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Solid State Sciences 11 (2009) 628–634

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Solid State Sciences

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Syntheses and crystal structures of new vanadium(IV) oxyphosphatesM(VO)2(PO4)2 with M¼ Co, Ni

S. Kaoua a, P. Gravereau b, J.P. Chaminade b,*, S. Pechev b, S. Krimi a, A. El Jazouli c

a Laboratoire de Physico Chimie des Materiaux Inorganiques, Faculte des Sciences Aın Chock, Casablanca, Moroccob Institut de Chimie de la Matiere Condensee de Bordeaux (ICMCB-CNRS), Universite Bordeaux I, 33608 Pessac, Francec Laboratoire de Chimie des Materiaux Solides, Faculte des Sciences Ben M’Sik-UH2M, Casablanca, Morocco

a r t i c l e i n f o

Article history:Received 25 July 2008Received in revised form21 November 2008Accepted 27 November 2008Available online 13 December 2008

Keywords:Co(VO)2(PO4)2

Ni(VO)2(PO4)2

Vanadium(IV) oxyphosphate compoundSingle crystal and powder X-ray diffraction

* Corresponding author. Tel.: þ33540006265; fax:E-mail address: chamin@icmcb-bordeaux.cnrs.fr (

1293-2558/$ – see front matter � 2008 Elsevier Masdoi:10.1016/j.solidstatesciences.2008.11.011

a b s t r a c t

We have extended our research interest on titanium oxyphosphates (MII(TiO)2(PO4)2, with MII¼Mg, Fe,Co, Ni, Cu, Zn) to vanadium oxyphosphates MII(VIVO)2(PO4)2 (MII¼ Co, Ni). For each compound twophases, named a and b according to synthesis conditions, have been stabilized at room temperature, thencharacterized. The four crystal structures M(VO)2(PO4)2 (a and b for M¼ Co, Ni) have been determined inmonoclinic P21/c space group using X-ray single crystals diffraction data. Structure of the a phase isderived from the Li(TiO)(PO4) (orthorhombic Pnma) and LiNi0.50(TiO)2(PO4)2 (monoclinic P21/c) types,with cell parameters: a¼ 6.310(1) Å, b¼ 7.273(1) Å, c¼ 7.432(1) Å, b¼ 90.43(1)� for M¼ Co, anda¼ 6.297(2) Å, b¼ 7.230(2) Å, c¼ 7.421(2) Å, b¼ 90.36(2)� for M¼Ni. Structure of the b phase is derivedfrom the Ni(TiO)2(PO4)2-type (monoclinic P21/c) with cell parameters: a¼ 7.2742(2) Å, b¼ 7.2802(2) Å,c¼ 7.4550(2) Å, b¼ 120.171(2)� for M¼ Co, and a¼ 7.2691(2) Å, b¼ 7.2366(2) Å, c¼ 7.4453(2) Å,b¼ 120.231(2)� for M¼Ni. All these structures consist of a three dimensional (3D) framework built up ofinfinite chains of tilted corner-sharing [VO6] octahedra, cross-linked by corner-sharing [PO4] tetrahedra.The M2þ ion (M¼ Co, Ni) is located in a triangular based antiprism which shares faces with two [VO6]octahedra. Structural filiation is discussed based on a common structural unit, a sheet where divalentcations M2þ (M¼ Co, Ni) are inserted. A thermal study of the a 4 b transition is also presented.

� 2008 Elsevier Masson SAS. All rights reserved.

1. Introduction

The research on phosphate materials is currently in progress dueto their exceptional optical and laser properties [1,2]. The potas-sium titanyl phosphate KTiOPO4 [3,4] is a well-known material forits nonlinear optical properties which is of technological impor-tance. The synthesis of vanadyl phosphates M(VO)2(PO4)2 wasundertaken in order to look for new material analogues toKTiOPO4, in which Kþ has been replaced by divalent ions M2þ andTi4þ by V4þ. Two new compounds Co(VO)2(PO4)2 andNi(VO)2(PO4)2 were obtained, with respectively two phases nameda (low temperature) and b (high temperature). Recently thesecompounds were announced in a publication devoted to catalystsfor selective oxidations of light hydrocarbons [5]. In this paper cellparameters and space group are given and structural relationshipsare discussed based on Lazulite-types, but structure determinationsare not yet published. Here we present methods of syntheses in

þ33540002761.J.P. Chaminade).

son SAS. All rights reserved.

both forms, powder and single crystals. Single crystals are obtainedby grain growth in the solid phase without chemical vapourtransport as used by Glaum et al. [5]. Atomic structural parametersfor the four phases, description filiations based on our precedentresults on titanyl phosphates [6–9] and a thermal study of thea 4 b transition have been performed.

