High-pressure study of the Sr2CoWO6 ordered double perovskite tungstate oxide

Using synchrotron radiation and Raman spectroscopy and a diamond anvil cell we measured the pressure dependence of the lattice parameters and Raman modes of polycrystalline Sr2CoWO6. Angle-dispersive X-ray diffraction patterns were indexed and showed that at 2.2 GPa, the material transforms from the I4/m tetragonal structure to the P2(1)/n monoclinic structure. For pressure values between 2.2 and 12.7 GPa only monoclinic symmetries were found. We had difficulties with the convergence of the profile matching of the diffractogram. taken at 12.7 GPa, indicating that the second phase-transition took place. To get more informations about these structural changes, an in-situ Raman spectroscopic study was conducted to explore the pressure-induced phase-transition sequence of Sr2CoWO6 to pressures of 40.8 GPa at room-temperature. Group theory yields nine Raman-active modes for Sr2CoWO6 (I4/m), all the predicted bands are observed at ambient conditions (phase I). The experimental results indicate that structural changes are observed at 2.15 and 11.15 GPa, which we attribute to phase transitions; thus, giving rise to two new phases, named as phase II and phase III, respectively. In the 9.33-14.84 GPa interval a coexistence of phases II and III is observed. (C) 2007 Elsevier B.V. All rights reserved.