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Solid State Sciences 10 (2008) 991e997www.elsevier.com/locate/ssscie

Synthesis, crystal structure and thermal behaviour of trihydrated acidholmium diphosphate HHoP2O7$3H2O

Sana Hraiech a,b, Fathia Chehimi-Moumen a,*, Christelle Goutaudier b,D. Ben Hassen-Chehimi a, Malika Trabelsi-Ayadi a

a Laboratoire d’Application de la Chimie aux Ressources et Substances Naturelles et a l’Environnement, Departement de Chimie,

Faculte des Sciences de Bizerte, Zarzouna, Bizerte 7021, Tunisiab Laboratoire des Materiaux Luminescents, Universite Claude Bernard Lyon 1, Cedex, France

Received 21 February 2007; received in revised form 11 July 2007; accepted 12 July 2007

Available online 21 July 2007

Abstract

Synthesis, structure refinement and thermal behaviour are reported for a new diphosphate HHoP2O7$3H2O. This salt crystallizes in the tri-clinic system with space group P1and with lattice parameters: a¼ 6.425(5), b¼ 6.912(5), c¼ 9.809(5) A; a¼ 81.554(5)�, b¼ 80.329(5)�,g¼ 88.485(5)� and Z¼ 2. The crystal structure is refined to R¼ 0.0443 using 859 reflections. The corresponding atomic arrangement is builtup by thick layers of HP2O7

3� anions and HoO8 polyhedra parallel to the (ab) plane.IR and Raman spectra of the HHoP2O7$3H2O are interpreted on the basis of characteristic vibrations of OeH and P2O7 groups. No coin-

cidence of most of Raman and IR band positions confirm the centrosymmetric structure for this salt. TG and DTA investigations show that theelimination of the crystallization water takes place in two stages between 30 and 340 �C. The stability of the anhydrous salt is very limited. Itdecomposes between 340 and 900 �C to give Ho2P4O13.

The holmium orthophosphate HoPO4 (Xenotime) is obtained when heating HHoP2O7$3H2O in a static air furnace at 1200 �C.� 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Inorganic compound; Chemical synthesis; X-ray diffraction; Crystal structure; Thermogravimetric analysis

1. Introduction [7,8] are identified. The first one concerns the HLnP2O7$3H2O

The fluorescence properties of lanthanide diphosphates ofgeneral formula MILnP2O7 (MI¼ Rb, Cs; Ln¼Y, Dy, Gd)have been investigated during the last 20 years [1,2]. Thesephosphates are found to show very weak concentrationquenching. Studies concerning the protonated lanthanide di-phosphates of formula HLnP2O7$xH2O show that these com-pounds crystallize with various hydration states [3e9]. Thedehydration of some of them leads to anhydrous diphosphateHLnP2O7 [4] which can have good optical proprieties. Singlecrystal structures are reported only for trihydrated and tetrahy-drated salts. Two types of trihydrates, denoted by type I and II

* Corresponding author. Tel.: þ216 22 866 307; fax: þ216 72 491 526.

E-mail address: fathia.chehimi@fsb.rnu.tn (F. Chehimi-Moumen).

1293-2558/$ - see front matter � 2007 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.solidstatesciences.2007.07.021

with Ln¼ LaeSm (type I) crystallizing in the orthorhombicsystem (space group Aba2). Their atomic arrangement isdescribed for the lanthanum salt [8]. From Sm to Yb, trihydratesdisplay the triclinic symmetry (type II), space group P1. The singlecrystal X-ray analysis is developed for the gadolinium saltHGdP2O7$3H2O [7]. Tetrahydrates HLnP2O7$4H2O are obtained,until now, for Ln¼ Sm [9], Gd [10] and Eu [11]; these compoundscrystallize in the monoclinic system, space group P21/n.

The formation of well crystallized dihydrates, monohy-drates and hemihydrates is also reported [12,13]. They are ob-tained by thermal dehydration of the corresponding trihydrateddiphosphates.

