The crystal structure of the low-temperature phase of (NO)2SnCl6 and ordering of the NO+ ions

Abstract

Abstract At ambient conditions (NO)2SnCl6 is isostructural with the corresponding titanium and vanadium salts which all crystallize in space group P4/mnc (Z = 2) and display a fourfold disorder of the NO+ ions. Below T c = 229 K the diffraction pattern splits into the components of a quadruplet twin, but unlike for (NO)2VCl6 remarkable discontinuities of the lattice parameters are not observed. The transition from P4/mnc to space group P21/n (Z = 2) occurs in only one step which at T c = 153 K, the temperature of the crystal structure determination, consists of an adaptive tilt (5.3°) of the SnCl6 2– octahedra leading to a reduction of the cationic disorder from four to merely two N—O alignments, which are differently populated. Despite of the continuously proceeding transition, the phase change from the high- to the low-temperature regime has to be classified as a 1st order process.