [30] and Bi12ðFe3þ0:35P5þ0:59ÞO20 [31], as well as

sillenite phases in the Bi2O3eSiO2eV2O5 [32] and Bi2O3eSiO2e

MnO2 [33,34] systems. However, no systematic investigations howthe presence of two dopants influences the formation of solid so-lutions with both dopants sharing the same 2a (or 8c) positionwerepublished.

pots (b) found in 12Bi2O3$MnO2 (3 h at 750 �C) system. Ti-rich (c) and Ti-poor spots (d)3$0.4V2O5 (3 h at 800 �C) system. Markers represent points and areas analyzed by EDX.

Fig. 4. (a) HRTEM image of Ti-rich grain in [001] zone axis (b) HRTEM image of Ti-poor grain in [111] zone axis (c) HRTEM image of V-rich grain in [001] zone axis.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e10296

To explore the phase-coexistence phenomenon among sillenites,we have investigated six pseudo-binary and six pseudo-ternarysystems. All products were synthesized by a high-temperature re-action and analyzed by XRD. Selected products were additionallycharacterized using SEM/EDX, HRTEM, SAED and DTA.

2. Experimental procedure

Six pseudo-binary Bi2O3eMxOy (M ¼ Mn, Ti, V) and six pseudo-ternary Bi2O3eM1xOyeM2xOy (M1 ¼ Pb, Zn, Ti and M2 ¼ Zn, Ti, Si)mixtures with different oxide proportions (Tables 1 and 3) wereprepared from the following starting materials: a-Bi2O3 (Alfa Aesar,99.975%), V2O5 (Merck, 99.6%), TiO2 (Fluka, 99%), SiO2 (Aldrich,

Fig. 5. SAED pattern of thermally treated pseudo-binary mixtures. Ti-rich (a) and Ti-poor grfound in 12Bi2O3$0.4V2O5 (3 h at 800 �C) system. The zone axes are shown in the upper le

99.6%), MnO2 (Merck, 99.99%), PbO (Merck, 99.99%) and ZnO (AcrosOrganics, 99.5%).

All mixtures were dry homogenized for about 30min in an agatemortar, heated at 690 �C for 1.5 h (heating rate 4 �C min�1) in anopen Pt crucible and then annealed to room temperature. Afterthat, the samples were reground and examined on a PhilipsPW1710 X-ray powder diffractometer using monochromatic Cu Karadiation (l ¼ 1.5418 �A) and step-scan mode (2q ¼ 4e90�).Depending on phase composition some samples were additionallytreated for 1.5 and 3 h at 690 �C and further, in a similar manner, at750 and 800 �C. The treatments were considered as finished if twosimultaneous XRD patterns did not show significant differences.The unit cell parameters were calculated by least-squares method

ains (b) found in 12Bi2O3$TiO2 (3 h at 800 �C) system. V-rich (c) and V-poor grains (d)ft corners.

Table 1Heat treatment, phase composition and doped g-Bi2O3 unit cell parameters in pseudo-binary systems.a

Nominal composition Time (h)/Temperature (�C) Phase composition (wt.%) Unit cell parameter ofdoped g-Bi2O3 phase, a (�A)

6Bi2O3$MnO2 3/750 gMn-Bi2O3 (75) þ a-Bi2O3 (22) þ BiMn2O5 (3) aMn ¼ 10.220(1)3/800 gMn-Bi2O3 (93) þ BiMn2O5 (7) aMn ¼ 10.221(1)

12Bi2O3$MnO2 6/690 gMn-Bi2O3 (49) þ a-Bi2O3 (51) aMn ¼ 10.223(1)3/750 gMn-Bi2O3 (65) þ gBi-Bi2O3 (32) þ a-Bi2O3 (3) aMn ¼ 10.2181(6); aBi ¼ 10.2627(7)

6Bi2O3$TiO2 3/750 gTi-Bi2O3 (89) þ gBi-Bi2O3 (11)b aTi ¼ 10.1801(7); aBi ¼ 10.2583(9)6/750 gTi-Bi2O3 aTi ¼ 10.1839(6)

12Bi2O3$TiO2 6/750 gTi-Bi2O3 (57) þ gBi-Bi2O3 (38) þ a-Bi2O3 (5) aTi ¼ 10.1874(6); aBi ¼ 10.2630(7)3/800 gTi-Bi2O3 (50) þ gBi-Bi2O3 (47) þ a-Bi2O3 (3) aTi ¼ 10.1837(7); aBi ¼ 10.2617(8)

6Bi2O3$0.4V2O5 1.5/750 gV-Bi2O3 (98) þ a-Bi2O3 (2) aV ¼ 10.2259(6)3/750 gV-Bi2O3 aV ¼ 10.2251(3)

12Bi2O3$0.4V2O5 6/750 gV-Bi2O3 (62) þ a-Bi2O3 (38) aV ¼ 10.2255(9)3/800 gV-Bi2O3 (40) þ gBi-Bi2O3 (60) aV ¼ 10.2310(7); aBi ¼ 10.257(1)

a To avoid the redundancy, the results of initial treatments at 690 �C are not shown in Table 1, if no sillenite phases were found.b Some traces of an unknown phase were also found.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102 97

using the LSUCRIPC program [35]. The program PowderCell [36]was used for determination of an approximate phase ratio in aRitveld-like refinement. The choice of 690 �C for an initial heatingtemperature was based on the results of Levin and Roth [13],showing that nearly all solidus lines in the Bi2O3eMxOy systems arepositioned above 700 �C, i.e. in the first stage, the reactions wereassumed to occur in the solid state. To prevent a possible loss ofcomponents, a prolonged heating and temperatures above 800 �Cwere avoided.

