Successive Phase Transitions and High Ionic Conductivity of Trichlorogermanate (II) Salts as Studied by 35C1 NQR and Powder X-Ray Diffraction*

A series of trichlorogermanate(II) salts (AGeCl3, A = Rb, Cs, C H 3N H 3, and (CH 3)4N) have been synthesized and characterized by 35C1 NQ R, 35C1 NM R, AC conductivity, DTA, and X-ray diffrac­ tion techniques. In the temperature range studied two, two, five, and four phases were confirmed for the Rb, Cs, C H 3N H 3, and (CH 3)4N salts, respectively. From the 35C1 NQ R and structural data, isolated pyramidal G eC l3 anions were recognized in the low temperature phases. With increasing temperature the relaxation times of the 35C1 NQR decreased exponentially and the signals disap­ peared 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 C H 3N H 3G eC l3 and (CH 3)4N G eC l3 increased drastically at the phase transitions to their cubic perovskite phases (CH 3N H 3G eC l3: a = 10-1 S m -1 at 400 K, (CH3)4N G eC l3: a = 5 x 10-2 S m _1 at 420 K). The mobile ion was confirmed to be the chloride ion by means of 35C1 NM R and X-ray diffraction.