On the existence of a second modification of K 4SrSi 3O 9 – X-ray single crystal diffraction, Raman spectroscopical and high temperature studies

Abstract

Single crystals of a new modification of K 4SrSi 3O 9 (form II) have been obtained from slow cooling of a stoichiometric melt. Structure analysis was based on X-ray diffraction data collected on a twinned specimen containing three different individuals. The compound crystallizes in the orthorhombic space group Ama2 with one formula unit in the cell. The lattice constants are a = 10.9840(15) Å, b = 7.9535(10) Å and c = 11.916(2) Å. Concerning the silicate anions, the new modification can be classified as a sechser single chain silicate. The crankshaft-like chains are running parallel to [100]. Linkage between the chains is provided by K and Sr cations which are distributed among five crystallographically independent sites. Site occupancy refinements indicated the absence of cation mixing on different positions. The coordination environment of two of the non-tetrahedral cations can be described by distorted octahedra. These octahedra share opposite triangular faces, building non-intersecting columns parallel to [011] and [ 0 1 ¯ 1 ] , respectively. The remaining three potassium sites have more irregular coordination environments with 6–10 next oxygen neighbors. Structural investigations on the new form are completed by solid state Micro-Raman spectroscopy. High temperature X-ray powder diffraction was employed to establish the stability fields of the two modifications. Up to the melting point no direct evidence for a phase transformation from the cubic form I to form II was found. The features of the thermal behavior of the lattice constant and the coefficient of thermal expansion of form I have been determined.