Structural phase transitions of clinoatacamite and the dynamic Jahn-Teller effect

The crystal structure of clinoatacamite, ideally Cu-2(OH)(3)Cl, has been studied using single-crystal X-ray diffraction and Raman scattering at variable temperature between 150 and 490 K in natural samples with different degrees of Co and Ni substitution. The monoclinic clinoatacamite crystal structure is closely related to the trigonal crystal structures of herbertsmithite, Cu3Zn(OH)(6)Cl-2, and paratacamite, Cu-3(Cu, Zn)(OH)(6)Cl-2. Disappearance of clinoatacamite superstructure reflections close to a temperature of 400 K indicates the transition to a trigonal herbertsmithite-type phase in all crystals containing Co and Ni impurities. In these crystals, which are invariably twinned at room temperature, an intermediate phase is observed in the temperature range 360-400 K, which has the trigonal paratacamite crystal structure. Untwinned end-member clinoatacamite directly transforms to the herbertsmithite-type at T-c = 445 K. Probability density functions of the oxygen and copper atoms as well as details from the Raman scattering measurements indicate that the interlayer Cu-coordination environment remains locally Jahn-Teller-distorted in the trigonal crystal structures with space group symmetries and and that the transition from monoclinic to trigonal symmetry is driven by the dynamic Jahn-Teller effect.