Structural and Raman Spectroscopic Investigations of K4BaSi3O9 and K4CaSi3O9

Abstract

AbstractThe crystal structures of K4BaSi3O9 and K4CaSi3O9 have been characterized by X‐ray diffraction techniques as well as Raman spectroscopy. The structure of K4CaSi3O9 has been refined from powder diffraction data via the Rietveld method using polycrystalline material prepared from solid state reactions. The compound is isostructural with form I of K4SrSi3O9. It crystallizes with 16 formula units in a cubic primitive cell (a = 15.94014(3) Å, V = 4050.20(1) Å3) and adopts space group $Pa{\bar 3}$. K4CaSi3O9 belongs to the group of cyclosilicates and contains highly puckered twelve‐membered [Si12O36]‐rings centered on the ${\bar 3}\rm -axes$. Five of the seven crystallographically independent alkaline and alkaline earth cations are surrounded by six oxygen ligands in the form of distorted octahedra, which share opposite triangular faces and form non‐intersecting columns parallel to the body diagonals of the cubic unit cell. This arrangement corresponds to one of the cubic cylinder or rod packings. The two remaining sites have more irregular coordination environments with eight next oxygen neighbors. High temperature X‐ray powder diffraction data have been collected to determine the thermal expansion of this material: between room temperature and 700 °C the coefficient of thermal expansion has a value of α = 12.9(2) × 10−6 [°C−1]. Single crystals of K4BaSi3O9 have been obtained from the devitrification of a glass with the same composition. The structure was determined from a single crystal diffraction data set collected at −100 °C and refined to a final R index of 0.0298 for 1288 observed reflections (I > 2σ(>I)). The compound is isostructural with modification II of K4SrSi3O9. Basic crystallographic data are as follows: space group Ama2, a = 11.0695(15) Å, b = 8.0708(10) Å, c = 11.905(2) Å, V = 1063.6(3) Å3, Z = 4. With respect to the silicate anions the material can be classified as a sechser single chain silicate. The crankshaft‐like chains run parallel to [100] and are linked by K and Ba cations, which are distributed among five crystallographically independent sites. The coordination polyhedra of two of the non‐tetrahedral cations can be described by distorted octahedra sharing opposite triangular faces. They build non‐intersecting columns parallel to [011] and [0‐11], respectively. The other sites exhibit more irregular coordination spheres with 7‐9 neighbours.