Synthesis and crystal structure of a new form of potassium–bismuth polyphosphate KBi(PO3)4
Introduction
KBi(PO3)4 (type: IV) is a member of a family of compounds with general formula MIMIII(PO3)4 (where MI is a monovalent cation: Li, Na, Ag, K, etc. and MIII is a trivalent cation: Nd, La, Sm, Ce, Y, etc.). The common chemical features of these polyphosphates [1], [2], [3], [4], [5], [6], [7] indicate that they are relatively stable under normal conditions of temperature and humidity. They can be kept for many years in a perfect state of crystallinity, they are not water soluble as may be inferred from their estimated molecular weights and they all produce glasses when heated to their melting points [8].
The literature dealing with these compounds was rather confusing for a long time, but it is currently well established that the MIMIII(PO3)4 compounds can be classified into seven different structural types, nowadays usually denoted by roman numerals I to VII. This nomenclature, first proposed by Palkina et al. [9] is today generally accepted. In addition, many of these compounds are isotypic and some of them are polymorphic. The main crystallographic data for some trivalent–monovalent polyphosphates [1], [2], [3], [4], [5], [6], [7] which are gathered in Table 1, can explain the difference between these seven structure types.
An examination in the literature shows that a few polyphosphates are synthesized in the ternary M2IO–Bi2O3–P2O5 system, and their structures were determined. Bukhalova et al. [10] elaborate the compound NaBi(PO3)4 and on the basis of the powder diffraction data and the examination of the single crystals by Bagieu-Beucher [11], this compound appears to be isotypic to NaNd(PO3)4 [2]. Crystals of NH4Bi(PO3)4 were obtained by Averbuch-Pouchot and Bagieu-Beucher [12] during the course of an investigation of the P2O5–(NH4)2O–Bi2O3 system. The crystal structure was determined, and NH4Bi(PO3)4 was recognized as belonging to form IV.
The structure of the polyphosphate KBi(PO3)4 belonging to form III which is isotypic to KNd(PO3)4 [3] was well discussed by Palkina et al. [13] and up to now, the synthesis and the crystal structure of this type IV material are unknown.
Our own work has focused on studying the P2O5–K2O–Bi2O3 system. This study allowed the finding of a new form of polyphosphate KBi(PO3)4 (type IV) whose chemical preparation and crystal structure were determined. In addition, the titled compound has been characterized by IR and Raman spectroscopy.
Section snippets
Synthesis and characterization
Single crystal of a new form of KBi(PO3)4 was prepared by a flux method. At room temperature, 2 g of KH2PO4 and 0.52 g of Bi2O3 were slowly added to 2.5 ml of phosphoric acid H3PO4 (85%). The mixture was then slowly heated to 823 K and kept at this temperature for 24 h. After a few days of decreasing temperature, colorless, transparent and parallelepiped crystals were separated from the excess phosphoric acid by washing the product in boiling water. Subsequently, a second washing with nitric
Description of the structure
Fig. 1 shows a projection on the ac plane of the structure of a new form of KBi(PO3)4. A projection on the bc plane is depicted in Fig. 2. The basic structural units are helical ribbons (PO3)n, formed by corner-sharing of PO4 tetrahedra. The ribbons (two per unit cell) run along the [101] direction with a period of eight tetrahedra. Every two chains deduct themselves by 21 symmetry (Fig. 1). These chains are joined to each other by NdO8 dodecahedra, giving to a three-dimensional framework
Conclusion
The crystal structure is described for a new form of potassium bismuth polyphosphate KBi(PO3)4. The structure is determined by a single crystal X-ray analysis, and shows that this compound crystallizes in a monoclinic system P21/n, with a structural type IV. The main geometrical feature of this structure is the existence of two infinite (PO3)n chains, with a period of eight PO4 tetrahedra that forms two-dimensional zigzag sheets along the [101] direction. In this structure, bismuth atoms are in
Acknowledgements
The authors are most grateful to Professor Mohamed Rzaigui for the X-ray data collection.
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