X-Ray Diffraction Study on the Crystal Structure and the Incommensurate-Commensurate Phase Transition in {N(CH₃)₄}₂CuBr₄

Abstract

The superstructures were found in the low temperature phase of {N(CH 3 ) 4 } 2 CuBr 4 . With decreasing temperature, the cell dimension changes as b 0 , ∼2 b 0 , 2 b 0 and b 0 , respectively. The space group of each phase is Pmcn, incommensurate, Pbc2 1 and P12 1 /cl. As the temperature decreases the modulation wave number q 0 ≈0.62 b * at -1°C decreases continuously to ∼0.57 b * at -31°C and jumps to the commensurate value of 1/2 b * in the ferroelectric phase.