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

International audienceWe have studied the structural evolution of monoclinic BaZr(PO4)2 during heating up to 835K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730K 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 D3d ð¯3mÞ site, the P and two O atoms at a C3v(3m), and six O atoms at a Cs(m) site in the D3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either D13 d ðP¯31mÞ, D33 d ðP¯3m1Þ, or D53 d ðR¯3mÞ