2. Experimental section

2.1. Powder syntheses

The reagents used for the synthesis of M(VO)2(PO4)2 (M¼ Co,Ni) were VO2 (or V2O3 and V2O5) and M(PO3)2 (M¼ Co, Ni). M(PO3)2

metaphosphates were prepared from stoichiometric proportions ofM(NO3)2$6H2O (M¼ Co, Ni) and NH4H2PO4 dissolved in distilledwater, then heated progressively until 600 �C. Crystalline powdersof M(VO)2(PO4)2 (M¼ Co, Ni) were synthesized by solid statereaction, according to the following reaction: M(PO3)2þ 2VO2 /

M(VO)2(PO4)2 (M¼ Co, Ni). Stoichiometric proportions of thereagents were put in a gold tube. The gold tube was introduced intosilica tube which was sealed under vacuum and heated at 700 �C for

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 629

40 h for a phase, and at 900 �C for 40 h for b phase. The powders ofM(VO)2(PO4)2 (M¼ Co, Ni) are green. Their purity was checked byX-ray diffraction analysis with a ‘‘PANALYTICAL X’PERT PRO’’ q–q-diffractometer (Cu Ka1,2 radiation). The use of the ‘‘profile match-ing’’ option of FULLPROF program [10] confirms that all theobserved diffracted lines are taken into account by the proposedspace group, P21/c. In the case of a phase of Ni compound a Rietveldrefinement has been done using FULLPROF and a PANALYTICALX’PERT PRO’’ q–2q (Cu Ka1-Ge incident monochromator) diffrac-tometer data. The quality of this refinement (cRp¼ 0.113 and

Fig. 1. Powder X-ray diffraction patterns of Ni(VO)2(PO4)2. a. Powder X-ray diffraction p

cRwp¼ 0.142 (without background); c2¼ 2.04; Rb¼ 0.044 andRf¼ 0.041) constitute an ‘‘a posteriori’’ method for checkingchemical composition and structural model. The patterns aredifferent between a and b-ones but are very similar when changingM (Co or Ni). Fig. 1 shows results for Ni compounds.

2.2. Thermal study

DTA investigation was performed with MDTA 85 ‘‘SETARAM’’,5 �C/min.

attern of a-Ni(VO)2(PO4)2. b. Powder X-ray diffraction pattern of b-Ni(VO)2(PO4)2.

Table 2Crystal data and structure refinements for b-M(VO)2(PO4)2 with M¼ Co, Ni.

Formula b-Co(VO)2(PO4)2 b-Ni(VO)2(PO4)2

Formula weight 382.75 382.52Crystal size (mm) 80� 60� 50 85� 65� 65Colour Opaque with metallic

brightnessGreenish

Crystal system MonoclinicSpace group P21/cTemperature (K) 293(2)a (Å) 7.2742(2) 7.2691(2)b (Å) 7.2802(2) 7.2366(2)c (Å) 7.4550(2) 7.4453(2)b (�) 120.171(2) 120.231(2)Volume (Å3) 341.32(2) 338.39(2)Z 2Calculated density

(g/cm3)3.724 3.754

Wavelength (Å) 0.7107Diffractometer BRUKER–KAPPA–CCDScan method CCD ScansAbsorption coefficient

(mm�1)5.60 5.98

F(000) 366 368q range (�) 3.5–32.0 3.5–32.1Index ranges �10� h� 10, �10� k� 10,

�11� l� 11�10� h� 10,�10� k� 10, �11� l� 11

Reflections collected(I> 0s(I))

4294 4357

Independent reflections(I> 0s(I))

1185 [R(int)¼ 0.026] 1179 [R(int)¼ 0.025]1075 reflectionswith I> 2s(I)

1069 reflectionswith I> 2s(I)

Absorption correction Empirical SCALEPACK

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634630

High-temperature X-ray diffraction study has been done witha ‘‘PANALYTICAL X’PERT MPD’’ q–q-diffractometer (Cu Ka1,2 radia-tion) with a HTK16 Anton Paar furnace. Platinum heating filamentis filled with powder dusted with 20 mm sieve and sample surface iscorrected with a razor blade.

