The development of cyclophosphate crystal chemistry

Solid State Sciences 7 (2005) 760–766 www.elsevier.com/locate/ssscie

The development of cyclophosphate crystal chemistry André Durif LEDSS, Université Joseph Fourier, Grenoble, France Received 4 November 2004; accepted 9 November 2004 Available online 18 April 2005

Abstract Among the various condensation schemes occurring in phosphoric anions one of them leads to the formation of cyclic entities. The corresponding salts, for a long time called “metaphosphates” are now denominated cyclophosphates. Built up by a ring of corner-sharing PO4 tetrahedra, the general formula of their anions is Pn O3n n− . Today these anions are known for n = 3, 4, 5, 6, 8, 9, 10, and 12. The development of the chemistry of cyclophosphates was very slow, spreading along almost two centuries. We report first a short history of the development of each class of cyclophosphates, then we examine their general properties  2005 Elsevier SAS. All rights reserved.

1. Introduction During the nineteenth century the German school of chemistry has been very active in the field of phosphates. Starting from the pioneer works of Berzelius [1], Clark [2] and Graham [3] the German chemists elaborated, along this century, a great number of condensed phosphates, and some of them as Lindbom [4], Glatzel [5], and Tammann [6] suspected for some of them the cyclic nature of the anion, but these assumptions remained hypotheses till the development of X-ray structural analysis. Today, after the optimization of the chemical preparations of good starting materials, and the generalization of the flux methods several hundreds of cyclophosphates are well characterized.

It is not before 1955 that the optimization of this reaction for the production of pure sodium cyclotriphosphate was performed by Thilo and Grunze [7]: 3NaH2 PO4 → Na3 P3 O9 + 3H2 O. If the reaction is conducted at 823 K for five hours one obtains an almost pure Na3 P3 O9 , sometimes contaminated by a very small percentage of insoluble impurities easily removable after dissolution in water. Some years before these experiments Boullé [8] suggested for the syntheses of cyclotriphosphates a metathesis reaction based upon the use of the silver salt, Ag3 P3 O9 ·H2 O. He applied this reaction for the production of the calcium salt: 2Ag3 P3 O9 ·H2 O + 3CaCl2

2. Development of cyclotriphosphate chemistry The main component of the water soluble Graham salt is most of time sodium cyclotriphosphate. Its percentage in this salt is nevertheless very dependent of the calcination temperature used for its production by the following reaction NaH2 PO4 → NaPO3 + H2 O. E-mail address: [email protected] (A. Durif). 1293-2558/$ – see front matter  2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.11.022

H O

−−2−→ Ca3 [P3 O9 ]2 ·9H2 O + 6AgCl. After 1955, when large amounts of Na3 P3 O9 became available through the Thilo and Grunze process [7], this metathesis reaction was extensively used and lead to the characterization of a large number of new cyclotriphosphates. Experimental details on these two processes can be found in Averbuch and Durif [9]. In this same book are also described the use of flux methods and some much less conventional processes used for the synthesis of cyclotriphosphates.

A. Durif / Solid State Sciences 7 (2005) 760–766

Fig. 1. The P3 O9 group with no internal symmetry as observed in Na2 KP3 O9 . Black circles figurate phosphorus atoms, open circles the oxygen atoms. This convention applies also to the following drawings.

Today more than one hundred and fifty cyclotriphosphates have been well characterized by using these various processes. We now examine briefly the main general geometrical features of the P3 O9 group. In a previous analysis [10] of the internal symmetry of the P3 O9 groups covering 56 different structure types of cyclotriphosphates we observed that most of them (41) have no internal symmetry, six have a mirror symmetry, five adopt a ternary symmetry, two a symmetry 3/m and two a twofold configuration. But in this type of anion the most striking feature is the value of one of the condensation parameter, the P–P–P angle, always close by 60◦ . The constraints due to the cyclization phenomenon can explain such small value, up to now never observed in the other classes of condensed phosphoric anions. Fig. 1 gives the representation of a P3 O9 group with no internal symmetry as observed in Na2 KP3 O9 [11].