Thermal studies, carried out in our laboratory for some ofthese salts, show that their thermal decompositions occur indifferent pathways even for isostructural compounds studiedunder the same conditions (nature of the atmosphere above

Table 1

Crystal data and experimental parameters for the X-ray intensity data

collection

I Crystal data

Chemical formula: HHoP2O7$3H2O Formula weight: 393.93 g mol�1

Crystal system: Triclinic Space group: P1

a¼ 6.425(5), b¼ 6.912(5),

c¼ 9. 809(5) A

V¼ 424.7(7) A3

a¼ 81.554(5)�; b¼ 80.29(5)�;

g¼ 88.485(5)�Z¼ 2

rcal¼ 3.080(4) g cm�3 F(000)¼ 368

Absorption coefficient m¼ 9.722 mm�1

Crystal size (0.10� 0.09� 0.07) mm3

II Intensity measurement

Temperature: 298 K Wavelength: Ka (Mo), l¼ 0.107 A

Diffractometer: Nonius

Kappa CCD

Scan mode: u/2q

Monochromator: graphite Theta range: 4.26< q< 28.72�

Measurement area 0� h� 8, �6� k� 8, �12� l� 13

Total number

of scanned reflections

1077

III Structure determination

Program used WINGX [14]

SHELXS 93 [15]

SHELXL 97 [16]

Unique reflection include 859 pour I> 2s(I )

Weighting scheme WGHT¼ 1/[s2(Fo2)þ (0.676P)2]

where P¼ (Fo2þ 2Fc

2)/3

Unweighted agreement factor R¼ 0.0443

Weighted agreement factor Rw¼ 0.1029

Table 2

Final atomic coordinates for HHoP2O7$3H2O, Ueq (A2) for non-hydrogen

atoms and Uiso (A2) for hydrogen atoms

Atoms x (s) y (s) z (s) Ueq (s)

Ho 0.62696(7) 0.7396(1) 0.54700(6) 0.0143(3)

P1 0.1947(4) 0.7662(7) 0.4375(3) 0.016(1)

P2 0.9420(5) 0.8601(8) 0.2117(4) 0.016(1)

O(E12) �0.016(1) 0.742(2) 0.5313(8) 0.016(4)

O(E11) 0.332(1) 0.587(1) 0.4443(9) 0.017(3)

O(E13) 0.330(1) 0.923(2) 0.468(1) 0.028(5)

O(E21) 0.933(1) 0.073(2) 0.164 (1) 0.029(6)

O(E22) 0.990(1) 0.738(2) 0.086 (1) 0.033(4)

O(E23) 0.755(1) 0.771(2) 0.3117(9) 0.030(4)

O(L) 0.151(1) 0.820(2) 0.282(1) 0.026(5)

O(W1) 0.335(1) 0.633(2) 0.7257(9) 0.026(5)

O(W2) 0.710(1) 0.726(2) 0.776(1) 0.034(4)

O0(W3)a 0.493(6) 0.31(1) 0.969(5) 0.10(2)

O00(W3)a 0.06(6) 0.12(2) 0.988(6) 0.23(9)

Uiso

992 S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

the sample during its decomposition and heating rate). In orderto explain their decomposition process, it seems to be neces-sary to examine their structural arrangement and search forthe origin of these differences. We report in this paper thechemical preparation, the vibrational spectra and the thermalbehaviour of holmium compound HHoP2O7$3H2O whichexhibits triclinic symmetry and is isostructural with the trihy-drated gadolinium diphosphate previously reported [7,12].

2. Experimental

Single crystals of HHoP2O7$3H2O are obtained by mixing10 mL of holmium oxide solution Ho2O3 (8� 10�2 M) with10 mL of a sodium diphosphate Na4P2O7$10H2O aqueous so-lution (10�1 M). The mixture is evaporated at room tempera-ture. The crystallization of HHoP2O7$3H2O thin plates startsfrom the solution after a few days. The single crystals arethen isolated and washed with distilled water.

