Structure and phase transitions of CaTaOAlO4

The structure of CaTaOAlO 4 (CTAO) has been determined using X-ray powder diffraction and density functional methods in combination with 27 Al MAS NMR spectroscopy. A structural phase transition occurs near room temperature in CTAO as indicated by heat-capacity measurements, lattice strain data and infrared spectroscopy. Rietveld analysis of the powder diffraction data does not indicate deviation from monoclinic symmetry C2/c. But the observed quadrupolar coupling of the Al atom is reproduced by the electronic structure calculations only in a structure with space-group symmetry P2 1 /n, distinguished by two different Ta coordination environments. The atomic coordinates of this low-temperature structure of CTAO are obtained by computational force relaxation within the experimental unit cell determined at 170K.