3. Development of cyclotetraphosphate chemistry For a long time the chemistry of cyclotetraphosphates has been the object of many controversies. As far as we know Maddell [12] was the first to prepare a series of compounds of general formula M(PO3 )2 for M = Mn, Cu, Co, Ni, Mg, and later on, Glatzel [5] suspected the cyclic nature of their anions. Today all of them are recognized as cyclotetraphosphates. Several other chemists of the nineteenth century resumed and developed this investigation and using the copper salt, Cu2 P4 O12 , elaborated a process for the preparation of the sodium salt based on the following reaction: Cu2 P4 O12 + 2Na2 S → Na4 P4 O12 + 2CuS. The process described above for the preparation of the sodium cyclotetraphosphate, was the only possible one for the production of this starting material for a long time. One must wait till the middle of the past century for the elaboration of a more convenient process based on the low-

761

Fig. 2. The centrosymmetrical P4 O12 group in Cu2 P4 O12 .

temperature (273 K) hydrolytic cleavage of P4 O10 , leading to cyclotetraphosphoric acid with very high yields, according to the following scheme: P4 O10 + 2H2 O → H4 P4 O12 . This reaction was mentioned and used several times during the first part of past century and was finally carefully analysed and optimized by Bell et al. [13] and by Thilo and Wicker [14]. The first structural proof for the cyclic nature of the anion was given by Pauling and Sherman [15] in 1937. Today more than one hundred cyclotetraphosphates are well characterized. Forty-nine types of P4 O12 groups have been observed. Among them more than ten, built up by four cristallographically independent PO4 tetrahedra, have no internal symmetry, while the other ones can adopt a centrosymmetrical configuration or various symmetries as 2, 2/m, m, mm, −4, −4/2m. As for cyclotriphosphates, and for the same reasons, the P–P–P angles are small, never departing significantly of 90◦ . Fig. 2 gives the representation of the centrosymmetrical cyclotetraphosphoric anion observed in Cu2 P4 O12 [16].

4. Development of cyclopentaphosphate chemistry Cyclopentaphosphates are very rare. Chromatographic experiments demonstrated their existence in the water soluble part of the Graham salt. Thilo and Schülke [17] isolated small quantities of sodium cyclopentaphosphate and later on Jost [18] described a poor quality crystal structure of Na4 NH4 P5 O15 ·4H2 O. On must wait till 2000 to record a second example, when Murashova et Chudinova [19] prepared Na2 MnP5 O15 by a flux method, and reported an accurate structure determination for this salt. Fig. 3 gives the representation of the P5 O15 ring as observed in Na2 MnP5 O15 . The P5 O15 group has here a mirror symmetry. In front of

762

A. Durif / Solid State Sciences 7 (2005) 760–766

Fig. 3. The P5 O15 group as observed in Na2 MnP5 O15 . This anion has a mirror symmetry.

so few examples no further comments can be added for this class of compounds.

5. Development of cyclohexaphosphate chemistry The chemistry of cyclohexaphosphates made very rapid progresses during the last fifteen years. For a long time, the lack of a reliable process to produce large amounts of an appropriate starting material can probably explain this relatively late development. Nevertheless, before 1985, some cyclohexaphosphates, Na6 P6 O18 ·6H2 O, Cu2 Li2 P6 O18 , Cr2 P6 O18 or Cs2 (UO2 )2 P6 O18 , for instance, were discovered by various methods and clearly characterized. The relatively low development in this domain is difficult to explain for, as early as 1965, Griffith and Buxton [20] described a reproducible process for the preparation of useful amounts of sodium or lithium cyclohexaphosphates. This process was greatly improved in 1985 when Schülke and Kayser [21] performed a detailed investigation of the thermal behavior of LiH2 PO4 and proposed a convenient procedure to prepare large amounts of pure Li6 P6 O18 . A detailed description of this process is described in [9]. Still more recently, Averbuch-Pouchot [22] described the preparation and an accurate structural characterization of Ag6 P6 O18 ·H2 O, opening the way to metathesis reactions deriving, from the Boullé’s one [8], for the preparation of water-soluble cyclohexaphosphates. The lithium salt being the usual starting material one can use the very weak solubility of lithium fluoride to prepare, with generally very good yields, a good number of cyclohexaphosphates. Schematically the reaction is: 6M I F + Li6 P6 O18 → M I6 P6 O18 + 6LiF and was used for the elaboration of all alkali cyclohexaphosphates.

