A Raman spectroscopic study of the phase transition of BaZr(PO4)(2): Evidence for a trigonal structure of the high-temperature polymorph

摘要

We have studied the structural evolution of monoclinic BaZr(PO4)(2) during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO4 vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a D-3d((3) over barm) site, the P and two O atoms at a C-3p(3m), and six O atoms at a C-s(m) site in the D-3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either D-3d(1)(P(3