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Microporous and Mesoporous Materials 101 (2007) 66–72

Synthesis and property of three novel organically templatedlayered cerium materials

Dan Wang a,*, Ranbo Yu b, Hong Wang c, Xiaotian Li c, Xianran Xing b

a Multi-phase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, PR Chinab Department of Physical Chemistry, University of Science and Technology, Beijing, PR Chinac School of Materials Science and Engineering, Jilin University, 130023 Changchun, PR China

Received 28 July 2006; received in revised form 2 November 2006; accepted 2 November 2006Available online 10 January 2007

Abstract

Three novel organically templated layered cerium materials, [enH2]0.5[CeIVF3(HPO4)] (CeFPO), [enH2]0.5[CeIII(PO4)(HSO4)(H2O)](CePSO), and [enH2]0.5[CeIII(SO4)2] (CeSO), have been synthesized by hydrothermal synthesis techniques and characterized by single-crystal and powder XRD, TG-DTA, IR, ICP, CHN, and magnetic susceptibility measurement. These compounds possess unique cer-ium-centered polyhedra CeO3F5, CeO8(H2O), and CeO9, and their layers are separated by the ethylenediaminium cations.� 2006 Elsevier Inc. All rights reserved.

Keywords: Hydrothermal synthesis; Organically templated; Layered structure; Cerium materials

1. Introduction

Lanthanide materials with potential applications as ionexchanger, moisture sensor, fluorescence material, andion conductor, have attracted considerable research atten-tions [1,2]. Synthesis of open-framework and layeredlanthanide materials is of great interest because the inter-spaces of three-dimensional channels and lamellar spaceswould be capable of enhancing some properties of this classof compounds, such as ion conductivity and ion exchange,and widening the application area as well [3,4].

The research of meta-stable lanthanide material wasstarted from 1950s. The meta-stable hexagonal phase ofREIIIPO4 (RE = La, Ce, Pr, Nd) with one-dimensionalopen tunnels was obtained in an aqueous solution at mod-erate temperature [5]. In the following years, some relatedhexagonal phases of AIBIICIII(PO4)2 (AI = K, Rb, Cs; BII =Ca, Sr, Eu; CIII = RE and Bi) were prepared successively[6–9]. Recently, hydrothermal synthesis of M2CeIV(PO4)2 Æ

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.11.025

* Corresponding author. Tel./fax: +86 10 62631141.E-mail address: danwang@home.ipe.ac.cn (D. Wang).

H2O (M = NH4, Na, K) powders under alkaline conditionswere reported, but the structures of them are not clarifiedyet [10]. In addition, active research attention was also paidto cerium polyphosphates [11,12]. However, the explora-tion for novel open-framework and layered lanthanidematerials could not go further smoothly, and the exampleof this class of compound is still rather few so far.

We are interested in exploring the synthesis of novel lan-thanide materials, especially cerium materials for a numberof reasons. First, as a lanthanide element, cerium couldgive the high coordination numbers and the variety ofcoordination geometries. Hydrothermal synthesis in thepresence of bulky organic molecules could be expected toresult in the formation of new, complex framework archi-tectures. For actinide materials investigation, Clearfieldand co-workers have demonstrated that in the synthesisof uranyl phosphonates, the presence of sterically demand-ing organic groups on the phosphonate ligands leads to theformation of novel structure types, including porous struc-tures [13–17]. O’Hare and co-workers focused on uraniummaterial synthesis, and a number of hybrid organic–inor-ganic materials, including uranium phosphates, uraniumfluorides, uranium oxyfluoride, and uranium molybdates,

D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72 67

with layered or open-framework structures have been pre-pared [18–21]. Second, cerium materials may be envisionedto exhibit useful catalytic, ion-exchange and intercalationproperties. Finally, the existence of manifold stable oxida-tion states in cerium material offers the possibility of syn-thesizing materials with unique magnetic properties.