Fig. 4. The P6 O18 group as observed in K6 P6 O18 ·3H2 O. This anion has a mirror symmetry.

More than fifty accurate determinations of cyclohexaphosphate atomic arrangements are now performed authorizing the comparison of a good number of different P6 O18 ring-anions. A survey of their internal symmetry shows that among all the presently known examples, one has no internal symmetry, but a strong pseudo-m conformation, one has m symmetry, one a 2/m symmetry, two a 3 symmetry, seven a −3 and eighteen are centrosymmetrical. In the two categories of cyclophosphate anions we have already analyzed, cyclotri- and cyclotetraphosphates, the P–P–P angles never depart significantly from their ideal values, 60 ± 2◦ for cyclotriphosphates and 90 ± 4◦ for cyclotetraphosphates. For the P6 O18 ring anions, in which geometrical constraints are less important one observes very large deviations from the ideal value of 120◦ , these P–P–P angles spreading from 85 to 145◦ . To illustrate this class of compounds we report in Fig. 4 the P6 O18 group observed in K6 P6 O18 ·3H2 O [23].

6. Development of cyclooctaphosphate chemistry From paper-chromatography experiments, the existence of phosphoric ring anions larger than [P6 O18 ]6− was clearly established more than fifty years ago. In 1968, following a careful investigation of the thermal behavior of Pb2 P4 O12 ·4H2 O, Schülke [24,25] reported the preparation of a crystalline cyclooctaphosphate, Pb4 P8 O24 , and the possibility to prepare the corresponding sodium salt. In spite of this possibility it is only since 1992 that a renewal of interest for this class of cyclophosphates appeared. Between these two dates, the great majority of the well characterized cyclooctaphosphates were discovered during the study of various phase-equilibrium diagrams and prepared as crystals

A. Durif / Solid State Sciences 7 (2005) 760–766

763

Fig. 6. The P9 O27 group as observed in Al3 P9 O27 .

Fig. 5. The P8 O24 group as observed in Ga2 K2 P8 O24 .

by flux methods at relatively high temperature. During the same period it was also discovered that the thermal treatment of some diphosphates can lead to cyclooctaphosphates [27]: 603 K −−−→ Ga2 K2 P8 O24 + 4H2 O. 2GaK(H2 P2 O7 )2 −

The structure of the gallium–potassium salt, was later on, determined by Palkina et al. [28]. The first structural characterization of a [P8 O24 ]8− ring anion did not occur before 1975, when Laügt and Guitel [26] determined the crystal structure of the copper–ammonium salt, Cu3 (NH4 )2 P8 O24 . Among the ten cyclooctaphosphates presently known one can notice that two of the [P8 O24 ]8− ring anions have the 2/m internal symmetry, seven are centro-symmetrical and one has no symmetry. The P–P–P angles ranging from 92.1 to 146.7◦ are quite comparable to those observed in cyclohexaphosphates. We illustrate this category of anion by the representation in Fig. 5 of the P8 O24 group observed in Ga2 K2 P8 O24 [29].

clononaphosphate, but the poor quality of the crystals did not authorize an accurate structure determination. Some months later an accurate structure of the aluminum salt by Fratzky, Schneider and Meisel [31] confirmed the previous investigation. Fig. 6 reports a drawing of this unique example of a P9 O27 group. The internal symmetry is −3.