The selected products were also characterized by scanningelectron microscope connected with an energy-dispersive X-rayunit (SEM/EDX), high resolution transmission electron microscopy(HRTEM) analysis and selected area electron diffraction (SAED). Thesamples were examined by a JEOL 2010F TEM equipped with a fieldemission gun and operated at 200 kV. Energy dispersive X-rayspectroscopy (LINK ISIS-300, with an UTW SieLi detector and JEOLJSM-6610 LV with an INCA Energy 350) was employed for thechemical composition determination and phase analyses. The dif-ferential thermal analysis (DTA) was carried out on an SDT Q600instrument (TA Instruments) in N2 atmosphere (flow rate:100 cm3 min�1; heating rate: 10 �C min�1). Since no significantthermal effects were observed below 500 �C, the selected productswere analyzed cyclically ranging from 500 to 900 �C.

3. Results and discussion

3.1. Pseudo-binary systems

As shown in Table 1, the heat treatment of pseudo-binaryBi2O3eMxOy (M ¼ Mn, Ti, V) starting mixtures resulted in incor-poration of dopants into the M-site of sillenite structures.Depending on the initial composition and heating conditions, theproducts contained one or two sillenite-like phases. The unit cellparameters of these phases are in agreement with the reportedvalues for microcrystalline and/or single crystal samples [20,37e43]. Therefore, it can be concluded that the experimental

Table 2EDX analysis of thermally treated pseudo-binary mixtures.

Nominal composition Dopant content, at.%

12Bi2O3$MnO2 (1.59 at.% Mn) Mn-rich grains 1.90 � 0.27Mn-poor grains 0.17 � 0.07

12Bi2O3$TiO2 (1.59 at.% Ti) Ti-rich grains 1.07 � 0.07Ti-poor grains 0.19 � 0.08

12Bi2O3$0.4V2O5 (1.27 at.% V) V-rich grains 1.28 � 0.27V-poor grains 0.18 � 0.08

a-parameters are a very good indicator for the chemical compo-sition of obtained sillenites. The coexistence of several sillenitephases will be discussed below in detail.

The 6Bi2O3$MnO2 and 6Bi2O3$TiO2 compositions are in accor-dance with the Formula (1), and 6Bi2O3$0.4V2O5 with the Formula(3). Among these three systems, 6Bi2O3$TiO2 and 6Bi2O3$0.4V2O5starting mixtures yielded single-phase samples after the finaltreatment, i.e. Ti-doped (gTi-Bi2O3) and V-doped (gV-Bi2O3) sillen-ite, respectively (Table 1). The third one, 6Bi2O3$MnO2, gave oneMn-doped sillenite (gMn-Bi2O3) as a major phase, but a smallBiMn2O5 impurity was observed as well. According to the estab-lished approximate linear relationship between the unit cell pa-rameters of doped g-Bi2O3 and the dopant ionic radii [13,20], gTi-Bi2O3 possess the smallest and gV-Bi2O3 the largest unit cell.

The remaining three pseudo-binary mixtures, i.e. 12Bi2O3$MnO2,12Bi2O3$TiO2 and 12Bi2O3$0.4V2O5, contained only one-half of thedopant quantity comparing to the Formulas (1) and (3). The corre-sponding XRD patterns clearly showed peak doublets (Fig. 2), con-firming that the final thermal treatments produced the sampleshaving two sillenite-like phases, but with different unit cell param-eters. Based on peak positions, it is obvious that one of the phases ineach systemwas gMn-Bi2O3, gTi-Bi2O3 and gV-Bi2O3, respectively. Theunit cell parameters of the other phases (gBi-Bi2O3) are larger, butmutually identical, with the value of about 10.26�A, which is close tothe reported values for the nominally undoped g-Bi2O3 [1,2,5,44].

The scanning electron micrographs are given in Fig. 3. Althoughobtained at the mildest conditions, the system with Mn [Fig. 3(a)and (b)] consists of well sintered grains. On the contrary, the systemwith Ti is mostly dispersed, which could be related to its “fragility”during the product grinding. In general, all three specimens consistin agglomerated grains of similar morphology, and it was notpossible to distinguish different phases. As two levels of dopantconcentration, dopant-rich and dopant-poor regions were found inall three cases, the coexistence of two different sillenites wasconfirmed (Table 2). The completely undoped one was not found.The dopant concentrations in dopant-poor grains are almostidentical for all three systems with the amount of about 0.2 at.%,which is close to the detection limit of EDX analysis. The dopantcontent in dopant-rich regions generally agree with the nominalcompositions, but not with the Formulas (1) and (3) according towhich the maximum dopant amount is 3.03 at.% for Mn4þ and Ti4þ,and 2.44 at.% for V5þ ions.

