With respect to their phase transitions, Cs2CdBr4 and Cs2HgBr4 are the first known representatives of a new group having the fl-K2SO4-type structure. Upon coolmg they show the following phase sequence. Phase around the a axis by 7 °. In P1 there is also a slight distortion of the structure.
The structure of Cu2CdSiS4 is a superstructure of the wurtzite type. Crystallizing in the space group Pmn21 with the lattice parameters a = 7.60 (~-2awurtz,te), b = 6"48 (-~ l/3awurtz,e) and c = 6"25/~ ( = C,,,urtz,,c), the unit cell contains 2 formula units Cu2CdSiS4 (calculated densityD~,=4-26). The structure was postulated by geometrical considerations and by taking into account systematic and pseudo-selection rules; it was confirmed by Fourier syntheses. Half the tetrahedral voids of the hexagonal close sulphur packing are regularly occupied by two Cu, one Cd and one Si atom, while the coordination of the electropositive elements by sulphur is also tetrahedral. This structure is a 'normal tetrahedral' one, [after Parthr, E., Crystal Chemistry of Tetrahedral Structures, New York, Gordon & Breach (1964)], and represents a limiting member of the series wurtzite-Cu2CdSiS4, just as Cu2FeSnS4 does in the series sphalerite-stannite with cubic close sulphur packing.
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