The X-ray crystal diffraction data are collected on a Noniuskappa CCD diffractometer equipped with molybdenum radia-tion (l¼ 0.71073 A).

The crystal structure determination is performed using thePatterson heavy atom method for the location of the heavieratom (Ho) and successive difference-Fourier syntheses forphosphorus and oxygen atoms. At this stage, the introductionof the anisotropic thermal factors leads to R¼ 0.0452. A finaldifference-Fourier synthesis reveals the position of five hydro-gen atoms. Because of their refinement instability, the posi-tions and the thermal factors of these atoms are fixed. Thefinal cycle of least-squares refinement including 859 reflec-tions leads to R value of 0.0443 and Rw¼ 0.1029. Some re-finement details are given in Table 1. The final atomicpositions and anisotropic thermal parameters for the non-hy-drogen atoms are given in Tables 2 and 3, respectively. Theatom numbering scheme, used in this work, corresponds tothat in our report concerning HGdP2O7$3H2O in order to sim-plify the direct comparison between the two compounds.

IR absorption spectra are recorded on KBr pressed pelletsof the powdered samples in the range 4000e400 cm�1, usinga PerkineElmer FTIR 1000 spectrophotometer.

Raman spectrum is recorded using a Renishaw RM 1000spectrometer associated with a microscope (Leica) allowingthe selection of a region of good optical quality in the crystal-line sample. The wavelength available is 514 nm provided byargon ion laser.

Thermal analyses are performed using a TGA 851 of Met-tler Toledo thermobalance in the range 25e900 �C. Weighedsamples of 40e50 mg are placed in platinum crucible andheated in argon atmosphere, with a flow of 40 mL min�1, at5 �C min�1 heating rate.

3. Results and discussion

H(a) 0.8885 0.7223 0.0338 0.050

H(11) 0.1991 0.6844 0.8092 0.050

H(12) 0.3331 0.6249 0.5976 0.050

3.1. Structure analysis H(21) 0.7025 0.6452 0.7406 0.050

H(22) 0.7632 0.6967 0.8263 0.050

a Atoms with occupancy rate of 0.5.

The examination of the crystal coordinates of the Ho, P andO atoms in the present arrangement (Table 2) shows that they

are equivalent to those reported for the Gd, P and O atoms,respectively, in the HGdP2O7$3H2O structure. Although, thecoordinates of some of the hydrogen atoms are shifted signif-icantly, in particular those of the acidic hydrogen. This latter islinked to the O(E22) atom in both structures.

Table 3

Anisotropic thermal parameters (A2) for HHoP2O7$3H2O

Atoms U11 U22 U33 U23 U13 U12

Ho 0.0121(3) 0.0108(7) 0.0202(4) �0.0013(3) �0.0041(3) �0.0013(3)

P(1) 0.012(1) 0.017(4) 0.022(2) �0.009(2) �0.006(1) 0.001(1)

P(2) 0.013(1) 0.014(4) 0.020(1) �0.003(2) 0.001(1) �0.001(1)

O(E11) 0.016(4) 0.000(10) 0.031(5) 0.004(5) �0.004(4) 0.004(4)

O(E12) 0.001(3) 0.03(1) 0.014(4) 0.006(5) 0.004(3) �0.009(4)

O(E13) 0.017(4) 0.04(1) 0.030(5) 0.001(6) �0.009(4) �0.017(5)

O(E21) 0.032(5) 0.02(1) 0.033(6) �0.003(7) �0.006(4) 0.018(6)

O(E22) 0.028(5) 0.05(1) 0.026(6) �0.009(6) �0.002(4) �0.014(5)

O(E23) 0.009(4) 0.06(1) 0.019(5) �0.001(6) �0.004(4) �0.001(5)

O(L) 0.027(5) 0.02(1) 0.034(6) �0.016(6) �0.013(4) 0.030(5)