2.3. Single crystals and X-ray diffraction

Observation by optical microscope shows that the powders of a-M(VO)2(PO4)2 (M¼ Co, Ni) contain very small single crystals(w20 mm – Table 1). For b-M(VO)2(PO4)2 (M¼ Co, Ni) single crystals(Table 2) were obtained by heating the corresponding crystallinepowders at 1100 �C for 2 h until melting, followed by slow coolingat the rate of 5 �C h�1 until 500 �C. The furnace was then put offuntil room temperature.

Taking into account the small size of single crystals for a-Co(VO)2(PO4)2 and a-Ni(VO)2(PO4)2, X-ray measurements weredone on a RIGAKU R–AXIS RAPID equipped with a rotating Cuanode. For b-Co(VO)2(PO4)2 and b-Ni(VO)2(PO4)2 compounds datacollections have been done with a Nonius Kappa CCD diffractom-eter (Mo Ka radiation with a graphite monochromator). Details aresummarized in Tables 1 and 2.

3. Structure determinations

The extinction conditions observed for M(VO)2(PO4)2 (M¼Ni,Co) agree with the space group P21/c. The data were corrected forLorentz-polarization effects, and empirical absorption corrections

Table 1Crystal data and structure refinements for a-M(VO)2(PO4)2 with M¼ Co, Ni.

Formula a-Co(VO)2(PO4)2 a-Ni(VO)2(PO4)2

Formula weight (g) 382.75 382.52Crystal size (mm) 25� 25� 10 25� 25� 15Colour Light green GreenCrystal system MonoclinicSpace group P21/cTemperature (K) 293(2)a (Å) 6.310(1) 6.297(2)b (Å) 7.273(2) 7.230(2)c (Å) 7.432(2) 7.421(2)b (�) 90.43(2) 90.36(2)Volume (Å3) 341.1(1) 337.8(2)Z 2Calculated density

(g/cm3)3.727 3.760

Wavelength (Å) 1.5418Diffractometer RIGAKU R–AXIS RAPID (rotating Cu-anode)Scan method Image Plate-multi-scansAbsorption coefficient

(mm�1)46.4 31.1

F(000) 366 368q range (�) 6.5–68.0 6.5–71.5Index ranges �7� h� 7, �8� k� 8,

�8� l� 8�7� h� 7, �8� k� 8,�8� h� 9

Reflections collected(I> 0s(I))

1844 1904

Independent reflections(I> 0s(I))

623 [R(int)¼ 0.058] 645 [R(int)¼ 0.040]596 reflectionswith I> 2s(I)

623 reflections withI> 2s(I)

Absorption correction Empirical multi-scansRefinement method Full-matrix

least-squares on F2

Data/restraints/parameters 623/0/71 645/0/71Goodness of fit on F2 1.123 1.105Final R indices R1 [I> 2s(I)]¼ 0.031 R1 [I> 2s(I)]¼ 0.0301

wR2[I> 0s(I)]¼ 0.080 wR2[I> 0s(I)]¼ 0.081Extinction coefficient 0.0014(6) 0.0040(8)Largest diff. peak

and hole (e �3)0.67 (near V) and�0.51 (near O(3))

0.68 (near O3) and�0.60 (near O(3))

Refinement method Full-matrixleast-squares on F2

Data/restraints/parameters 1185/0/71 1179/0/71Goodness of fit on F2 1.269 1.090Final R indices R1 [I> 2s(I)]¼ 0.023 R1 [I> 2s(I)]¼ 0.019

wR2 [I> 0s(I)]¼ 0.090 wR2 [I> 0s(I)]¼ 0.047Extinction coefficient 0.046(5) 0.025(2)Largest diff, peak

and hole e �31.04 (near V) and�0.66 (near Co)

0.47 (near O(3)) and�0.52 (near V)

were carried out using SCALEPACK program [11]. The structures ofboth crystals were solved with SHELXS-97 program by deconvo-lution of the Patterson function and the heavy-atom method, andrefinements with anisotropic displacement parameters for allatoms were done with SHELXL-97 program [12] (Tables 1 and 2).

The final atomic coordinates with the equivalent isotropicdisplacement parameters are presented in Tables 3 and 4. Selectedinteratomic distances and angles and bond valence sums are listed in

Table 3Atomic coordinates and equivalent isotropic displacement parameters (Å2�104) fora-M(VO)2(PO4)2 with M¼ Co (first line), Ni (second italic line).