8. Development of cyclodecaphosphate chemistry The first crystalline cyclodecaphosphate, Ba2 Zn3 P10 O30 , was obtained in 1982 by Bagieu-Beucher et al. [32,33] during an investigation of the Ba(PO3 )2 Zn(PO3 )2 phaseequilibrium diagram. Ten years later Schülke et al. [34] described a process for the production of alkali cyclodecaphosphates using the barium–zinc compound as starting material. Following this investigation several cyclodecaphosphates were synthesized providing a better, but still fragmentary, knowledge of the geometry of the P10 O30 ring anions. Only five of them are presently known, two have a binary symmetry and three are centrosymmetrical. The range of values for the P–P–P angles (86.6 to 144.1◦ ) is similar to those observed in large phosphoric rings. Fig. 7 reports the P10 O30 ring observed in potassium cyclodecaphosphate tetrahydrate, K10 P10 O30 ·4H2 O [35].

7. Development of cyclononaphosphate chemistry Forty years ago, condensed phosphates of general formula M(PO3 )2 with M = Al, Fe, Cr were systematically investigated. Some of them possess five crystalline forms denoted then A, B, C, D, E. The crystal structure of the E form, difficult to crystallize, could not be performed at this time, but Bagieu [30] from the observation of its mechanical properties suspected a cyclic anion. It is not before 1996 that Averbuch-Pouchot and Durif [9], using a crystal of (Fe, V)3 P9 O27, proved it was a cy-

9. Development of cyclododecaphosphate chemistry Very little information is available on cyclododecaphosphates. The first to be characterized with certainty was V3 Cs3 P12 O36 , discovered in 1981 by Lavrov et al. [36,37], during an investigation of various V2 O3 –M2 O–P2 O5 –H2 O systems. Very recently, Schülke [38] successfully performed the synthesis of some alkali cyclododecaphosphates by using

764

A. Durif / Solid State Sciences 7 (2005) 760–766

10. General considerations on cyclophosphates

Fig. 7. The P10 O30 group observed in K10 P10 O30 ·4H2 O given in projection along the [111] direction.

Fig. 8. The P12 O36 group of Cs3 V3 P12 O36 . A VO6 octahedron is inserted at the center of the ring.

In this class of compounds acidic anions are very rare. One can only report the existence of one acidic cyclotriphosphoric anion in Na2 HP3 O9 [41]. Cyclophosphoric anions form easily adducts with telluric acid or with nitrates. The cyclophosphate-tellurates are the most common. One can report among several dozens of such compounds, Te(OH)6 ·K8 P8 O24 ·2H2 O [42] recently investigated. The repartition of cyclophosphates among the various classes of these salts is very unequal. If cyclotri- and cyclotetraphosphates are each represented by more than one hundred compounds, one knows only two cyclopentaphosphates. More than one hundred and fifty cyclohexaphosphates are now well characterized but only one cyclononaphosphate. This fact can probably be explained by the lack of the appropriate starting materials for systematic investigations in the case of cyclopenta- and cyclononaphosphates and by the very recent optimizations of such materials for cyclodeca- and cycloduodecaphosphates. Substituted anions are common for cyclotri- and cyclotetraphosphates. One or several oxygen atoms can be substituted either by atoms as, S or by groups of atoms as NH or NH2 without any significant alteration of the geometrical configuration of the anions. Some examples are: (NH4 )3 P3 O6 S3 and K3 P3 O6 S3 [43,44] for the O → S substitution and (NH4 )3 (PO2 NH)3 ·H2 O [45] for an O → NH substitution. In the case of an O → S substitution, this substitution can be total, leading for example to (NH4 )3 P3 S9 [46]. Along this article, for each class of cyclophosphates, we discussed briefly the geometry of the rings comparing mainly one of the condensation parameter, the P–P–P angle. From a more detailed investigation performed using the three condensation parameters (P–O–P and P–P–P angles and P–P distances) [9] one can simply observe that when the size of the ring increases its local configuration adopts a geometry more and more similar to that observed for long chain polyphosphates.

