Studies on structural and thermal expansion behavior of alkali-thorium phosphates

The structure of RbTh2(PO4)3 and CsTh2(PO4)3, were derived by Rietveld profile analysis of the powder X-ray diffraction data of polycrystalline materials in a monoclinic cell in the space group C2/c. The atomic positions of isostructural KTh2(PO4)3 were used as the basis of the refinement of atomic parameters of RbTh2(PO4)3 and CsTh2(PO4)3. The structure is made of framework of [Th2(PO4)3]∞ with edge and corner sharing PO4 tetrahedra and ThO9 polyhedra. Rb and Cs atoms sit on two-fold axis in the large tunnels running along c-axis.The thermal expansion behavior of MTh2(PO4)3 (M = Na, K, Rb or Cs) were investigated between room temperature and 1273 K using a high temperature powder X-ray diffraction method. The lattice parameters of the four compounds were fitted into polynomial expression with respect to temperature. The average thermal expansion coefficients of the four compounds were derived from the high temperature data.