The high-resolution TEM images (Fig. 4) of dopant-rich anddopant-poor grains showed the absence of any defects, whichmeans that the dopants are well incorporated into the crystal lat-tice. The SAED analysis was then employed in order to identify themicrocrystalline phases and verify above obtained results. The

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e10298

samples for this analysis were selected as two extremes: with thelargest (12Bi2O3$TiO2 system) and the smallest (12Bi2O3$0.4V2O5)difference between the unit cell parameters of doped and nomi-nally undoped g-Bi2O3 phase. The presence of dopant-rich anddopant-poor grains was revealed, as well (Fig. 5). The patterns of Ti-and V-rich grains, Fig. 5(a) and (c), respectively, could be indexed asTi-doped [37] and V-doped [38] g-Bi2O3. The patterns of the Ti-poor and V-poor grains shown in Fig. 5(b) and (d), respectively,could be indexed as nominally undoped g-Bi2O3 using Radaev’sdescription [1].

The thermal analysis also confirmed the coexistence of two pha-ses. As shownbycyclic DTA curves (Fig. 6),when the sample obtainedfrom 12Bi2O3$MnO2 system is heated, two g-Bi2O3 / d-Bi2O3 tran-sitions occur, one corresponding togMn-Bi2O3/ d-Bi2O3 and anotherto gBi-Bi2O3/ d-Bi2O3. As expected, when the sample obtained from6Bi2O3$MnO2systemisheatedonlygMn-Bi2O3/ d-Bi2O3 transition isvisible at approximately the same temperature comparing to thecorresponding transition for12Bi2O3$MnO2system. This temperatureagrees well with the value reported for transition of Mn-dopedg-Bi2O3 [13]. After further heating, the melting of d-Bi2O3 takesplace at 761 and 787 �C for the sample obtained from 6Bi2O3$MnO2

and 12Bi2O3$MnO2, respectively. Both temperatures are slightlylower thanexpected [13]. Thehighermeltingpoint for 12Bi2O3$MnO2

is probably due to the larger Bi-content in the sample. On cooling,d-Bi2O3 crystallizes first, and its further transformation to the g-Bi2O3

phase(s) occurs in two steps (d-Bi2O3 / gMn-Bi2O3 and d-Bi2O3 / gBi-Bi2O3) for 12Bi2O3$MnO2, and in only one step (d-Bi2O3 / gMn-Bi2O3) for 6Bi2O3$MnO2 system.

A similar thermal behavior is found for the vanadium-containing systems (Fig. 7). On heating, two phase transitionswere found for 12Bi2O3$0.4V2O5, and only one for 6Bi2O3$0.4V2O5system. Detailed diagrams for both phases, stable and metastable,present in Bi2O3-rich region of Bi2O3eV2O5 system was describedby Kargin and Voevodskii [28]. Although they claimed that a stableV-containing sillenite phase has a formula 12Bi2O3$V2O5, thetemperature of 776 �C corresponds well to the reported gV-Bi2O3 / d-Bi2O3 transition at 765 �C. On the other hand,two transitions, i.e. gBi-Bi2O3 þ gV-Bi2O3 / g-Bi2O3 (solidsolution) / d-Bi2O3, predicted in the phase diagram for meta-stable phases [28] occur at about 645 and 710 �C, which is close tothe temperatures observed in our system (Fig. 7). However, the

Fig. 6. Cyclic DTA curves of thermally treated pseudo-binary mixtures: 6Bi2O3$MnO2

(3 h at 800 �C) and 12Bi2O3$MnO2 (3 h at 750 �C).

temperature of 652 �C could also easily be assigned to the tran-sition of the nominally undoped g-Bi2O3 to d-Bi2O3, because thisvalue is close to 663 and 650 �C reported by Harwig [2] and Levinand Roth [5], respectively.

To summarize, if the quantity of dopants is in accordance withthe Formulas (1) and (3), the single doped g-Bi2O3 phase is formed.Their unit cell parameters agree with the reported values for thecorresponding sillenites. If a half (or even less [22]) of dopantquantity is applied, two coexisting sillenite-like phases are found.The same phenomenon, i.e. the coexistence of two isostructuralphases was occasionally observed for the non-bismuth containingcompounds, too. Shanker et al. showed that the synthesis ofBa2�xSrxTiO4 with x � 1 lead to a biphasic mixture of two ortho-rhombic Ba2TiO4-type phases with slightly different latticeparameter [45]. Prchal et al. described the coexistence of two iso-structural hexagonal ZrNiAl-type phases as structural anomaly,which was observed in the pseudo-ternary Ce1�xYxPdAl series for0.4 � x � 0.6 [46]. Within the Bi2O3-containing systems, thecoexistence of two sillenites is not so uncommon, but it is observedmore or less accidentally [23e26]. Our detailed research showedthat one of two coexisting phases in each system is the sillenite, ordoped g-Bi2O3. It should be emphasized that the unit cell param-eters remained basically unchanged in regard to the literature data,although the dopant contents did not satisfy the Formulas (1) and(3). This means that those formulas actually show the maximumpossible dopant quantity.

On the other hand, the unit cell parameters of the secondsillenite-like phase are mutually identical (about 10.26 �A) and, asalready mentioned, in good agreement with the reported data forthe nominally undoped g-Bi2O3 [1,2,5,44]. The value of 10.268(1)�Awas extrapolated by Harwig [2] from high temperature data ofnominally undoped g-Bi2O3. The identical value was also reportedby Levin and Roth [5] with the comment that the preparation of g-Bi2O3 is not possible without quenching or the presence of impu-rities. A slightly lower value, 10.2501(5) �A, was determined byRadaev et al. [1]. This nominally undoped g-Bi2O3 was obtained byheating a-Bi2O3 containing 0.1 wt.% CdO. Khachani et al. [44] foundthe unit cell parameter of 10.255 and 10.256 �A for g-Bi2O3 with 0.1at.% V and 0.1 at.% As, respectively. Apparently, in these three cases,the small percentage of mentioned elements should be consideredas accidentally present impurities and not as intentionally addedcomponents. Here presented EDX results (Table 2) indicate that a

Fig. 7. Cyclic DTA curves of thermally treated pseudo-binary mixtures: 6Bi2O3$0.4V2O5

(3 h at 750 �C) and 12Bi2O3$0.4V2O5 (3 h at 800 �C). (The melting was not examinedsince d-Bi2O3 that crystallizes from the melts remains stable to the room temperature[28].)