Atom Wyckoff site x y z U(eq)

Co/Ni 2a 0 0 0 77(3)0 0 0 65(3)

V 4e 0.73831(9) 0.21958(10) 0.32731(9) 57(3)0.74064(9) 0.21948(9) 0.32690(8) 47(3)

P 4e 0.24957(14) 0.12245(13) 0.37563(14) 53(3)0.24987(13) 0.12349(13) 0.37382(12) 46(3)

O(1) 4e 0.7594(4) 0.1565(4) 0.1178(4) 101(7)0.7645(4) 0.1528(4) 0.1174(4) 92(6)

O(2) 4e 0.7781(4) 0.0059(4) 0.7909(4) 84(7)0.7795(4) 0.0047(4) 0.7930(4) 76(6)

O(3) 4e 0.2625(4) 0.4918(4) 0.0455(4) 108(7)0.2622(4) 0.4916(4) 0.0441(4) 92(6)

O(4) 4e 0.4405(4) 0.2519(4) 0.8548(4) 85(6)0.4419(4) 0.2515(4) 0.8537(4) 73(6)

O(5) 4e 0.9479(4) 0.7494(4) 0.1170(4) 76(6)0.9469(4) 0.7525(4) 0.1176(4) 61(6)

Table 4Atomic coordinates and equivalent isotropic displacement parameters (Å2�104) forb-M(VO)2(PO4)2 with M¼ Co (first line), Ni (second italic line).

Atom Wyckoff site x y z U(eq)

Co/Ni 2a 0 0 0 75(2)0 0 0 57(1)

V 4e 0.73645(5) 0.22110(5) 0.54141(5) 55(1)0.73893(4) 0.22085(4) 0.54314(4) 45(1)

P 4e 0.25041(8) 0.12163(7) 0.74838(8) 50(2)0.25066(6) 0.12292(6) 0.74971(6) 41(1)

O(1) 4e 0.7581(2) 0.1541(2) 0.7602(2) 96(3)0.7636(2) 0.1505(2) 0.7630(2) 82(2)

O(2) 4e 0.7787(3) 0.0049(2) 0.0988(3) 86(3)0.7806(2) 0.0039(2) 0.0982(2) 72(2)

O(3) 4e 0.4384(3) 0.2493(2) 0.8627(3) 102(3)0.4406(2) 0.2495(2) 0.8652(2) 92(3)

O(4) 4e 0.2702(3) 0.0064(2) 0.5905(3) 116(4)0.2691(2) 0.0060(2) 0.5913(2) 104(3)

O(5) 4e 0.0495(3) 0.2536(2) 0.1390(3) 81(3)0.0510(2) 0.2504(2) 0.1406(2) 70(2)

Table 5Bond distances (Å), bond valences (BV) and angles (�) for a-M(VO)2(PO4)2.

M¼ Co M¼Ni

Distance (Å) BV Distance (Å) BV

2�M–O(5)i,ii 2.047(3) 0.383 2.020(3) 0.3722�M–O(2)iii,iv 2.084(3) 0.347 2.064(3) 0.3302�M–O(1)v,vi 2.095(3) 0.336 2.048(3) 0.345

C2.075D

Ps¼ 2.13 C2.044D

Ps¼ 2.09

V–O(1) 1.630(3) 1.516 1.636(3) 1.492V–O(4)vii 1.903(3) 0.725 1.905(3) 0.721V–O(3)viii 1.907(3) 0.717 1.905(3) 0.721V–O(2)vii 2.031(3) 0.513 2.025(3) 0.521V–O(5)ix 2.031(3) 0.513 2.022(3) 0.526V–O(1)x 2.342(3) 0.221 2.349(3) 0.217

C1.974D

Ps¼ 4.20 C1.974D

Ps¼ 4.20

P–O(3)x 1.513(3) 1.325 1.515(3) 1.317P–O(4)vii 1.521(3) 1.296 1.518(3) 1.307P–O(5)viii 1.552(3) 1.192 1.552(3) 1.192P–O(2)iii 1.559(3) 1.170 1.557(3) 1.176

C1.536D

Ps¼ 4.98 C1.536D

Ps¼ 4.99

Angle (�) Angle (�)

O(5)ii–M–O(2)iii 77.0(1) 77.1(1)O(5)ii–M–O(1)vi 79.2(1) 80.1(1)O(2)iii–M–O(1)vi 79.4(1) 79.8(1)

O(4)vii–P–O(5)viii 106.5(2) 106.4(2)O(2)iii–P–O(5)viii 107.4(1) 107.4(1)O(3)x–P–O(5)viii 109.6(2) 109.4(2)O(2)iii–P–O(3)x 109.8(2) 110.0(2)O(2)iii–P–O(4)vii 111.3(2) 111.6(2)O(3)x–P–O(4)vii 112.1(2) 111.9(1)