Chemical synthesis of cerium materials by using mildmethods including hydrothermal method was started early[10]. However, most of these syntheses were limited to theknown phases. In this work, we investigated the hydrother-mal synthesis of new cerium materials in the CeO2–H3PO4/H2SO4–organic amine–mineralizer–H2O system. Threenovel organically templated layered cerium materials,[enH2]0.5[CeIVF3(HPO4)] (CeFPO), [enH2]0.5[CeIII(PO4)-(HSO4)(H2O)] (CePSO), and [enH2]0.5[CeIII(SO4)2] (CeSO),have been hydrothermally crystallized. Herein, the syn-thesis, structure, and properties of these compounds arereported.

2. Experimental

2.1. Synthesis

CeFPO was synthesized hydrothermally from a start-ing composition of 1.0Ce(SO4)2 Æ 2(NH4)2SO4 Æ 4H2O/3.6H3PO4/2.6H2N(CH2)2NH2/10HF/220H2O at 160 �Cfor 3 days. CePSO was synthesized hydrothermally froma starting composition of 1.0Ce(SO4)2 Æ 2.0(NH4)2SO4 Æ4H2O/3.0H3PO4/4.0H2N(CH2)2NH2/8.0H2SO4/240H2O at110 �C for 4 days. CeSO was synthesized hydrothermallyfrom a starting mixture of 1.0Ce(SO4)2/16.75H2SO4/2H3PO4/3.25H2N(CH2)2NH2/235H2O at 160 �C for 7days. The crystalline products were recovered by filteringand washing with deionized water. All of these three com-pounds could be synthesized as pure phases and their yieldscould reach 85%, 80% and 70%, respectively.

Table 1Crystal data and structure refinement

Compounds CeFPO

Empirical formula [C2N2H10]0.5[CeIVF3(HPO4)]Formula weight 324.17Crystal system TriclinicSpace group P1Lattice parametersa (A) 6.248(2)b (A) 7.079(2)c (A) 8.794(3)a (�) 103.92(2)b (�) 100.84(2)c (�) 110.28(2)V (A3) 338.0(2)Z 1Number of reflection measured 10144 5072Number of observations (I > 3.00r(I)) 4566Number of variables 199Residuals: R, Rw 0.024, 0.021

R ¼PjjF oj � jF cjj=

PjF oj.

Rw ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP

wðjF oj � jF cjÞ2=P

wF 2o

q.

2.2. Crystal structure determination

The prismatic crystals of the three compounds wereselected for single-crystal X-ray analysis. Crystal structuredata were collected on a Rigaku AFC7R diffractometerby using molybdenum Ka radiation (k = 0.71069 A, graph-ite monochromator). Further details of crystal data andstructure refinement are summarized in Table 1.

2.3. Characterizations

Powder X-ray diffraction patterns were taken on a MacScience MXP3 X-ray diffractometer, using Ni-filtered CuKa radiation. Thermogravimetric analysis (TGA) was car-ried out on a Rigaku Thermoflex TAS 200 thermal analysissystem with a heating rate of 10 �C/min over 20–1000 �Ctemperature range. Infrared absorption spectra (IR) wererecorded on a JASCO FT/IR-410 spectrometer usingtransparent KBr pellets: 1–2 mg of the sample was crushedand mixed with 300 mg KBr. ICP analysis was carried outon a HITACHI P-4010 Inductively Coupled PlasmaAtomic Emission Spectrometer. CHN elemental analysiswas carried out on a Yanaco MT-5 CHN CORDER, byusing antipyrine as the standard sample. Magnetic suscep-tibility was measured using a Quantum Design MPMS-XLSQUID magnetometer in the 5–300 K range at 1000 Oe.

3. Results and discussion

3.1. Synthesis

Fluoride ion was proved to be an effective mineralizerfor crystallization of open-framework cerium materials inour previous research [22–24]. In this research fluoride min-eralizer was successfully used for the crystallization of

CePSO CeSO

[C2N2H10]0.5[CeIII(PO4)(HSO4)(H2O)] [N2C2H10]0.5[CeIII(SO4)2]381.24 687.48Monoclinic TriclinicP21/a (No.14) P1

12.999(5) 5.519(3)7.150(4) 7.259(4)9.212(5) 9.783(6)90 87.21(3)95.33(4) 82.88(3)90 80.43(3)852.5(7) 383.4(4)3 1total: 7712 unique: 7335 (Rint = 0.051) 4897 1007(Rint = 0.05)5624 7501128 2370.049, 0.084 0.0472, 0.0604

68 D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72

CeFPO. On the other hand, syntheses without fluoride ionhave also been investigated. It was observed that higheracidity of the synthesis system (pH < 1) and lower synthesistemperature were benefit for novel cerium material s crys-tallization. These conditions might on one hand stabilizethe protonated organic amine and on the other hand helpto incorporate the organic species into the frameworks.Correspondingly, two fluoride-free layered cerium materi-als CePSO and CeSO were obtained.