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102 99

negligible dopant content of about 0.2 at.% is sufficient for thestabilization of nominally undoped g-Bi2O3. This suggests that thesynthesis of the completely undoped g-Bi2O3 phase very likely isnot possible. Also, the unit cell parameters signify that Bi3þ ionsshare M-site with other cations and hence the second phasesshould be considered as solid solutions. This approach was alsoproposed by Guha et al. [26] in their research of the binary Bi2O3e

ZnO system. As it was stated in the Introduction, they found thecoexistence of a sillenite phasewith composition Bi38ZnO58 (5mol%ZnO), which is the maximum content for M2þ ions predicted by theFormula (2), and g-Bi2O3 solid solution with 2 mol% ZnO. It shouldbe emphasized that such g-Bi2O3 solid solution was obtained byquenching. However, if the same mixture was slowly cooled, ittransforms into a-Bi2O3 and Bi38ZnO58 [26]. The other three authorsthat have detected the coexistence of two sillenite-like phases [23e25] were certain that one of the phases was sillenite, but did notdefine the composition of the second phase. In two cases [23,24],the products were obtained by quenching and only in the third oneby slow cooling [25]. To compare, all here described products wereobtained by annealing, which means that these solid solutions arestable at room temperature.

With one exception [28], among numerous published data[13,47e54], detailed phase diagrams showing Bi2O3-rich region forthe pseudo-binary Bi2O3eMxOy systems (M ¼ Mn, Ti, V) do notexist. Still, in two cases [28,48], the authors mentioned some kindof g-Bi2O3 solid solutions. Kargin and Voevodskii [28] defined thatg-Bi2O3 solid solution could contain 0.5e4.0 mol% V2O5 at roomtemperature, while Miyazawa and Tabata [48] claimed that g-Bi2O3solid solution containing slightly less TiO2 (13.85 mol% TiO2) thanpredicted with Formula (1), exists at 853 �C. At low temperatures,the mentioned phase diagrams [13,47e54] sometimes assume thepresence of a-Bi2O3 solid solution with less than 0.5 mol% of dop-ants. Instead, we have demonstrated that an extremely smalldopant (or impurity) percentage of less than 0.2 at.% could be adriving force for the stabilization of g-Bi2O3 at room temperature.Thus, the area of stability of g-Bi2O3 solid solutions should beplaced in low-temperature regions of existing phase diagrams, verylikely not only in the case of dopants used in this study. This,however, requires additional and detailed studies in many othersystems.

3.2. Pseudo-ternary systems

In order to test whether the simultaneous application of twodifferent dopants produces one or two sillenite phases, six differentpseudo-ternary compositions were prepared and treated asdescribed in Experimental. The choice of dopant was based on therequirement to cover as wide range of dopants’ radii as possible.

Table 3Heat treatment, phase composition and doped g-Bi2O3 unit cell parameters in pseudo-te

Nominal composition Time (h)/Temperature (�C) Phase composition (wt

12Bi2O3$PbO$ZnO 3/690 gPb/Zn-Bi2O3

12Bi2O3$PbO$TiO2 3/690 gPb-Bi2O3 (67) þ gTi-Bi3/750 gPb-Bi2O3 (24) þ gTi-Bi

12Bi2O3$PbO$SiO2 1.5/690 gPb-Bi2O3 (51) þ gSi-Bi12Bi2O3$ZnO$TiO2 3/690 gZn/Ti-Bi2O3

12Bi2O3$ZnO$SiO2 3/690 gZn-Bi2O3 (75) þ gSi-Bi3/750 gZn-Bi2O3 (46) þ gSi-Bi3/800 gZn-Bi2O3 (50) þ gSi-Bi

12Bi2O3$TiO2$SiO2 3/690 gTi-Bi2O3 (46) þ a-Bi2O1.5/750 gTi-Bi2O3 (60) þ gSi-Bi23/750 gTi-Bi2O3 (69) þ gSi-Bi26/750 gTi-Bi2O3 (75) þ gSi-Bi23/800 gTi/SieBi2O3

Hence, the following dopants with quite different ionic radii havebeen selected: Pb2þ (ri¼ 0.98�A), Zn2þ (ri¼ 0.60�A), Ti4þ (ri¼ 0.42�A)and Si4þ (ri ¼ 0.26�A) [55]. The phase compositions of samples andunit cell parameters of obtained g-Bi2O3 phases are listed in Table 3.