11. Chemical properties of cyclophosphates Fe3 Cs3 P12 O36 as starting material. Some experiments carried out by Averbuch-Pouchot with these alkali derivatives showed that they are extremely water soluble and hence very difficult to crystallize. From these materials, Schülke and Averbuch-Pouchot were, nevertheless, able to produce crystals of some other cyclododecaphosphates, guanidinium cyclododecaphosphate hexahydrate [39] and two adducts with telluric acid. The crystal structure of one of these adducts, (gua)12 P12 O36 ·12Te(OH)6 ·24H2 O, was recently determined [40]. We illustrate this class of compounds with the representation, in Fig. 8, of the P12 O36 group as observed in V3 Cs3 P12 O36 .

One could expect specific properties for each family of cyclophosphates. In fact and with rare exceptions, the general chemical properties of cyclophosphates do not vary significantly with the ring size. Their chemical behavior seems mainly dependent on the nature of the associated cations, a fact already observed in the other condensed phosphate families. Thus, the water solubility of alkali cyclophosphates seems to be independent on the ring size. For instance, potassium cyclodecaphosphate and potassium cyclododecaphosphate, K10 P10 O30 ·4H2 O and K12 P12 O36 ·19/2H2 O, show

A. Durif / Solid State Sciences 7 (2005) 760–766

very high water solubility compared with the potassium cyclotri- or cyclotetraphosphates. The thermal behavior of cyclophosphates seems also to be very dependent on the nature of the associated cations. As far as we know, all monovalent-cation cyclophosphates transform into long chain polyphosphates when heated. The evolution is different for salts of divalent cations. Some of them, like calcium, cadmium, strontium, barium, and lead cyclophosphates, transform also into long-chain polyphosphates, but the magnesium, cobalt, nickel, copper, manganese salts are stable as cyclotetraphosphates up to their melting points. For mixed-cation cyclophosphates, the thermal evolution does not obey any rules. Some of them, as BaNaP3 O9 or SrK2 P4 O12 , for instance, are stable as cyclophosphates up to their melting points, whereas others including all the incongruent melting salts, decompose. For instance, calcium– potassium cyclotetraphosphate, CaK2 P4 O12 , decomposes into a cyclotriphosphate and a long-chain polyphosphate when heated up to 973 K: CaK2 P4 O12 → CaKP3 O9 + KPO3 . As all condensed phosphoric anions, the Pn O3n rings decondense by hydrolysis. The water-soluble cyclophosphates are generally stable in an approximately neutral aqueous solution at room temperature. The rate of hydrolysis is rather low under these conditions and it seems that this stability increases with the ring size. Nevertheless, as in all condensed phosphates, the cyclophosphates hydrolyze upon departure from these conditions. This decyclization process can take various aspects, depending on the nature of the associated cation. For instance, potassium cyclotriphosphate is rapidly transformed into KH2 PO4 when heated slowly in a wet atmosphere or simply kept in hot water, K3 P3 O9 + 3H2 O → 3KH2 PO4 but, for lanthanum cyclotriphosphate, this hydrolysis is accompanied by the formation of phosphoric acid: LnP3 O9 ·3H2 O → LnPO4 + 2H3 PO4 . It was thought for a time, that the opening of the phosphoric rings by hydrolysis in a basic medium could be a proper way to prepare oligophosphates according to the following schematic process:

hydrolysis of cyclotetraphosphates based on the following scheme: H4 P4 O12 + H2 O → H6 P4 O13 . Various investigations were performed by Thilo and Ratz [48], Westman and Scott [49], Quimby [50] and Watters et al. [51] using this process. The conditions of preparation by this method are tedious, time consuming and deceptive most of time. For instance, Thilo and Ratz [48] described an attempt to prepare Na6 P4 O13 by hydrolytic decyclization of sodium cyclotetraphosphate, Na4 P4 O12 + 2NaOH(H2 O) → Na6 P4 O13 ·xH2 O but could not characterize the final oily product which decomposes rapidly: Na6 P4 O13 ·xH2 O → 2Na3 HP2 O7 ·H2 O. It was observed that the addition of zinc chloride to an aqueous solution of sodium cyclotriphosphate provokes a rapid opening of the ring anion and that crystals of a zinc–sodium triphosphate, Zn2 NaP3 O10 ·9H2 O, appear rapidly in the solution. [Co(NH3 )6 ]2 P4 O13 ·5H2 O reported by Schulz and Jansen [52] is till now the only example of preparation of a crystalline tetraphosphate by controlled hydrolysis of a cyclotetraphosphate. At the end of this survey one must confess that if our knowledge of the crystal chemistry of cyclophosphates is relatively satisfactory very little is known of their basic physical and chemical properties.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Mn Pn O3n + 2MOH → M(n+2) Pn O(3n+1) + H2 O. Probably because it was difficult to master, this type of reaction is most of time deceiving. Only a few examples of oligophosphates prepared by this way can be reported. Sotnikova-Yuzhik et al. [47] prepared Li5 P3 O10 ·5H2 O, a crystalline lithium triphosphate, by hydrolytic decyclization of Li3 P3 O9 :

765

[11] [12] [13]

313–353 K Li3 P3 O9 + 2LiOH − −−−−−→ Li5 P3 O10 + H2 O.

[14] [15] [16]

For a long time, the only suggested method for the preparation of water-soluble tetraphosphates was the alkaline

[17] [18]

J. Berzelius, Ann. Physik 54 (1816) 31–52. T. Clark, Edinburgh J. Sci. 7 (1827) 298–314. T. Graham, Phil. Trans. Roy. Soc. 123A (1833) 253–284. C.G. Lindbom, Berichte 8 (1875) 122–124. A. Glatzel, Inaugural Dissertation, Würzburg University, Germany, 1880. G. Tammann, Z. Phys. Chem. 6 (1890) 122–140. E. Thilo, H. Grunze, Z. Anorg. Allg. Chem. 281 (1955) 263–283. A. Boullé, C. R. Acad. Sci. 206 (1938) 517–519. M.T. Averbuch-Pouchot, A. Durif, Topics in Phosphate Chemistry, World Scientific, Singapore, 1996. M.T. Averbuch-Pouchot, A. Durif, Crystal chemistry of cyclophosphates, in: Chain, Clusters, Inclusion Compounds, Paramagnetic Labels, and Organic Rings, vol. 5, Elsevier Science, Amsterdam, 1994, pp. 1–160. I. Tordjman, A. Durif, C. Cavéro-Ghersi, Acta Crystallogr. B 30 (1974) 2701–2704. R. Maddrell, Liebig Ann. 61 (1847) 53–63. R.N. Bell, L.F. Audrieth, O.F. Hill, Ind. Eng. Chem. 44 (1952) 568– 572. E. Thilo, W. Wicker, Z. Anorg. Allg. Chem. 277 (1952) 27–36. L. Pauling, J.S. Sherman, Z. Kristallogr. 96 (1937) 481–487. M. Laügt, J.C. Guitel, I. Tordjman, G. Bassi, Acta Crystallogr. B 28 (1972) 201–208. E. Thilo, U. Schülke, Z. Anorg. Allg. Chem. 344 (1965) 293–307. K.H. Jost, Acta Crystallogr. B 28 (1972) 732–738.

