Crystal growth, structural characterization and high temperature behavior of Na3TmSi3O9

Abstract

Abstract Single-crystals of Na3TmSi3O9 were obtained in the course of systematic studies on the synthesis of rare earth element containing silicates from alkali fluoride flux systems. At 25 °C, the compound has the following basic crystallographic data: space group P212121, a = 15.1681(3) Å, b = 15.0855(3) Å, c = 14.9937(3) Å, V = 3430.83(12) Å3, Z = 16. Structure solution was based on direct methods. The subsequent refinement calculations resulted in a residual of R(∣F∣) = 0.039 for 9159 independent observed reflections with I > 2σ(I) and 579 parameters. From a structural point of view, Na3TmSi3O9 belongs to the group of single-chain silicates. The periodicity of the extremely folded chains has a value of 24. The compound is isostructural with the corresponding yttrium phase. Alternatively, the structure can be described as a mixed octahedral-tetrahedral framework, for which a detailed topological analysis is presented. Furthermore, a distinct pseudo-symmetry was observed. The sub-structure of the silicon and thulium atoms fulfills the symmetry requirements of P41212 within a few tenths of an Ångstrom. The high-temperature behavior of Na3TmSi3O9 was investigated by in-situ single-crystal X-ray diffraction in the range between 25 and 700 °C. No anomalies in the lattice parameters and unit cell volume vs. temperature curves were observed, pointing to the absence of structural phase transitions. From the evolution of the lattice parameters, the thermal expansion tensor αij has been determined. Thermal expansion shows a pronounced anisotropy. In the whole temperature region, the largest values are observed along [010] (parallel to the single-chains).