C109.5D C109.5D

O(1)x–V–O(5)ix 73.9(1) 73.2(1)O(1)x–V–O(2)vii 74.8(1) 73.7(1)O(2)vii–V–O(5)ix 78.5(1) 77.9(1)O(1)x–V–O(3)viii 83.0(1) 83.1(1)O(1)x–V–O(4)vii 84.8(1) 85.3(1)O(3)viii–V–O(5)ix 89.9(1) 90.7(1)O(2)vii–V–O(4)vii 91.7(1) 91.4(1)O(3)viii–V–O(4)vii 92.0(1) 91.8(1)O(1)–V–O(5)ix 97.9(1) 97.7(1)O(1)–V–O(2)vii 98.0(1) 99.2(1)O(1)–V–O(4)vii 102.8(1) 103.2(1)O(1)–V–O(3)viii 103.3(1) 102.9(1)O(2)vii–V–O(3)viii 157.0(1) 156.3(1)O(4)vii–V–O(5)ix 158.2(1) 157.9(1)O(1)–V–O(1)x 169.9(1) 169.3(1)

Symmetry transformations used to generate equivalent atoms: i�xþ 1, �yþ 1, �z;iix� 1, y� 1, z; iii�xþ 1, �y, �zþ 1; ivx� 1, y, z� 1; vx� 1, y, z; vi�xþ 1, �y,�z;viix, �yþ 1/2, z� 1/2; viii�xþ 1, y� 1/2, �zþ 1/2; ix�xþ 2, y� 1/2, �zþ 1/2;xx, �yþ 1/2, zþ 1/2.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 631

Tables 5 and 6. Bond valences are calculated using the ‘‘Brown method’’[13]: Vi¼

PjVij; Vij¼ exp [(Rij� dij)/b] with b¼ 0.37 Å; Rij characterized

a cation–anion pair (O2�: 1.774 Å for V4þ, 1.654 Å for Ni2þ, 1.680 Å forCo2þ and 1.617 Å for P5þ); dij is the distance between i and j atoms. Theresults are in good agreement with the theoretical values for theexpected formal oxidation states of Ni2þ, Co2þ, V4þ, P5þ and O2� ions.

4. Structure descriptions

The structures of M(VO)2(PO4)2 (M¼ Co, Ni), in both forms a andb, consist of a three dimensional 3D-framework built up from [VO6]octahedra, triangular based antiprism [MO6] and isolated [PO4]tetrahedra. The [VO6] octahedra share corners and form infinitechains along the c-axis (Fig. 2). These chains are linked by phos-phate tetrahedra to constitute the 3D-framework. Two successive[VO6] octahedra of the same chain are linked via the oxygen O(1)atom, which does not belong to the [PO4] tetrahedron. Co2þ andNi2þ cations are located in a triangular based antiprism sharingfaces with two [VO6] octahedra.

4.1. a-M(VO)2(PO4)2 (M¼ Co, Ni)

4.1.1. VO6 octahedraVanadium atoms are displaced from the centre of the octahedron

giving rise to an alternating long (Co: 2.342(3); Ni: 2.349(3) Å) andshort (Co: 1.630(3); Ni: 1.636(3) Å) V–O(1) distances while the fourremaining V–O bond distances have intermediate values rangingbetween 1.903(3) and 2.031(3) Å for Co (average value of 1.974 Å); andbetween 1.905(3) and 2.022(3) Å for Ni (average value of 1.974 Å).Theshortest V–V distance is 3.742 Å for M¼ Co and 3.737 for M¼Ni.

4.1.2. MO6 polyhedra (M¼ Co, Ni)Co2þ and Ni2þ cations are located in a triangular based antiprism

sharing faces with two [VO6] octahedra. This connection by facesimplies short Co–V (2.916 Å) and Ni–V (2.899 Å) distances. M–Odistances are ranging from 2.047(3) to 2.095(3) Å (average value:2.075 Å) for Co, and from 2.020(3) to 2.048(3) Å (average value:2.044 Å) for Ni. Average values are in good agreement with the sumof the ionic radii of O2� (CN¼ 3) and M2þ ions (CN¼ 6): 2.11 Å (HS)for Co and 2.05 Å for Ni. The shortest Co–Co and Ni–Ni distances are5.199 Å and 5.180 Å respectively.

