Crystal growth and phase transitions of solid solutions in [N(CH3)4]2MX4 isomorph class

Abstract

Abstract Homogeneous single crystals of tetramethylammonium tetrahalogenometallate solid solutions have been grown: a temperature difference growth method with thermally enforced convection was used during the material transport from an equilibrated feed material to the growing crystals. The smallest crystals were mostly studied by differential scanning calorimetry and X-ray crystallography. Some bigger crystals were available for dielectric measurements and Raman scattering. The temperature-concentration phase diagrams of solid solutions look like the temperature-pressure phase diagrams of pure tetramethylammonium salts. The progressive substitution seems equivalent to a change of the pressure, and is understood as a distortion of the metal-halide tetrahedron and a change of interactions with the first, second and third neighbours. The structure instabilities seem to have their origin within the b−c layers, and the increase of pressure (or equivalent concentration) should correspond to a hardening of the...