SUCCESSIVE PHASE-TRANSITIONS AND HIGH IONIC-CONDUCTIVITY OF TRICHLOROGERMANATE (II) SALTS AS STUDIED BY CL-35 NQR AND POWDER X-RAY-DIFFRACTION

A series of trichlorogermanate(II) salts (AGeCl3, A = Rb, Cs, CH3NH3, and (CH3)4N) have been synthesized and characterized by Cl-35 NQR, Cl-35 NMR, AC conductivity, DTA, and X-ray diffraction techniques. In the temperature range studied two, two, five, and four phases were confirmed for the Rb, Cs, CH3NH3, and (CH3)4N salts, respectively. From the Cl-35 NQR and structural data, isolated pyramidal GeCl3- anions were recognized in the low temperature phases. With increasing temperature the relaxation times of the Cl-35 NQR decreased exponentially and the signals disappeared far below the melting point. This suggests that the reorientation of the anion about the pseudo three-fold axis is excited. With further increase in temperature, the ionic conductivity of CH3NH3GeCl3 and (CH3)4NGeCl3 increased drastically at the phase transitions to their cubic perovskite phases (CH3NH3GeCl3: sigma = 10(-1) S m-1 at 400 K, (CH3)4NGeCl3: sigma = 5 x 10(-2) S m-1 at 420 K). The mobile ion was confirmed to be the chloride ion by means of Cl-35 NMR and X-ray diffraction.