3.2. Crystal structure description

In our previous research we reported four new cerium-centered polyhedra, CeO6F2 [22], CeO4F4 [23], and CeO7

[24]. In the present work, three types of basic building unit,CeO5F3, CeO8(H2O) and CeO9 were obtained (Fig. 1).

3.2.1. CeFPO

The structure of CeFPO crystal consists of macro-anionic [CeF3(HPO4)]� layers and interlamella diproto-nated ethylene-diammonium cations. The macroanionic[CeF3(HPO4)]� layer is formed by cerium-centered poly-hedral CeO3F5 and phosphorus-centered tetrahedral PO4.Along the b-axis of the structure, CeO3F5 polyhedra are

Fig. 1. Stick-and-ball representa

connected with each other via Ce2F2 rings to give the cor-rugated chains (Fig. 2a). Each PO4 group shares its oxygencorners with three CeO3F5 polyhedra and leave the forthoxygen as –OH group to link up the chains of cerium-cen-tered polyhedra and form the two-dimensional layers(Fig. 2b). The macroanionic layers were separated byorganic diprotonated ethylenediaminium cations. In theplanes vertical to the c-axis, there is six-ring windowcontains four CeO3F5 polyhedra and two PO4 tetrahedrawith the polyhedron arrangement as –Ce–Ce–P–Ce–Ce–P– and has diagonal varied between 3.86 and 7.08 A(Fig. 2b), which is similar to that of six-ring channels of(NH4)[CeIVF2(PO4)] [23]. Ethylenediaminium cations arelocated near the six-ring windows to balance the frame-work negative charge and direct the windows structure.

The Ce–O distances vary in the range from 2.207(8) to2.35(1) A, which is comparable with the range of 2.188–2.407 A for cerium atoms coordinated by eight oxygenatoms in CeIVOSO4(H2O) [25]. The average Ce–F bondlength of 2.300 A is coincident with that observed inNH4CeF7(H2O) [26]. The average P–O distance of1.537 A is a little longer than 1.53 A observed in cal-cium–cerium phosphate, Ca19CeIV(PO4)14 [27]. The organictemplate is connected with the inorganic layers through

tion of basic building unit.

Fig. 2. Stick-and-ball representation of [C2N2H10]0.5[CeIVF3(HPO4)]showing (a) arrangement of organic template between the adjacent layers;(b) layer structure with six-membered ring and the organic templatelocation.

Fig. 3. Asymmetric unit of [C2N2H10]0.5[CeIII(PO4)(HSO4)(H2O)] withatoms labelled.

D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72 69

hydrogen bonds. N(1) and N(2) are involved in hydrogenbonds with framework terminal F(4) and F(5) withN(1)� � �F(5) = 2.934(3) A and N(2)� � �F(4) = 2.909(3) A,respectively.

Differential thermal analysis and thermogravimetricanalysis of CeFPO shows that the template ethylenedia-mine decomposed endothermically at 265 �C, which givesrise to a marked weight loss. Then the CeFPO undergoescomplicated oxidization decomposition at 300–800 �Cand converts into a crystalline phase of CePO4 at highertemperature [28]. The total weight loss is ca. 27.14% whichagrees well with the calculated result (27.46%) according toEq. (1):

½ðCH2Þ2ðNH3Þ2�0:5½CeIVF3ðHPO4Þ� þ 1:25O2 ! CO2 "þNH3 " þ1:5F2 " þ0:5H2O " þCePO4 ð1Þ