After the final treatment, the single double-doped sillenites,gPb/Zn-Bi2O3, gZn/Ti-Bi2O3, gTi/Si-Bi2O3, were found in three systems:12Bi2O3$PbO$ZnO, 12Bi2O3$ZnO$TiO2 and 12Bi2O3$TiO2$SiO2,respectively. Their identification was based on the unit cell pa-rameters, which were very close to the arithmetic mean of thecorresponding values for single-doped sillenites [37,39,41,42]. Thisindicates a random distribution of two dopants over the M-site.Similarly, the unit cell parameter of microcrystallineBi12(B3þ0.5P5þ0.5)O20 is 10.1515 �A [25], which is also between theunit cell parameters of B- [56] and P-doped [27] g-Bi2O3. On theother hand, the single crystal of Bi12(Fe3þ0.35P5þ0.59)O20 was foundto have a ¼ 10.167(14) �A [31], which is lower than the unit cellparameters of both corresponding single-doped g-Bi2O3 phases[27,38]. However, due to rather diverse dopant radii, there are twocrystallographically independent MO4-tetrahedra (M¼ Fe and P) inthis structure with quite different MeO distances. Therefore, thisphase can be treated as a kind of co-crystal, and it is hard to predictthe influence of such situation on its unit cell parameter.

As shown in Table 3, gPb/Zn-Bi2O3 and gZn/Ti-Bi2O3 were obtainedalready at 690 �C, unlike gTi/Si-Bi2O3 whose formation required800 �C. This behavior is probably caused by the diverse stability ofthe prepared phases. As discussed previously [20], the dopantshave a different influence on this stability, giving the followingorder: Pb2þ > Zn2þ > Ti4þ > Si4þ, in which the stability of phasesdecrease. Thus, the difficulty to prepare doped g-Bi2O3 phases in-creases with the decreasing radii of dopants.

This explains the highest temperature necessary for the syn-thesis of gTi/Si-Bi2O3, and the complex pathway for its acquiring(Table 3, Fig. 8). Initially, at 690 �C, only gTi-Bi2O3 was found, be-sides a-Bi2O3. After next two thermal treatments at 750 �C, eventhree coexisting sillenites were formed: gTi-Bi2O3, gSi-Bi2O3 andgBi-Bi2O3. This is the first time that the coexistence of three iso-structural phases was found in the same sample. The amount of gBi-Bi2O3 gradually decreases with increasing the reaction time, and,consequently, after 6 h at 750 �C, only two single-doped phases, gTi-Bi2O3 and gSi-Bi2O3, remained. Finally, at 800 �C, the double-dopedgTi/Si-Bi2O3 was obtained. This is as expected and should be corre-lated with an orderedisorder transition, i.e. with solid solutioncharacter of gTi/Si-Bi2O3, where Ti and Si share the same site at hightemperatures.

Thefinal treatmentof three remainingmixtures,12Bi2O3$PbO$TiO2,12Bi2O3$PbO$SiO2 and 12Bi2O3$ZnO$SiO2, resulted in two coexistingsillenites.According totheunit cell parametersofdopedg-Bi2O3phases[39,41,42], gPb-Bi2O3 and gSi-Bi2O3, as well as gSi-Bi2O3 and gZn-Bi2O3,

rnary systems.

.%) Unit cell parameter ofdoped g-Bi2O3 phase, a (�A)

aPb/Zn ¼ 10.2375(3)2O3 (27) þ a-Bi2O3 (6) aPb ¼ 10.2482(8); aTi ¼ 10.200(1)2O3 (76) aPb ¼ 10.2593(6); aTi ¼ 10.2055(6)2O3 (49) aPb ¼ 10.256(3); aSi ¼ 10.113(3)

aZn/Ti ¼ 10.2012(7)2O3 (27) aZn ¼ 10.1906(9); aSi ¼ 10.118(1)2O3 (54) aZn ¼ 10.2113(6); aSi ¼ 10.1205(8)2O3 (50) aZn ¼ 10.2143(8); aSi ¼ 10.1182(5)3 (54) aTi ¼ 10.1948(7)O3 (19) þ gBi-Bi2O3 (21) aTi ¼ 10.1818(1); aSi ¼ 10.097(2); aBi ¼ 10.253(1)O3 (23) þ gBi-Bi2O3 (8) aTi ¼ 10.1869(7); aSi ¼ 10.116(1); aBi ¼ 10.267(1)O3 (25) aTi ¼ 10.1810(8); aSi ¼ 10.105(1)

aTi/Si ¼ 10.1500(3)

Fig. 10. SEM micrographs of the sample obtained from the 12Bi2O3$PbO$TiO2 systemat 750 �C in back scattering mode. gTi-Bi2O3 is shown as dark gray phase, gPb-Bi2O3 aslight gray phase and Pb-rich phase (indicated by arrows) as darkest gray.

Fig. 8. Parts of the XRD patterns of 12Bi2O3$TiO2$SiO2 sample thermally treated underdifferent conditions.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102100

are formed in the 12Bi2O3$PbO$SiO2 and 12Bi2O3$ZnO2$SiO2 systems,respectively. Moreover, the phase composition, which is about 50wt.%of each component supports this conclusion. As an illustration, XRDpattern of 12Bi2O3$PbO$SiO2 system is shown in Fig. 9, because of thelargest difference between a-parameters.

However, the unit cell parameters of two sillenites obtained inthe 12Bi2O3$PbO$TiO2 system were not consistent after twodifferent heating treatments (Table 3). After the first heating cycle,the unit cell parameter of gPb-Bi2O3 was lower, while for gTi-Bi2O3 itwas higher than expected [37,41]. Although gPb-Bi2O3 unit cellparameter reached the literature value after the second thermaltreatment, the a-parameter of gTi-Bi2O3 was still higher. Moreover,the gTi-Bi2O3 content was favored, i.e. 76 wt.% of this phase wasfound. To clarify this behavior, SEM/EDX analysis was employed.

