High pressure study of NH4Pb2Br5 type compounds.: I -: Structural parameters and their evolution under high pressure

We have investigated the nature of the interactions of ns(2)-cations and the possible structure-determining,role of the ns(2)-electron pair at ambient and high pressure in several AB(2)X(5) (A = K, Rb, Cs, In, TI; B = Sri, Ph, Sr; X = Cl, Br, I) compounds. Structural parameters are obtained by high pressure x-ray diffraction as well as by quantum mechanical methods (DFT-GGA-calculations). The structural parameters at ambient and high pressure are discussed and compared to those of Tl(5)Se(2)l crystallising in the antitype structure. Short cation-cation distances in the NH4Pb2Br5 type structure enable direct cation-cation interactions and the existence of an ns(2)-cation in the B-position is crucial for the stability of these structures. The effect of pressure on the structural parameters of these compounds gives new insights into the interactions of lone pair cations. The pronounced decrease of the cation-cation distances with pressure points to strongly increasing bonding interactions between the lone pair cations.