X-Ray Study on the Thermal Hysteresis of the Modulation Wavevector in (Rb1-xKx)2ZnCl4

The modulation wavevector \(kz{=}(1/3{-}δ)c*\) exhibits remarkable thermal hysteresis over a wide temperature range including the incommensurate-commensurate transition point in the mixed crystal system (Rb 1- x K x ) 2 ZnCl 4 with x =0, 0.005, 0.02, 0.28, 0.92, 0.997 and 1. This hysteresis seems to correspond to the thermal hysteresis observed in the dielectric constant and to support the proposal by Hamano et al . that the pinning of the modulation wave by defects causes the hysteresis. In the Rb rich case ( x ≦0.02), the parameter δ has a tendency to be fixed to the value realized at the normal-incommensurate transition point. In the K rich case ( x =0.997), it decreases monotonically with decreasing temperature, with a plateau in a middle range of the incommensurate phase. In the middle range of x , δ is pinned to a constant value down to low temperature.