Zero thermal expansion in a flexible, stable framework: Tetramethylammonium copper(I) zinc(II) cyanide

(Figure Presented) Tetramethylammonium copper(I) zinc(II) cyanide, which consists of N(CH₃)₄⁺ ions trapped within a cristobalite-like metal cyanide framework, has been studied by variable-temperature powder and single-crystal X-ray diffraction. Its coefficient of thermal expansion is approximately zero over the temperature range 200-400 K and comparable with the best commercial zero thermal expansion materials. The atomic displacement parameters, apparent bond lengths, and structure of a low-temperature, low-symmetry phase reveal that the low-energy vibrational modes responsible for this behavior maintain approximately rigid Zn coordination tetrahedra but involve significant distortion of their Cu counterparts.