The micrographs (Fig. 10) mainly confirmed the XRD results.Two major phases are found with a greater presence of dark gray

Fig. 9. XRD pattern of the final products from 12Bi2O3$PbO$SiO2 system.

areas showing that gTi-Bi2O3 is a dominant phase. However, sometraces (less than about 3%) of an unknown Pb-rich phase were alsofound. All phases were intimately mixed inside of the sample withno evident crystal faces. The EDX analysis (Table 4) also confirmedthe coexistence of two sillenites with different composition andone Pb-rich phase. The dopant concentrations found are in agree-ment with Formulas (1) and (2), within one or two standard de-viations, for gPb-Bi2O3 and gTi-Bi2O3, respectively. The appearanceof a Pb-rich phase could be the consequence of lead excessremained after the formation of gPb-Bi2O3 phase. Apparently, thecontent of this phase was below the detection limit of XRDtechnique.

The position of Pb-rich phase within gTi-Bi2O3 grains only couldindicate a kind of exsolution process that occurs during samplecooling. This is also a possible cause of strains and enlargement ofgTi-Bi2O3 unit cell parameter, due to accommodation of crystallattices.

It is also interesting to comment the changes in phase compo-sition observed in the 12Bi2O3$ZnO$SiO2 system. Two dopedg-Bi2O3 were obtained at all three temperatures, but in differentproportions. At lowest temperature, gZn-Bi2O3 is a predominantphase, but with the temperature increasing, its quantity decreases

Table 4EDX data of the sample obtained from 12Bi2O3$PbO$TiO2 system at 750 �C.

Element content, at. %

Pb Ti Bi O

12Bi2O3$PbO$TiO2 (nominalcomposition)

1.54 1.54 36.92 60.00

6Bi2O3$0.32PbO (idealcomposition)a

1.04 0 39.16 59.80

6Bi2O3$TiO2 (ideal composition)a 0 3.03 36.36 60.61gPb-Bi2O3 (light gray phase) 0.92(12) 0 39.45 (11) 59.63(6)gTi-Bi2O3 (dark gray phase) 0 2.1(5) 37.5(2) 60.4(8)Pb-rich phase (darkest

gray phase)4.4(2) 0 36.56(15) 59.03(6)

a According to the Equations (1) and (2).

Fig. 11. Unit cell parameter of sillenites obtained in pseudo-ternary systems versusdopant ionic radius. In the upper part, the allowed combinations yielding single-phasedouble-doped samples are indicated by the lefteright arrows, and the forbidden onesare shown by dashed lines.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102 101

in favor to gSi-Bi2O3. The quantities of two compounds becameequal after the final treatment, as it was expected considering thenominal composition. This is an another confirmation of abovementioned dopant influence on the relative stability of sillenitephases, according to which the synthesis of Si-doped g-Bi2O3 re-quires the highest and the synthesis of Zn-doped g-Bi2O3 thelowest temperature.

Generally, all pseudo-ternary systems, no matter whether oneor two sillenites were obtained, were consistent with our previousfindings [20]: the unit cell parameter increases with increasingionic radius of Mnþ. If the unit cell parameters of here produced andsome previously published sillenites in pseudo-ternary systems areplotted versus ri (Fig. 11), the anticipated approximately linearrelationship was found. (In the case of mixedM-sites, ri values werecalculated as the arithmetic mean of two ionic radii.) The linearfit using the least-squares method yielded the equation:a ¼ 10.11(1) þ 0.16(2)ri, which is very close to the equation foundpreviously [20]: a ¼ 10.09(1) þ 0.18(2)ri.

As shown in Fig. 11, among six investigated systems, there arethree allowed dopant combinations that give a double-doped sil-lenite. The main reason is a difference in ionic radii of presentdopants. The explanation basically lies in HumeeRothery rule [57],which, although developed for alloys, was also used to predict thepossibility of the solid solution formation in the case of ceramicmaterials [58]. According to this rule, two atoms can form the solidsolution if their radii differ by no more than 15%. However, in ourcase, the differences between ionic radii of applied cations thatform the double-doped sillenite are significantly greater and reacheven 40%. This is due to great capability of the sillenite structure toaccommodate various dopants, because its lattice demonstrates aquite unexpected and enormous flexibility, and must be correlatedto the Bi1EO(5þ2) coordination polyhedron with two very long andadjustable BieO distances [20] and the softness of Bi 6s2 loneelectron pair [59,60].

4. Conclusions

The various single- and double-doped sillenites are easily pre-pared by high-temperature solid state reaction between a-Bi2O3and different oxides in some pseudo-binary Bi2O3eMxOy (M ¼ Mn,

Ti, V) and pseudo-ternary Bi2O3eM1xOyeM2xOy (M1 ¼ Pb, Zn, Tiand M2 ¼ Zn, Ti, Si) systems. This resulted in incorporation ofdifferent cations into the M-site of the sillenite structure. The re-sults based on the XRD, SEM/EDX, HRTEM and SAED characteriza-tions showed that the unit cell parameters of all synthesized phasesare in agreement with the reported values, no matter if one double-doped or several single-doped sillenite-like phases were obtained.It is again shown that a-parameter increases linearly withincreasing ionic radius of Mnþ.