4.1.3. PO4 tetrahedra[PO4] tetrahedra share oxygen atoms with four [VO6] octahedral

groups. O(2) and O(4) belong to the same chain while O(3) and O(5)belong to two different chains. Therefore, one [PO4] group connects

three different chains.–V–O(1)–V–O(1)–V–. The [PO4] tetra-hedra are isolated from each others and are rather regular with O–P–O angles varying from 106.5(2)� to 112.1(2)� for Co, and from106.4(2)� to 111.9(1)� for Ni. P–O bond distances are rangingbetween 1.513(3) Å and 1.559(3) Å for Co and between 1.515(3) Åand 1.557(3) Å for Ni.

4.2. b-M(VO)2(PO4)2 (M¼ Co, Ni)

b-M(VO)2(PO4)2 (M¼ Co, Ni) was found to be isostructural withNi(TiO)2(PO4)2 [6].

4.2.1. VO6 octahedraHere also vanadium atoms are displaced from the centre of the

octahedron giving rise to an alternating long (Co: 2.362(2); Ni:2.372(1) Å) and short (Co: 1.633(1); Ni: 1.636(1) Å) V–O(1)distances while the four remaining V–O bond distances haveintermediate values ranging between 1.903(2) and 2.031(2) Å forCo (average value of 1.977 Å); and between 1.900(1) Å and

Table 6Bond distances (Å), bond valences (BV) and angles (�) for b-M(VO)2(PO4)2.

M¼ Co M¼Ni

Distance (Å) BV Distance (Å) BV

2�M–O(5)i 2.058(2) 0.372 2.031(1) 0.3612�M–O(2)ii,iii 2.080(2) 0.350 2.062(1) 0.3322�M–O(1)iv,v 2.095(2) 0.336 2.049(1) 0.344

C2.078D

Ps¼ 2.12 C2.047D

Ps¼ 2.07

V–O(1) 1.633(2) 1.508 1.636(1) 1.492V–O(3)vi 1.903(2) 0.725 1.900(1) 0.731V–O(4)v 1.914(2) 0.704 1.908(1) 0.715V–O(5)vii 2.021(2) 0.527 2.014(1) 0.537V–O(2)viii 2.031(2) 0.513 2.026(1) 0.520V–O(1)vi 2.362(2) 0.210 2.372(1) 0.204

C1.977D

Ps¼ 4.19 C1.976D

Ps¼ 4.20

P–O(4) 1.510(2) 1.335 1.511(1) 1.332P–O(3) 1.513(2) 1.325 1.513(1) 1.325P–O(5)viii 1.558(2) 1.173 1.556(1) 1.179P–O(2)v 1.561(2) 1.167 1.561(1) 1.163

C1.536D

Ps¼ 5.00 C1.535D

Ps¼ 5.00

Angle (�) Angle (�)

O(2)ii–M–O(5) 77.1(1) 77.2(1)O(1)iv–M–O(5) 79.0(1) 80.0(1)O(1)iv–M–(O(2)ii 79.9(1) 80.5(1)

O(3)–P–O(5)viii 106.3(1) 106.5 (1)O(2)v–P–O(5)viii 106.9(1) 106.7(1)O(2)v–P–O(4) 110.0(1) 109.9(1)O(4)–P–O(5)viii 110.3(1) 110.4(1)O(3)–P–O(4) 111.5(1) 111.6(1)O(2)v–P–O(3) 111.6(1) 111.6(1)

C109.4D C109.4D

O(1)vi–V–O(5)vii 73.7(1) 72.9(1)O(1)vi–V–O(2)viii 74.8(1) 73.9(1)O(2)viii–V–O(5)vii 79.1(1) 78.4(1)O(1)vi–V–O(4)v 82.7(1) 82.7(1)O(1)vi–V–O(3)vi 84.0(1) 84.7(1)O(4)v–V–O(5)vii 89.8(1) 90.6(1)O(3)vi–V–O(4)v 90.7(1) 90.5(1)O(2)viii–V–O(3)vi 92.0(1) 91.8(1)O(1)–V–O(5)vii 98.2(1) 97.8(1)O(1)–V–O(2)viii 98.9(1) 100.0(1)O(1)–V–O(4)v 102.7(1) 102.5(1)O(1)–V–O(3)vi 103.7(1) 104.2(1)O(2)viii–V–O(4)v 156.9(1) 156.1(1)O(3)vi–V–O(5)vii 157.4(1) 157.3(1)O(1)–V–O(1)vi 170.4(1) 169.6(1)

Symmetry transformations used to generate equivalent atoms: i�x, �y, �z; iix� 1, y,z; iii�xþ 1, �y, �z; ivx� 1, y, z� 1; v�xþ 1, �y, �zþ 1; vix, �yþ 1/2, z� 1/2 ;viixþ 1,�yþ 1/2, zþ 1/2; viiix, �yþ 1/2, zþ 1/2.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634632

2.026(1) Å for Ni (average 1.976 Å). The shortest V–V distance is3.751 Å for M¼ Co and 3.746 for M¼Ni.