O(W1) 0.015(4) 0.02(1) 0.029(6) 0.007(6) 0.014(4) �0.011(5)

O(W2) 0.044(5) 0.04(1) 0.025(5) �0.007(6) �0.018(4) �0.010(6)

O(W3) 0.060(2) 0.5(2) 0.16(5) �0.3(1) �0.04(2) 0.12(7)

O00(W3) 0.04(1) 0.18(8) 0.09(2) �0.08(4) �0.01(2) 0.03(2)

993S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

As shown in Fig. 1, which is a projection on the (ac) plane,the HHoP2O7$3H2O atomic arrangement can be described asa succession of thick layers formed by diphosphoric groupand HoO8 polyhedra parallel to (ab) plane. Between theselayers is located the W3 water molecule, having the higherthermal factor and being split into two half-occupied positions(Tables 2 and 3). The two other crystallization water mole-cules are situated in the Ho environment.

As found in HGdP2O7$3H2O arrangement, the monohydro-gendiphosphate group observed in HHoP2O7$3H2O has no in-ternal symmetry and has a slightly staggered conformation.

Fig. 1. Projection in the (ab) plane of the a

The corresponding distorsion angles are: O(E23)eP1eP2eO(E11)¼ 16.45�, O(E22)eP1eP2eO(E12)¼ 10.71� andO(E21)eP2eP1eO(E13)¼ 12.62�.

The external PeO distances are comprised between 1.48(1)and 1.57(1) A, the two PeO distances of the PeOeP bridgeare 1.58(1) and 1.61(1) A (Table 4). The PeP distance isfound to be equal to 2.947(4) A and the PeOeP angle is134.8(6)�. All these distances and angles are in excellent ac-cordance with all the values commonly observed in condensedphosphate chemistry [17e22]. But it can be noted that thePeOeP angle is lower than that observed in HGdP2O7$3H2O

tomic arrangement in HHoP2O7$3H2O.

Table 4

Bond lengths (A) and angles (�) in the P2O7 group

P1 O(E11) O(E12) O(E13) O(L)

O(E11) 1.50(1) 2.53(1) 2.36(1) 2.50(1)

O(E12) 114.6(6) 1.505(8) 2.52(1) 2.48(1)

O(E13) 104.0(6) 114.1(6) 1.50(1) 2.50(2)

O(L) 108.3(5) 107.1(5) 108.6(7) 1.581(9)

P2 O(E21)a O(E22) O(E23) O(L)b

O(E21)a 1.48(1) 2.54(2) 2.53(2) 2.48 (1)

O(E22) 112.7(6) 1.57(1) 2.49 (1) 2.47(1)

O(E23) 116.9(7) 108.3(7) 1.50(1) 2.53 (1)

O(L)b 106.9(7) 101.7(6) 109.2(5) 1.61(1)

P1eP2 c 2.947(4) P1eO(L)eP2 134.8(6)�

O(E22)eH(a) 0.914(9) P2eO(E22)eH(a) 120.5(1)�

a Symmetry operation: x, 1þ y, z.b Symmetry operation: 1þ x, y, z.c Symmetry operation: �1þ x, y, z.

994 S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

(138.8�) considered being isostructural with HHoP2O7$3H2O.The POeH distance (0.914 (8) A and the PeOeH angle(120.5(1)�) observed in this arrangement are also slightly dif-ferent from those found in HGdP2O7$3H2O (0.848 A and118.4�, respectively).

The holmium cation is, as usual, eight-fold coordinatedsharing two oxygen atoms with water molecules (W1andW2) and six oxygens with four adjacent P2O7 groups belong-ing to the same layer (Fig. 2a). HoeO distances are comprisedbetween 2.25(1) and 2.602(9) A (Table 5). The HoO8 polyhe-dra assemble themselves, sharing two oxygen atoms, to pro-duce zigzag chains parallel to the b-axis (Fig. 2b). This factcan explain the short HoeHo distance observed in this ar-rangement (3.961(1) A).