If the quantity of dopants, for the pseudo-binary systems,

is in accordance with the formulas Bi112ðBi23þð4�nÞ=ð5�nÞMnþ1=ð5�nÞÞ

O19þ1=ð5�nÞ for n� 4 and Bi12ðM5þ0:8ÞO20 the single-doped g-Bi2O3 are

formed. If a half or less of dopant quantity is applied, two coexistingphases are found. One of these two phases is the doped sillenitecompound. The unit cell parameters of all second coexisting g-Bi2O3

phases are mutually identical and in good agreement with the re-ported data for the nominally undoped g-Bi2O3. However, the founddopant content of less than about 0.2 at.% suggests that the synthesisof the completely undoped g-Bi2O3 is probably impossible. This smalldopantpercentage is consideredasadriving force for the stabilizationof g-Bi2O3 at room temperature in the form of stable solid solutions.

The lattice flexibility of the sillenite structure was confirmed bythe formation of the single-phase double-doped sillenites, whichwere identifiedwithin the pseudo-ternary systemswhen using twodopants with the ionic radii that differs less than 40%. The unit cellparameters of obtained double-doped sillenites indicate the sta-tistic distribution of two dopants at M-site. Their values areapproximately the arithmetic mean of the unit cell parameters oftwo corresponding single-doped g-Bi2O3 phases. If the differencebetween the dopant ionic radii is more than 40%, two coexistingdifferently doped sillenites were obtained.

Acknowledgments

The authors gratefully acknowledge the financial support of theMinistry of Education, Science and Technological Development ofthe Republic of Serbia (Grant No. III 45007).

References

[1] S.F. Radaev, V.I. Simonov, Yu.F. Kargin, Acta Crystallogr. B 48 (1992) 604e609.[2] H.A. Harwig, Z. Anorg. Allg. Chem. 444 (1978) 151e166.[3] H.A. Harwig, A.G. Gerards, Thermochim. Acta 28 (1979) 121e131.[4] H.A. Harwig, J.W. Weenk, Z. Anorg. Allg. Chem. 444 (1978) 167e177.[5] E.M. Levin, R.S. Roth, J. Res. Natl. Bur. Stand. 68A (2) (1964) 189e195.[6] N.M. Sammes, G.A. Tompsett, H. Näfe, F. Aldinger, J. Eur. Ceram. Soc. 19 (1999)

1801e1826.[7] L.G. Sillén, Ark. Kemi Mineral. Geol. 12A (1937) 1e15.[8] G. Gattow, H. Schröder, Z. Anorg. Allg. Chem. 318 (1962) 176e189.[9] N. Cornei, N. Tancret, F. Abraham, O. Mentre, Inorg. Chem. 45 (2006) 4886e

4888.[10] A.F. Gualtieri, S. Imovilli, M. Prudenziati, Powder Diffr. 12 (1997) 90e92.[11] S. Ghedia, T. Locherer, R. Dinnebier, D.L.V.K. Prasad, U. Wedig, M. Jansen,

A. Senyshyn, Phys. Rev. B 82 (2010) 1e12.[12] T. Atou, H. Faqir, M. Kikuchi, H. Chiba, Y. Syono, Mater. Res. Bull. 33 (1998)

289e292.[13] E.M. Levin, R.S. Roth, J. Res. Natl. Bur. Stand. 68A (1964) 197e206.[14] P. Shuk, H.-D. Wiemhofer, U. Guth, W. Gijpel, M. Greenblatt, Solid State Ionics

89 (1996) 179e196.[15] R.V.K. Mangalam, R. Ranjith, A. Iyo, A. Sundaresan, S.B. Krupanidhi, C.N.R. Rao,

Solid State Commun. 140 (2006) 42e44.[16] M. Valant, D. Suvorov, J. Am. Ceram. Soc. 84 (2001) 2900e2904.[17] M. Valant, D. Suvorov, Chem. Mater. 14 (2002) 3471e3476.[18] http://www.ifpan.edu.pl/PERSONAL/borow/baz_sill.html.[19] D.C. Craig, N.C. Stephenson, J. Solid State Chem. 15 (1975) 1e8.[20] D. Poleti, Lj. Karanovi�c, A. Had�zi-Toni�c, Z. Kristallogr. 222 (2007) 59e72.[21] A. Dap�cevi�c, D. Poleti, Lj. Karanovi�c, Powder Diffr. 27 (2012) 2e7.[22] D. Poleti, Lj. Karanovi�c, M. Zduji�c, �C. Jovaleki�c, J. Serb. Chem. Soc. 76 (2011)

1e12.[23] A. Watanabe, H. Kodama, S. Takenouchi, J. Solid State Chem. 85 (1990) 76e82.[24] A. Rozaj-Brvar, M. Trontelj, D. Kolar, J. Less-Common Met. 68 (1986) 7e14.

A. Dap�cevi�c et al. / Solid State Sciences 25 (2013) 93e102102

[25] M. Valant, D. Suvorov, J. Am. Ceram. Soc. 85 (2002) 355e358.[26] J.P. Guha, S. Kunej, D. Suvorov, J. Mater. Sci. 39 (2004) 911e918.[27] J.P. Wignacourt, M. Drache, P. Conflant, J.C. Boivin, J. Chim. Phys. Phys. Chim.