4.2.2. MO6 polyhedra (M¼ Co, Ni)In the triangular based antiprism M–O distances are ranging

from 2.058(2) to 2.095(2) Å (average value: 2.078 Å) for Co, andfrom 2.031(1) Å to 2.062(1) Å (average value: 2.047 Å) for Ni. Herealso average values are in good agreement with the sum of ionic

Fig. 2. V–O–V–O– chain and PO4 tetrahedra in M(VO)2(PO4)2 (M¼ Co, Ni).

radii of O2þ and M2þ. Co–Co and Ni–Ni distances are 5.210 Å and5.191 Å respectively. Like in a phase, connection between MO6

polyhedra and VO6 octahedra by faces leads to short M–V distances,2.913 Å for Co–V and 2.898 Å for Ni–V. Investigations on possibleinduced magnetic interactions have been undertaken.

4.2.3. PO4 tetrahedraLike in the a-form [PO4] tetrahedra share oxygen atoms with

four [VO6] octahedral groups and connects three differentchains.–V–O(1)–V–O(1)–V–. They are rather regular with O–P–O angles varying from 106.3(1)� to 111.6(1)� for Co, and from

Fig. 3. Structural filiations for a-M(VO)2(PO4)2 (M¼ Co, Ni). a. Li(TiO)(PO4) ortho-rhombic type. b. LiNi0.50(TiO)2(PO4)2 monoclinic type. c. a-M(VO)2(PO4)2 (M¼ Co, Ni)type.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634 633

106.5(1)� to 111.6(1)� for Ni. P–O bond distances are rangingbetween 1.510(2) and 1.561(2) Å for Co and between 1.511(1) and1.561(1) Å for Ni.

Fig. 5. (A,B) sheet as common structural unit for a and b-M(VO)2(PO4)2 (M¼ Co, Ni).

5. Structural filiations

5.1. The common structural unit: (A,B) sheet

Structural filiations for a-M(VO)2(PO4)2 (M¼ Co, Ni) andbetween a and b varieties can be done based on ‘‘(A,B) sheets’’ ascommon structural unit (Fig. 5). One sheet is constituted of twoplanes (A and B) parallel to (1 0 0) of infinite chains of [VO6]octahedra (Fig. 2) linked together along b axis by [PO4] tetrahedra.Divalent cations M2þ (M¼ Co, Ni) are inserted inside these sheets,in trigonal antiprisms of the (2a) site.

5.2. Structural filiations for a-M(VO)2(PO4)2 (M¼ Co, Ni)

The three dimensional framework of the a phase can bedescribed by a succession along a of (A,B) sheets with M2þ cations(Fig. 3c). The (2b) sites between two successive (A,B) sheets areempty. This description can be correlated with the Li(TiO)(PO4)(orthorhombic Pnma) [14] and LiNi0.50(TiO)2(PO4)2 (monoclinicP21/c) [7] types.

In Li(TiO)(PO4) (2a) and (2b) sites are equivalent, correspondingto the (4a) site of the orthorhombic Pnma space group. This site isfully occupied by Liþ cations (Fig. 3a).

For LiNi0.50(TiO)2(PO4)2 the monoclinic deformation of the cell(P21/c) is quite similar to that we observed for the a phase: the (2a)site inside the (A,B) sheet is fully occupied by Liþ ions while the (2b)site between two sheets is partially occupied (50%) by Ni2þ ions(Fig. 3b).

5.3. Structural filiations between a and b varieties

In the b phase we can define a (A0 ,B0) sheet deduced from the(A,B) one by a c/2 translation. The three dimensional frameworkcan then be described by a succession of .(A,B)(A0 ,B0)(A,B)(A0 ,B0).sheets along a parameter, with a monoclinic deformation ofNi(TiO)2(PO4)2 type [6] (Fig. 4). Here also the (2b) site between twosuccessive sheets is empty.

In the b-cell, taking into account the origin translationOaOb¼ (0,1/2,1/2) and for atoms in the (A,B) sheet corresponding tox¼ 0 (Fig. 5), the atomic coordinate relations are the following:

xb ¼ xa; yb ¼ �ya þ 1=2; zb ¼ xa=2� za þ 1=2

Fig. 4. b-M(VO)2(PO4)2 (M¼ Co, Ni) type.