The examination of the hydrogen bond network, in theatomic arrangement of HHoP2O7$3H2O (Fig. 3, Table 6),shows that the acidic hydrogen atom is involved in a weak in-teraction with an oxygen of the W3 water molecule (O(E22)/O0(W3)¼ 3.271(2); O0(W3)/Ha¼ 2.472(2) A), and not withany oxygen of adjacent diphosphate anion as observed inHGdP2O7$3H2O. In fact, in the last compound, supposed tobe isotructural with HHoP2O7$3H2O, the H acid is engaged

Fig. 2. (a) The coordination of the holmium atom. (b) Proje

in a strong H-bond with an external oxygen atom of theHP2O7

3� group as to form finite clusters. This arrangement isonly observed in HGdP2O7$3H2O. In all previously reportedatomic arrangements of acidic diphosphates, the acidic anionsare interconnected through strong H-bonds involving theacidic hydrogen and external oxygen atoms of adjacentdiphosphate anions as to form infinite networks [17e20].

The difference between the hydrogen bond networks ofHHoP2O7$3H2O and HGdP2O7$3H2O can be due to the differ-ence observed between the coordinates of the hydrogen atomsin both structures. Such a case is observed withMgHPO4$3H2O [23] and CaHAsO4$3H2O [24] which areknown to be isostructural.

3.2. Infrared and Raman spectra

The IR spectrum of HHoP2O7$3H2O is shown in Fig. 4; itis identical to that given for HGdP2O7$3H2O [12]. The corre-sponding Raman spectrum is illustrated in Fig. 5. Bandsobserved in the regions 3700e3000 cm�1 and 1700e1600 cm�1 in both spectra are being attributed to OH/H2Ovibrations [25,26]. The multiplicity observed for the stretchingand bending frequencies of H2O indicates different hydrogenbond strengths in the atomic arrangement of HHoP2O7$3H2O.This fact is in good agreement with the X-ray analysis results.The H2O rocking band is observed at 626 cm�1 in infraredspectrum and at 650 cm�1 in Raman spectrum.

Bands of weak intensity observed between 2920 and2364 cm�1 would correspond to the PeOeH vibration modes[25].

The different diphosphate vibration modes are observed inthe range 1300e400 cm�1 and 1250e200 cm�1 in the IR andRaman spectra, respectively [25,26]. The most noteworthyfeature of this group is the appearance of a strong intensityband at 942 cm�1 in the IR spectrum and at 993 cm�1 in theRaman one, corresponding to the nas(POP) stretching vibra-tion. But one can note the absence of the symmetric ns(POP)IR absorption band, which appears normally at about750 cm�1. According to the proposed correlation between

ction in the (ab) plane of chains of HoO8 dodecahedra.

Table 5

Main interatomic distances (A) in the HoO8 dodecahedra

HoeO(E11) 2.602(9) HoeO(E13)c 2.33(1)

HoeO(E11)b 2.25(1) HoeO(E23) 2.96(8)

HoeO(E12)a 2.27(7) HoeO(W1) 2.393(9)

HoeO(E13) 2.43(1) HoeO(W2) 2.38(1)

a Symmetry operation: xþ 1, y, z.b Symmetry operation: �xþ 1, yþ 1, �zþ 1.c Symmetry operation: �xþ 1, �yþ 2, �zþ 1.

Table 6

Principal characteristics of the hydrogen bonding in the atomic arrangement of

HHoP2O7$3H2O

OeH/O OeH (A) O/H (A) OeH/O (�) O/O (A)

O(E22)eH(a)/O(W3)a 0.914 2.472 146.12 3.271

O(W1)eH(11)/O(E21)a 1.177 1.891 134.49 2.843

O(W1)eH(12)/O(E12) 1.267 1.564 172.96 2.826

O(W2)eH(21)/O(E12)a 0.712 2.630 165.87 3.325

a Symmetry operation: �xþ 1, yþ 1.

