Structures and Phase Transitions in Rb2MoO4 and Rb2WO4

The structural phase transitions of Rb2MoO4 and Rb2WO4 have been reinvestigated with the use of heat capacity measurement and synchrotron X-ray and neutron scattering techniques. In Rb2MoO4, the existence of polymorphous structures has been confirmed. These crystal structures at 290 K are a monoclinic beta-K2MoO4 type one and an orthorhombic beta-K2SO4 type one. In an annealed sample, two typical first-order-type anomalies were observed at 783 and 752 K. Furthermore, new lambda-type anomaly equivalent to the second normal-incommensurate phase transition in A(2)BO(4)-type ferroelectrics was observed at 223 K. On the other hand, the crystal structure at 290 K of Rb2WO4 was a monoclinic beta-K2MoO4 type one and no phase transition was detected down to 8 K.