Low temperature phase transition and crystal structure of CsMgPO4

CsMgPO4 doped with radioisotopes is a promising compound for usage as a radioactive medical source. However, a low temperature phase transition at temperatures close to ambient conditions (similar to-40 degrees C) was observed. Information about such kind of structural changes is important in order to understand whether it can cause any problem for medical use of this compound. The phase transition has been investigated in detail using synchrotron powder diffraction, Raman spectroscopy and DFT calculations. The structure undergoes a transformation from an orthorhombic modification, space group Pnma (RT phase) to a monoclinic polymorph, space group P2(1)/n (LT phase). New LT modification adopts similar to RT but slightly distorted unit cell: a= 9.58199(2) angstrom, b=8.95501(1) angstrom, c=5.50344(2) angstrom, beta=90.68583(1)degrees, V=472.198(3) angstrom(3). CsMgPO4 belongs to the group of framework compounds and is made up of strictly alternating MgO4- and PO4-tetrahedra sharing vertices. The cesium counter cations are located in the resulting channel-like cavities. Upon the transformation a combined tilting of the tetrahedra is observed. A comparison with other phase transitions in ABW-type framework compounds is given. (C) 2014 Elsevier Inc. All rights reserved.