995S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

the (POP) bridge angles in diphosphates and the difference(nasens) [26], the symmetric (POP) absorption band can be ab-sent only in the case of a (POP) angle equal to 180 �C. But it isnot the case for HHoP2O7$3H2O. Indeed, the POP angle in thisdiphosphate is found to be 134.8�. This vibration appears inthe Raman spectrum as a medium intensity band at 773 cm�1.

Vibrations occurring below 600 cm�1 can be assigned tothe PO3 deformation and rocking (400e600 cm�1), POP de-formation and external modes (400e200 cm�1).

The non-coincidence of the majority of the positions of theRaman and IR absorption bands confirms the centrosymmetricstructure [26] of HHoP2O7$3H2O.

3.3. Thermal behaviour

Fig. 6 shows both TG and DTA thermograms of HHo-P2O7$3H2O carried out in an argon atmosphere from roomtemperature to 900 �C.

TG curve shows that the decomposition process takes placein four stages between 30 and 900 �C. The first one is achievedat 125 �C, and corresponds to the loss of one water moleculeper formula unit (% exp¼ 4.8%; % th¼ 4.57%). It is accom-panied by an endothermic effect peaking at 130 �C. The sec-ond one occurs between 125 and 340 �C and is accompaniedby two endothermic peaks at 160 and 180 �C. The correspond-ing water loss is 8.84%. This value is comparable to the theo-retical percentage calculated for the removal of two watermolecules (9.13%).

Fig. 3. Hydrogen bond network in HHoP2O7$3H2O.

The third and the fourth steps occur in the ranges340e660 �C and 660e900 �C, respectively.

In order to identify and characterize the different phasesformed at the end of each thermal effect, heat treatment wascarried out. The elaborated product is heated in the DTA untila peak is obtained. When this happens, heating is stopped. Thesample is cooled down to room temperature then, subjected toX-ray and IR spectroscopic analysis. Heating is then, resumedwith a new sample of the same compound, and treatment, asdescribed above, was performed for each thermal effect foundin the DTA curve.

The results of the heat treatment are illustrated in Figs. 4and 7.

In the IR spectrum of the product heated at 125 �C (Fig. 4b)the OeH vibration bands are still present. The characteristicbands of the P2O7 group (ns(POP) and nas(POP)) are observedin the range 700e950 cm�1. One can note the appearance ofthe ns(POP) band at 760 cm�1, which was absent in the trihy-drate spectrum. According to the thermogravimetric analysis,this spectrum would correspond to the dihydrate HDy-P2O7$2H2O. The corresponding X-ray patterns (Fig. 7b)show that this salt is well crystallized and adopts a crystallinestructure different from that of the initial diphosphate.

After calcination at 340 �C, the IR spectrum of the heatedproduct (Fig. 4c) shows a decrease in the OeH absorptionbands’ intensity and the presence of the diphosphate vibration

4000 3000 2000 1000

HoPO4

Ho2P4O13

HHoP2O7

HHoP2O7.3H2O

HHoP2O7.2H2O

1650

3618

3655

f

2920

3450

2364

1624

1202 1118

1068

998 94

2

626 47

6ed

c

b

a

Wavenumbers (cm-1)

Fig. 4. IR spectra of HHoP2O7$3H2O at different temperatures: (a) dried at

room temperature; (b) calcined at 125 �C; (c) calcined at 340 �C; (d) calcined

at 660 �C; (e) calcined at 900 �C; (f) calcined at 1200 �C.

3500 3000 2500 2000 1500 1000 500

0

2000

4000

6000

8000

10000

Raman Schift (cm-1)

3681

3590

3488

3218

29138

941

156

164

977

399

31020

1096

1112

1204

Inte

nsity

(u.

a)

3000 400 800600

700

800

900

1000

1100

2380

1650

Fig. 5. Raman spectrum of HHoP2O7$3H2O.

