Structures and phase transitions in (K1-xRbx)2SeO4

The phase diagram of the mixed crystal (K1-xRbx)2SeO4 was determined by means of thermal analysis and neutron scattering experiments. The hexagonal to orthorhombic phase transition line exists for any x. The normal-incommensurate phase transition temperature decreases continuously with increasing Rb content. However, the incommensurate-commensurate phase transition was not observed except for K2SeO4. According to the clear softening of the Sigma2- Sigma 3 phonon branches and the finite frequency at 0 K for x > 0.34, an existence of the hypothetical phase transition was confirmed.