EFFECTS OF ALKALI SUBSTITUTION AND PRESSURE ON THE CHARGE-DENSITY WAVE TRANSITIONS OF 2-DIMENSIONAL METALS K3CU8S6 AND RB3CU8S6

K3Cu8S6 is a two-dimensional metal which has two phase transitions at T1 (153 K) and T2 (55 K), accompanied by an incommensurate and a commensurate superstructure, respectively. In order to investigate these phase transitions, the mixed crystals (K1-xRbx)3Cu8S6 have been synthesized and the effect of the alkali substitution and that of pressure have been studied by the measurements of their transport properties. With increasing x, T1 significantly decreases, reaching T1 of Rb3Cu8S6 (= 110 K). Pressure also reduces T1 of K3Cu8S6 and Rb3Cu8S6. On the other hand, the T2 transition becomes obscure with increasing x, and disappears thoroughly for x greater-than-or-equal-to 0.50. In Rb3Cu8S6, the T2 transition is absent at ambient pressure but induced by the pressure of at most 0.5 GPa. Around x = 0.06, a remarkable rise in the resistivity appears below T2. Its mechanism is discussed.