996 S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

bands. According to the thermogravimetric results the removalof the three crystallization water molecules occurs in the firsttwo steps between 30 and 340 �C. So, this spectrum can be at-tributed to the anhydrous salt. The corresponding X-ray pat-terns (Fig. 7c) agree with those given in the literature forHLnP2O7 (Ln¼Dy, Er, Yb, Y [4] and Gd [12]). The holmiumanhydrous salt, obtained at 340 �C, is then isostructural withthese compounds. The crystal structures of these latter are stillunknown. It can be noted that dehydration temperature ofHHoP2O7$3H2O (340 �C) is considerably lower than that re-ported for HGdP2O7$3H2O (500 �C).

The X-ray patterns obtained after heating HHoP2O7 at660 �C (Fig. 7d) show a decrease in the crystallinity of theproduct and the appearance of supplementary lines. This indi-cates that a decomposition begins in this step. The correspond-ing IR spectrum (Fig. 4d) does not differ significantly fromthat of the anhydrous compound. A complex IR spectrum(Fig. 4e) is obtained when the product is heated at 900 �C

Fig. 6. TG and DTA curve

for 2 h. The corresponding X-ray patterns are comparable tothose given for Y2P4O13 and Gd2P4O13 [27e29]. It can benoted that the formation of tetraphosphates of formulasMn2P4O13 and Bi2P4O13 through thermal decomposition ofMnHP2O7 and BiHP2O7$2H2O, respectively, is mentioned inthe literature [30,31].

According to the TG results, the weight loss observed be-tween 340 and 900 �C is of 2.27%. This value is close tothe theoretical loss of 0.5 water molecule. So we think thatthe two last steps are due to the decomposition of the anhy-drous salt formed at 340 �C and the removal of the acidic hy-drogen. This decomposition leads to a salt of composition(Ho2O3$2P2O5) which may be Ho2P4O13 according to thereaction:

HHoP2O7 / 1=2H2O þ 1=2Ho2P4O13

When the product is heated at 1200 �C for 30 h, in a staticair furnace, only HoPO4 (Xenotime) bands and lines were ob-served, respectively, in Figs. 4e and 7e [32,33].

In the case of the gadolinium diphosphate, the anhydrousHGdP2O7 salt, which is obtained at 530 �C, is found stable un-til 725 �C. It decomposes at this temperature to give the gado-linium diphosphate Gd4(P2O7)3. The difference observedbetween the thermal behaviours of the gadolinium and the hol-mium diphosphates is probably due to the difference found be-tween the arrangements of the hydrogen diphosphates anionsin both structures: clusters in HGdP2O7$3H2O and isolated an-ions in HHoP2O7$3H2O.

4. Conclusion

The chemical preparation and the crystal structure are de-scribed for HHoP2O7$3H2O. This compound crystallizes inthe triclinic space group P1. Its atomic arrangement is builtup by corrugated layers of diphosphate groups and HoO8

polyhedra.The study of the thermal behaviour of HHoP2O7$3H2O

shows that its dehydration takes place in two stages between

s of HHoP2O7$3H2O.

10 20 30 40 50 60

HoPO4

Ho2P4O13

HHoP2O7

HHoP2O7.2H2O

HHoP2O7.3H2O

f

e

d

c

b

a

2θ (°)

Fig. 7. X-ray patterns of HHoP2O7$3H2O at different temperatures: (a) dried at

room temperature; (b) calcined at 125 �C; (c) calcined at 340 �C; (d) calcined

at 660 �C; (e) calcined at 900 �C; (f) calcined at 1200 �C.

997S. Hraiech et al. / Solid State Sciences 10 (2008) 991e997

30 and 340 �C. The anhydrous salt obtained at 340 �C decom-poses between 340 and 900 �C to give the holmium tetraphos-phate Ho2P4O13.

On the whole, the structure of the HHoP2O7$3H2O resem-bles that of HGdP2O7$3H2O but some details in their hydro-gen bond networks make the two structures different andlead to different thermal behaviours of the two compounds.

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