PRESSURE-INDUCED MODULATED PHASES OF

[N(CH₃)₄]₂CuCl₄. S. Shimomura, N. Hamaya¹⁾, Y. Fujii²⁾, T. Fukui, H. Terauchi, Department of Physics, Kwansei-Gakuin University, Nishinomiya, Hyogo 662, Japan, ¹⁾Department of Physics, Ochanomizu University, Bunkyo-ku, Tokyo 112, Japan, ²⁾Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, Shirakata, Tokai, Ibaraki 319-11, Japan

The A₂BX₄ type dielectrics are known to have various commensurate (C) and incommensurate (IC) phases. Tetramethylammonium tetrachlorocuprate

[N(CH₃)₄]₂CuCl₄, which belongs to the A₂BX₄ type, undergoes the successive phase transitions with decreasing temperature from the prototypical normal (N) phase to the IC phase at 26deg.C and to the C phase characterized by q = c^*/3 at 19deg.C. We carried out x-ray diffraction measurements under hydrostatic pressure using a beryllium-cylinder pressure cell and a diamond-anvil cell in order to determine the pressure and temperature dependence of the modulation wave vector and to explore additional pressure-induced phases.

The IC phase characterized by q < c^*/3 decreases its stable range with increasing pressure and finally vanishes at a point (23.4MPa, 30.0deg.C), which is denoted by L*. Another IC region characterized by q > c^*/3 is found to appear at L* and widen with increasing pressure. The second-order N-IC phase line and the first-order IC-C phase line meet at L*, where the phase transition between the N and C phases is considered to take place directly. The modulation wave vector in both IC regions tends to c^*/3 monotonically as the L* point is approached. These facts suggest that the L* point is a multicritical one in incommensurate systems. Applying higher pressure at room temperature, we found the existence of three more phases up to about 4 GPa.

One of the authors (SS) would like to thank a postdoctoral fellowship of Kwansei-Gakuin University.