Biol. 88 (1991) 1939e1949.[28] Yu.F. Kargin, V.Yu. Voevodskii, Zh. Neorg. Khim. 42 (1997) 1547e1549.[29] S. Neov, V. Marinova, M. Reehuis, R. Sonntag, Appl. Phys. A 74 (Suppl.) (2002)

S1016eS1018.[30] G.V. Avdeev, T.I. Milenov, A.V. Egorysheva, K.P. Petrov, V.M. Skorikov,

R.Kh. Titorenkova, P.M. Rafailov, Russ. J. Inorg. Chem. 56 (2011) 913e918.[31] S.F. Radaev, V.I. Simonov, Kristallografiya 37 (1992) 914e941.[32] T.I. Melnikova, G.M. Kuzmicheva, V.B. Rybakov, N.B. Bolotina, A.B. Dubovskii,

A. Cousson, J. Struct. Chem. 52 (2011) 510e516.[33] T.I. Melnikova, G.M. Kuzmicheva, V.B. Rybakov, N.B. Bolotina, A.B. Dubovsky,

Crystallogr. Rep. 55 (2010) 229e232.[34] T.I. Melnikova, G.M. Kuzmicheva, V.B. Rybakov, N.B. Bolotina, A.B. Dubovskii,

Inorg. Chem. 50 (2011) 2002e2009.[35] R.G. . Garwey, LSUCRIPC, least squares unit-cell refinement with indexing on

the personal computer, Powder Diffr. 1 (1986) 114.[36] W. Kraus, G. Nolze, PowderCell for Windows, V.2.4, Federal Institute for Ma-

terials Research and Testing, Berlin, Germany.[37] D.C.N. Swindells, J.L. Gonzalez, Acta Crystallogr. B 44 (1988) 12e15.[38] A.N. Yudin, E.A. Pobedimskaya, L.E. Terenteva, I.V. Petrova, L.N. Kaplunnik,

G.V. Malakhova, Neorg. Mater. 25 (1989) 1715e1718.[39] H.S. Horowitz, A.J. Jacobson, J.M. Newsam, J.T. Lewandowski, M.E. Leonowicz,

Solid State Ionics 32/33 (1989) 678e690.[40] U. Delicat, S.F. Radaev, M. Troemel, P. Behrens, Yu.F. Kargin, A.A. Marin, J. Solid

State Chem. 110 (1994) 66e69.[41] A.D. Murray, C.R.A. Catlow, F. Beech, J. Drennan, J. Solid State Chem. 62 (1986)

290e296.

[42] S.F. Radaev, L.A. Muradyan, Yu.F. Kargin, V.V. Volkov, V.A. Sarin, E.E. Rider,V.I. Simonov, Kristallografiya 35 (1990) 1126e1132.

[43] Y. Wang, R. He, M. Yang, T. Wen, H. Zhang, J. Liang, Zh. Lin, Y. Wang, G. Li,J. Lin, CrystEngComm 14 (2012) 1063e1068.

[44] N. Khachani, M. Devalette, P. Hagenmuller, Z. Anorg. Allg. Chem. 533 (1986)93e98.

[45] V. Shanker, T. Ahmad, A.K. Ganguli, Bull. Mater. Sci. 27 (2004) 421e427.[46] J. Prchal, H. Kitazawa, O. Suzuki, J. Alloy. Compd. 437 (2007) 117e119.[47] Y. Masuda, H. Masumoto, A. Baba, T. Goto, T. Hirai, Jpn. J. Appl. Phys. Part 1 31

(1992) 3108e3112.[48] S. Miyazawa, T. Tabata, J. Cryst. Growth 191 (1998) 512e516.[49] T.M. Bruton, J. Solid State Chem. 9 (1974) 173e175.[50] E.I. Speranskaya, I.S. Rez, L.V. Kozlova, V.M. Skorikov, V.I. Slavov, Inorg. Mater.

1 (1965) 213e216.[51] M. Touboul, C. Vachon, Thermochim. Acta 133 (1988) 61e66.[52] Ya.N. Blinovskov, A.A. Fotiev, Russ. J. Inorg. Chem. 32 (1987) 145e146.[53] C.K. Lee, G.S. Lim, K.S. Low, A.R.West, Solid State Ionic Mater., in: B.V.R. Chowdari

(Ed.), Proc. Asian Conf. Solid State Ionics, 4th, Kuala Lumpur,Malaysia, August 2e6, 1994, World Scientific Press, Singapore, 1994, pp. 211e216.

[54] J. Lopez-Martinez, A. Romero-Serrano, A. Hernandez-Ramirez, B. Zeifert,C. Gomez-Yanez, R. Martinez-Sanchez, Thermochim. Acta 516 (2011) 35e39.

[55] R.D. Shannon, Acta Cryst. A 32 (1976) 751e767.[56] M. Burianek, P. Held, M. Muhlberg, Cryst. Res. Technol. 37 (2002) 785e796.[57] W. Hume-Rothery, H.M. Powell, Z. Kristallogr. 91 (1935) 23.[58] M.M. Wu, J. Peng, Y.Z. Cheng, H. Wang, Z.X. Yu, D.F. Chen, Z.B. Hu, Solid State

Sci. 8 (2006) 665e670.[59] E. Makovicky, Neues Jb. Miner. Abh. 160 (1989) 269e297.[60] E. Makovicky, in: E. Parthé (Ed.), Modern Perspectives in Inorganic Crystal

Chemistry, Kluwer Academic Publishers, Dordrecht, 1992, pp. 131e161.