6. Thermal study of the a-M(VO)2(PO4)2 4 b-M(VO)2(PO4)2

(M [ Co, Ni) transition

- As indicated in Section 2.1 obtaining of a or b phase depends onthe heating temperature of the reagents (VO2 and M(PO3)2)used for the synthesis.

- If the silica tube is heated at 700 �C for 40 h, the a phase isobtained, and if it is heated at 900 �C for 40 h, it is the b phase.Mixture of the two phases is observed when cooling fromintermediate temperatures.

- When the a phase undergoes a thermal treatment in silica tubeat 900 �C for 40 h, the a phase is conserved.

6.1. DTA study

DTA studies have been done starting with a or b phase forM(VO)2(PO4)2 with M¼ Co or Ni in sealed platinum tube underargon. Starting with the a phase, no phase transition effect isobserved until melting (1050 �C for Co and 1080 �C for Ni), thencooling to room temperature re-crystallization leads to the b phase.Fig. 6 illustrate results for M¼Ni. Starting with the b phase, nophase transition is observed until melting (w1080 �C), and b phaseis kept after re-crystallization.

6.2. High-temperature X-ray diffraction study

An X-ray diffraction study has been done for a-Ni(VO)2(PO4)2

with He gas-flow in the HTK16 furnace. Several data collectionshave been carried out during heating (25, 100, 200, 300, 400, 500,600, 700, 800, 900, 1000, 1020, 1040,. �C) with the followingconditions: angular range 10–80� (2q) ; steps of 0.02� (2q); countingtime of 5 s by step; slope of 2 �C/min and a holding time of 30 minfor each level. We particularly followed the evolution of the dif-fracted peak w22�(2q), which would disappear if a / b transitionoccurs (Fig. 1). Up to 1100 �C pattern’s evolution shows dilatation ofthe a-cell, lowering of intensities, but no transition. Fig. 7 shows cellparameters and volume dependence with increasing temperature.

Fig. 6. DTA study starting with a-Ni(VO)2(PO4)2.

0 200 400 600 800 10006,2

6,4

6,6

6,8

7,0

7,2

7,4

7,6

90,0

90,2

90,4

338

340

342

344

346

b

c

V

β

a, b

, c (Å

)

T (°C)

a

V (Å

3)

β (°)

Fig. 7. a-Ni(VO)2(PO4)2 cell parameters and volume dependence with increasingtemperature.

S. Kaoua et al. / Solid State Sciences 11 (2009) 628–634634

At 1120 �C a strong relative decrease of the diffracted peakw22�(2q) is observed but with the beginning of melting. At 1140 �Cmelting is done. After cooling down to room temperature, powderon Pt-filament is recovered and X-ray diffraction analysis confirmsit is the b phase. These observations agree with the DTA results. Thehigher values obtained for melting temperatures in the X-ray studycan be explained by systematic error between temperature controlof the HTK16 furnace and real surface temperature of the sample.

7. Conclusion

New vanadium oxyphosphates MII(VIVO)2(PO4)2 (MII¼ Co, Ni)have been obtained. According to synthesis temperature withstarting reagents, two phases named a and b have been prepared.The four crystal structures have been determined in monoclinicP21/c space group using X-ray single crystal diffraction data.

Structure of the a phase can be correlated with Li(TiO)(PO4) andLiNi0.50(TiO)2(PO4)2 types. Structure of the b phase is derived from

the Ni(TiO)2(PO4)2-type we have found for several titanium oxy-phosphates MII(TiO)2(PO4)2. Structural filiation has been discussedbased on a common structural sheet built up of parallel infinitechains of tilted corner-sharing [VO6] octahedra, cross-linked bycorner-sharing [PO4] tetrahedra. Divalent cations M2þ (M¼ Co, Ni)are inserted in trigonal antiprism sites of these sheets.

The thermal study, by DTA or by X-ray diffraction, shows thatthe a phase obtained from the mixture of starting reagents, atw700 �C, is stable until melting. Whatever the starting phase is(a or b), after melting the b phase is always obtained.

Acknowledgements

This work was supported by the ‘‘Programme International deCooperation Scientifique’’ (PICS) of CNRST/Morocco-CNRS/France.The authors thank E. Lebraud for high temperature X-rays diffrac-tion studies, P. Dagault and D. Denux for DTA study.

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