ANION-CENTRED TETRAHEDRA IN INORGANIC COMPOUNDS

Sergey Krivovichev, Stanislav Filatov

Department of Crystallography, St.Petersburg University University Emb. 7/9 199034 St.Petersburg Russia, E-mail: flt@cryst.geol.pu.ru

Keywords: anion-centred tetrahedra, cationic complexes, inorganic crystal structures

It is an usual way in crystal chemistry to consider cations as central atoms and anions as ligands. However, there is a number of inorganic structures containing structural units based on anion-centred polyhedra and, in particular, anion-centred tetrahedra [XA4], where X = O-2, N-3 and A = Cu, Pb, REE, etc. Their identification as units of special type is not only from geometry, but also because of the high strength of the X-A bonds. [XA4] tetrahedra may be isolated from each other or linked into finite polyions, chains, layers or frameworks. The consideration of these complexes is essential to describe the organization of crystal structure and to explain its physical anisotropy.

The aim of this report is to outline the answers on the following questions:

  1. How many types of [XA4]-complexes do exist?
  2. How many types of [XA4]-complexes may exist?
  3. What are the rules of their organization?
  4. How are the [XA4]-units related with others to give a whole structure?
  5. Are the principles of crystal chemistry of [XA4] tetrahedra similar to those of crystal chemistry of cation-centred tetrahedra (e.g., silicate)?

We do not consider our work as a first effort in the field of 'anion-centred' crystal chemistry. The references should be made on the great number of papers in which anion-centred tetrahedra were considered as building blocks in the concrete structures. However, a systematic approach was used only in a few works, which we reviewed in [1]. Our interest to this theme was generated by the crystal structure determinations of some exhalative minerals. As it turned out, they are based on the [OCu4] tetrahedra linked in variously shaped polycations [2].

The second our aim is to put two our own questions.

1. What is a nature of chemical bonds in [XA4] tetrahedra? M.Lulei and J.D.Corbett [3] consider their as A4 metal clusters interstitially stabilized by N, O central atoms for A represented by rare earth metals. Does this be the case for other A?
2. If yes, what are the electron rules governing their formation? And, generally, what are the conditions of their formation, in particular, in nature?

These questions are of general importance because the similar structures (anion-centred cationic complexes or cores) are well known in minerals, glasses, molten media, aqueous solutions, metal-organic compounds and biological systems.

This work was financially supported by the Russian Fund for Basic Researches (RFBR, grant # 96-05-65576).

  1. S.V. Krivovichev, S.K. Filatov, T.F. Semenova, Russ. Chem. Rev. 67 (1998) 155-174.
  2. S.K. Filatov, T.F. Semenova, L.P. Vergasova, DAN SSSR 322 (1992) 536-539; G.L. Starova, S.V. Krivovichev, V.S.Fundamensky, S.K. Filatov, Miner. Mag. 61 (1997) 456-461.
  3. M. Lulei, J.D. Corbett, Angew. Chem. 34 (1995) 2262-2264; ibid, Eur. J. Solid State Inorg. Chem. 33 (1996) 241-250; M. Lulei, S.J. Steinwand, J.D. Corbett, Inorg. Chem. 34 (1995) 2671-2677; M. Lulei, Inorg. Chem. 37 (1998) 777-781.