PRESSURE-INDUCED PHASE-TRANSITIONS AND CHANGE IN CRYSTAL COLOR IN [N(CH3)(4)](2)CUCL4

X-ray diffraction studies of [N(CH3)(4)](2)CuCl4 single crystals have been carried out under hydrostatic pressure at room temperature. With increasing pressure, phase transitions take place successively, to a m onoclinic phase with a unique axis along b (space group, C-2h(5)-P12(1 )/c1) at 1.4 GPa, to a monoclinic phase with a unique axis along c (C- 2h(5)-P112(1)/n) at 1.7 GPa, and then to an orthorhombic phase (D-2(4) -P2(1)2(1)2(1)) at 3.4 GPa. The transition to the highest-pressure pha se is accompanied by a discontinuous change in the crystal color from light yellow to dark green. The color change is considered to be a res ult of a change in CuCl42- anion geometry.