3.2.2. CePSOThe structure of CePSO is based on a network of cer-

ium-centered polyhedral CeO9, phosphorus-centered tetra-

hedral PO4, and sulfur-centered tetrahedral SO4. Theasymmetric unit of it is shown in Fig. 3. In its structure,without using F� mineralizer, a new type of basic buildingunit CeO8(H2O)9 was formed. CeO8(H2O) polyhedra areconnected with each other through common oxygen edgeto form the corrugated chains along the b-axis (Fig. 4a).The PO4 tetrahedra are connected to CeO8(H2O) polyhe-dra at four vertices by edge sharing to make a sheet asshown in Fig. 4a, and SO4 groups are stuck to the sheetsurface by connecting with CeO8(H2O) polyhedra at twovertices via corner sharing to give rise to the macroanionic[Ce(PO4)(HSO4)(H2O)]� layers as shown in Fig. 4b and c.The Ce coordination sphere is completed by a water mole-cule, namely H2O(9), which is indicated by the low bondvalence for O(9) of 0.386. The macroanionic layers are sep-arated by diprotonated ethylenediaminium cations(Fig. 5a). In the inorganic layer, there are two kinds offour-ring windows containing two CeO8(H2O) polyhedraand tow PO4 tetrahedra with diagonal varied between4.28–4.82 A, and 4.18–4.68 A (Fig. 5b). Ethylenediamin-ium cations locate near the larger four-ring windows todirect the window structure. Additionally, the ethylenedi-ammonium cations were observed contacting with adjacentlayers via hydrogen bonds of N� � �O(4) = 2.857(7) A,N� � �O(6) = 2.823(6) A, and N� � �O(7) = 2.948(6) A. Inour previous studies, ammonium cation was found alsohave template effect and often compete with organic speciesto direct the framework structures [29]. For the CePSOsynthesis, although ammonium cations are also existed inthe reaction mixture, they are not incorporated into theproduct. The system acidity is considered to be advanta-geous to enhance the competition ability of ethylenedia-minium cations. The Ce–O distances vary in the rangefrom 2.422(3) to 2.646(3) A, which is comparable with

Fig. 4. Structure of the inorganic layers showing (a) the sheet formed byone-dimensional chains of CeO9 polyhedra with PO4 tetrahedra; (b) and(c) SO4 tetrahedra sticking to the surface of the sheet of CeO9 chains andPO4.

Fig. 5. Structure of [C2N2H10]0.5[CeIII(PO4)(HSO4)(H2O)] showing (a) theorganic amine lying between the adjacent layers; (b) layer structure withtwo types of four-membered windows with organic amine located near oneof them.

70 D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72

those in CeIII(PO4) (2.336–2.658 A) [30]. All of the fourcoordination oxygen atoms of the PO4 tetrahedron bridgeto cerium atoms and have an average bond length of1.505 A. For sulfur-centered tetrahedron, S atom sharestwo of its coordination oxygen atoms with cerium atomsand has S–O distances of 1.496(3) and 1.480(3) A, respec-tively. Of the remaining S–O linkages, the longer one, S–O(6) = 1.469(3) A might be the S–OH linkage, and theshorter one, S–O(7) = 1.458(3) A might be considered asS@O bond due to enhanced d-p p-bonding [31].

Thermal analysis carried out in air gave a TG curve withseveral steps, which are difficult to be assigned due to thecomplex composition and structure of the title compound.It revealed that the compound could be stable up to ca.290 �C. The total weight loss is about 39%. Upon1000 �C an amorphous phase was obtained.

The IR absorption bands for CePSO are assigned as fol-lows: the manifold absorption bands at rang of 1218–918 cm�1 are associated with the stretching vibrations ofPO4 and SO4 units; the bands at rang of 670–538 cm�1 cor-respond to bending vibrations of PO4 and SO4 groups.Bands arising from NHþ3 groups are also seen. The bandsat 1641 and 1538 cm�1 may be due to dN-H of the NHþ3 ,

D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72 71

and the bands at 3418 and 3272 cm�1 are assigned tostretching vibration of H2O and NHþ3 groups.

The CePSO is paramagnetic and displays Curie–Weissbehavior in high temperature range, having Curie constantof C = 0.75 emu K�1 mol�1 and a Weiss constant ofh = �31.8 K, which corresponds to effective magneticmoments of leff = 2.45 BM, and is consistent with the valueexpected for a system consisting of isolated high-spin Ce3+,lcal = 2.54 BM.

