X-Ray Study of Phase Transitions of the Elpasolite-Like Ordered Perovskite Pb2CoWO6w

The phase diagram of the ordered perovskite Pb2CoWO6 is investigated by means of single crystal X-ray diffraction. The results confirm the Bravais lattices given by the electron microscopy studies for the two lower-temperature phases (II and III). Temperature dependence of the lattice parameters is measured accurately between 10 and 310 K. The volume of the incommensurate phase (II) is found smaller than those of phases I and III. The subvolume could explain the frequent metastability of phase II below the transition II-III. This point is discussed. A dependence of the transition temperatures on the samples is found and attributed to synthesis problems.