766

A. Durif / Solid State Sciences 7 (2005) 760–766

[19] E.V. Murashova, N.N. Chudinova, Cristallografiya 45 (2000) 625– 628. [20] E.J. Griffith, R.L. Buxton, Inorg. Chem. 4 (1965) 549–551. [21] U. Schülke, R. Kayser, Z. Anorg. Allg. Chem. 531 (1985) 167–175. [22] M.T. Averbuch-Pouchot, Z. Kristallogr. 189 (1990) 17–23. [23] L.N. Kholodkovskaya, L.A. Borodina, V.K. Trunov, N.N. Chudinova, Inorg. Mater. 25 (1989) 394–398. [24] U. Schülke, Z. Anorg. Allg. Chem. 360 (1968) 231–246. [25] U. Schülke, Angew. Chem., Int. Ed. Engl. 7 (1968) 71. [26] M. Laügt, J.C. Guitel, Z. Kristallogr. 141 (1975) 203–216. [27] N.N. Chudinova, M.A. Avaliani, L.S. Guzeeva, I.V. Tananaev, Izv. Akad. Nauk SSSR, Neorg. Mater. 14 (1978) 2054–2057. [28] K.K. Palkina, S.I. Maksimova, V.G. Kusznetsov, N.N. Chudinova, Dokl. Akad. Nauk SSSR 245 (1979) 1386–1389. [29] N.N. Chudinova, I.V. Tananaev, M.A. Avaliani, Izv. Akad. Nauk SSSR, Neorg. Mater. 13 (1977) 2234–2237. [30] M. Bagieu, Thèse, Univ. Grenoble, France, 1980. [31] D. Fratzky, M. Schneider, M. Meisel, Z. Kristallogr. NCS 215 (2000) 341–342. [32] M. Bagieu-Beucher, A. Durif, J.C. Guitel, J. Solid State Chem. 45 (1982) 159–163. [33] M. Bagieu-Beucher, A. Durif, J.C. Guitel, J. Solid State Chem. 40 (1981) 248. [34] U. Schülke, M.T. Averbuch-Pouchot, A. Durif, Z. Anorg. Allg. Chem. 612 (1992) 107–112. [35] M.T. Averbuch-Pouchot, U. Schülke, Z. Kristallogr. 210 (1995) 129–132.

[36] A.V. Lavrov, M.Ya. Voitenko, E.G. Tselebrovskaya, Izv. Akad. Nauk SSSR, Neorg. Mater. 17 (1981) 99–103. [37] A.V. Lavrov, V.P. Nikolaev, G.G. Sadikov, M.Y. Voitenko, Dokl. Akad. Nauk SSSR 259 (1981) 103–106. [38] U. Schülke, in preparation. [39] U. Schülke, M.T. Averbuch-Pouchot, Z. Anorg. Allg. Chem. 621 (1995) 1232–1236. [40] M.T. Averbuch-Pouchot, U. Schülke, Z. Anorg. Allg. Chem. 622 (1996) 1997–2002. [41] M.T. Averbuch-Pouchot, J.C. Guitel, A. Durif, Acta Crystallogr. C 39 (1983) 809–810. [42] U. Schülke, M.T. Averbuch-Pouchot, A. Durif, Z. Kristallogr. 204 (1993) 143–152. [43] T.B. Kuvshinova, M. Meisel, G.-U. Wolf, M.P. Pautina, Neorg. Mater. 30 (1994) 1151–1155. [44] M. Meisel, G.U. Wolf, M.T. Averbuch-Pouchot , Acta Crystallogr. C 47 (1991) 1368–1370. [45] N. Stock, W. Schnick, Acta Crystallogr. C 52 (1996) 2645–2647. [46] G.-U. Wolf, M. Meisel, Z. Anorg. Allg. Chem. 494 (1982) 49–54. [47] V.A. Sotnikova-Yuzhik, G.V. Pelsyak, E.A. Prodan, Russ. J. Inorg. Chem. 32 (1987) 1505–1507. [48] E. Thilo, R. Ratz, Z. Anorg. Allg. Chem. 260 (1949) 255–266. [49] A.E.R. Westman, A.E. Scott, Nature 168 (1951) 740. [50] O.T. Quimby, J. Phys. Chem. 58 (1954) 615–624. [51] J.I. Watters, P.E. Sturrock, R.E. Simonaitis, Inorg. Chem. 2 (1963) 765–767. [52] F. Schulz, M. Jansen, Z. Anorg. Allg. Chem. 543 (1986) 152–160.