Fig. 7. Polyhedral representation of [N2C2H10]0.5[CeIII(SO4)2] showing thearrangement of organic template between the adjacent layers along the (a)a-axis and (b) b-axis.

3.2.3. CeSO

The crystal structure of CeSO consists of sulfate groups,two cerium atoms with identical coordination numbers(CN = 9), which perform several structural functions.The framework of the CeSO is formed by cerium-centeredpolyhedral CeO9 and sulfur-centered tetrahedral SO4. Thestructure consists of macroanionic [Ce(SO4)2]� separatedby diprotonated ethylenediammonium cations. On the ab-plane of the structure, CeO9 polyhedra are connected viacommon edge to form two-dimensional layers with six-membered window (Fig. 6). Each SO4 group shares itsthree oxygen corners with CeO9 polyhedra to divide thesix-membered windows into smaller four- and three-mem-bered windows, and leave the fourth oxygen as terminalS@O group. Along the b-axis of the structure, the macro-anionic layers could be considered as a corrugated planesformed by edge-sharing CeO9 polyhedra (Fig. 7a) withthe sulfur-centered tetrahedral SO4 groups sticking to theirsurface.

The Ce–O distances vary in the range from 2.413(10) to2.694(9) A, which is comparable with those for ceriumatoms in CeIII(PO4) (2.336–2.658 A) [30]. All of the fourcoordination oxygen atoms of the SO4 tetrahedron bridgeto cerium atoms and have an average bond length of1.505 A. For sulfur-centered tetrahedron, S atom sharesthree of its coordination oxygen atoms with cerium atoms

Fig. 6. Stick-ball-polyhedral representation of [N2C2H10]0.5[CeIII(SO4)2]with atoms labelled.

and have an average bond length of 1.493 A, which is com-parable with those S–O distances. Of the remaining S–Olinkages with a shorter average bond length (1.444 A),might be considered as S@O double bond, and the longerone (1.469(3) A), S–O(6) might be the S–O–H bond. Theorganic template is connected with the inorganic layersthrough hydrogen bonds. N(1) and N(2) are involved inhydrogen bonds with framework terminal O(5), O(11),O(1) and O(14) with N(1)� � �O(5) = 2.894(17) A,N(1)� � �O(11) = 2.763(17) A and N(2)� � �O(1) = 2.778(17) A, N(2)� � �O(14) = 2.911(19) A respectively.

The IR absorption bands for CeSO are assigned as fol-lows: the manifold absorption bands at rang of 1098,�990 cm�1 are associated with the stretching vibrationsof SO4 units; the bands at rang of 655–598 cm�1 corre-spond to bending vibrations of SO4 groups. Bands arisingfrom NHþ4 groups are also seen. The bands at 1626 and1514 cm�1 may be due to N–H vibrations of the NHþ4 ,and the bands at 3430 and 3176 cm�1 are assigned tostretching vibration of NHþ4 groups.

Differential thermal analysis and thermogravimetricanalysis of CeSO shows that the template ethylenediamineand partial SO4 groups oxidization decomposed at about500 �C with corresponding exothermic effect, which givesrise to a marked weight loss about 22%. Then the CeSO

72 D. Wang et al. / Microporous and Mesoporous Materials 101 (2007) 66–72

undergoes complicated oxidization decomposition from700 �C and converts into a complex crystalline phase athigher temperature. Up to 800 �C, the total weight loss isca. 33%.

4. Conclusion

In summary, three novel organically templated layeredcerium materials with new type of basic building unit,CeO5F3, CeO8(H2O) and CeO9, has been synthesized underhydrothermal conditions. This synthesis work illustratesthat the acidity of the synthesis system is important forincorporation of organic amine template in to the struc-ture. With [enH2]2+ cations located in the interspace amongthe layered, the three novel organically templated layeredcompounds might probably have some interesting ion-exchange properties.

