BONDED INTERACTIONS IN GDFEO3-TYPE PEROVSKITES

C.T. Prewitt has advanced our understanding of the crystal chemistry in many important mineral groups, including the ABO₃ perovskites and related compounds. In spite of the importance of ABO₃ perovskites in earth science, relatively little is understood about the bonded interactions of the A atoms that occupy the sites within the octahedral framework. For example, not even the coordination numbers of the non-framework atoms has been established conclusively for most orthorhombic perovskites. We have approached this problem by calculating the electron density of oxide perovskites using both the procrystal model [1] and CRYSTAL98 [2] and analyzing the topological features of the electron density with SPEEDEN [3] and TOPOND [4]. We apply the criterion of Bader (1990) that a bonded interaction occurs between a pair of atoms if a topological feature, a (3,-1) critical point, exists in the electron density between the atoms. The calculations show that most non-framework atoms in oxide perovskites are surrounded by at least eight (3,-1) critical points. MgSiO₃ perovskite appears to be unique among orthorhombic oxide perovskites analyzed thus far in having only six bond paths from the non-framework cation to surrounding oxygen atoms. Location of (3,-1) critical points in ABO₃ perovskites also shows the effect of increased octahedral tilting on the bonded interactions around the A atom. The number of (3,-1) critical points observed around Ca in CaBO₃ perovskites (B=Ti,Ge,Sn,Zr), for example, decreases with increasing distortion. With increasing pressure, the bonded interactions of the non-framework cation is predicted to change if the dominant compression mechanism is achieved through tilting of the octahedra. The topological analysis also provides insight into the transition of the perovskite structure to the LiNbO₃ structure, first observed by Prewitt and co-workers in MnTiO₃ [5].

References:[1] Gibbs G.V., Spackman M.A., Boisen M.B. (1992) Am. Mineral. 77, 741-750 [2] Dovesi R., Saunders, V.R., Roetti C., Causà, M., Harrison, N.M., Orlando R., Zicovich-Wilson, C.M. (1998) CRYSTAL98 User’s Manual [3] Downs R.T., Andalman A., Hudasko M. (1996) Am. Mineral. 81, 1344-1349 [4] Gatti C. (1997) TOPOND96 User's Manual [5] Ross N.L., Ko J., Prewitt C.T. (1989) Phys. Chem. Mineral. 16, 621-629