High-Temperature Phase Transition in Lanthanum Titanate Perovskites

抄録

High-temperature phase transition of lanthanum niobium titanate La_0.64(Ti_0.92,Nb_0.08)O₃ has been studied using neutron powder diffraction and the Rietveld method (RIETAN-2001T). The material is orthorhombic (Cmmm) between 295 K and 534 K, while it is tetragonal (P4/mmm) at 637 K and 740 K. The tilt of the oxygen octahedron around Ti and Nb atoms was found to induce the tetragonal-to-orthorhombic phase transition. The angle of the tilt and the b/a ratio of cell parameters decrease with an increase of temperature and become 0 deg. and unity, respectively, at a transition temperature between 534 and 637 K.Both high- and low-temperature structures were in situ investigated by synchrotron radiation powder diffraction technique. Electron density distribution was obtained by the combination of the Rietveld refinement (RIETAN-2000) and maximum entropy method(Enigma and/or PRIMA).