Acknowledgments

This work was partly supported by the National NaturalScience Foundation of China (grant number 20401015 and50574082). D. Wang thanks the ‘‘Century Program orHundreds-Talent Program’’ of Chinese Academy of Sci-ences, and R. Yu thanks the ‘‘Beijing Nova Program’’(No.: 2005B20), and the Foundation of University of Sci-ence & Technology Beijing (No.: 20050214890).

References

[1] G.J. McCarthy, W.B. White, D.E. Pfoertsch, Mater. Res. Bull. 13(1978) 1239.

[2] I. Szczygiel, T. Znamierowska, J. Solid State Chem. 95 (1991) 260.[3] H.Y.-P. Hong, Mater. Res. Bull. 11 (1976) 173.[4] M.A. Subramanian, P.R. Rudolf, A. Clearfield, J. Solid State Chem.

60 (1985) 172.[5] R.C.L. Mooney, Acta Crystallogr. 3 (1950) 337.[6] M. Vlasse, Acta Crystallogr. 38 (1982) 2328.

[7] C. Parent, P. Bochu, A. Daouidi, G. Leflem, J. Solid State Chem. 43(1982) 190.

[8] L.P. Keller, G.J. McCarthy, R.G. Garvey, Mater. Res. Bull. 20 (1985)459.

[9] G. Wu, M. Jansen, K. Konigsten, J. Solid State Chem. 98 (1992)210.

[10] Y. Xu, S. Feng, W. Pang, G. Pang, Chem. Commun. (1996) 1305.[11] M. Rzaigui, K. Ariguib, M.T. Averbuch-Pouchot, A. Durif, J. Solid

State Chem. 50 (1983) 240.[12] V. Krasnikov, M. Vaivada, Z. Konstants, J. Solid State Chem. 74

(1988) 1.[13] R.J. Francis, M.J. Drewitt, P.S. Halasyamani, C. Ranganathachar,

D. O’Hare, W. Clegg, S.J. Teat, Chem. Commun. (1998) 279.[14] R.J. Francis, P.S. Halasyamani, D. O’Hare, Angrew. Chem., Int. Ed.

Engl. 37 (1998) 2214.[15] R.J. Francis, P.S. Halasyamani, D. O’Hare, Chem. Mater. 10 (1998)

3131.[16] P.S. Halasyamani, S.M. Walker, D. O’Hare, Chem. Mater. 121 (1999)

7415.[17] P.S. Halasyamani, R.J. Francis, S.M. Walker, D. O’Hare, Inorg.

Chem. 38 (1999) 271.[18] D. Grohol, M.A. Subramanian, D.M. Poojary, A. Clearfield, Inorg.

Chem. 35 (1996) 5264.[19] D.M. Poojary, D. Grohol, A. Clearfield, Angrew. Chem., Int. Ed.

Engl. 34 (1995) 1508.[20] D.M. Poojary, D. Grohol, A. Clearfield, J. Phys. Chem. Solids 56

(1995) 1383.[21] D.M. Poojary, A. Cabeza, M.A.G. Aranda, S. Bruque, A. Clearfield,

Inorg. Chem. 35 (1996) 1468.[22] R. Yu, D. Wang, N. Kumada, N. Kinomura, Chem. Mater. 12 (2000)

3527.[23] R. Yu, D. Wang, T. Takei, N. Kumada, N. Kinomura, J. Solid State

Chem. 157 (2001) 180.[24] R. Yu, D. Wang, et al., Chem. Lett. 33 (9) (2004) 1186.[25] O. Lindgren, Acta Crystallogr. B 32 (1976) 3347.[26] R.R. Ryan, R.A. Penneman, Acta Crystallogr. B 27 (1971) 1939.[27] B.I. Lazoryak, R.N. Kotov, S.S. Khasanov, Z. Neorganich. Khimii

41 (1996) 1281.[28] Powder Diffraction File, Inorganic and Organic, Set 32, 32-199,

JCSPD International Center Diffraction Data, USA.[29] D. Wang, R. Yu, N. Kumada, N. Kinomura, Chem. Mater. 12 (2000)

956.[30] K.M. Ghouse, Indian J. Pure Appl. Phys. 6 (1968) 265.[31] H.Y. Sung, J. Yu, I.D. Williams, J. Solid State Chem. 140 (1998) 46.