High-temperature synchrotron X-ray powder diffraction study of the orthorhombic-tetragonal phase transition in La0.63(Ti0.92,Nb0.08)O3

The crystal structure of La-0.63(Ti-0.92,Nb0.08)O-3 has been refined by the Rietveld analysis of CuKalpha X-ray powder diffraction data collected at 23degreesC. This material was confirmed to have an A-site deficient orthorhombic perovskite-type structure with double ideal perovskite AB03 units along the c-axis (space group Pinnim, Z = 2, a = 3.86036(5) Angstrom, b = 3.87222(5) Angstrom, c = 7.82609(9) Angstrom). Lattice parameters of the same sample have been investigated in situ in the temperature range from 25degreesC to 496degreesC by 1.37873(3) Angstrom synchrotron X-ray powder diffraction. The synchrotron X-ray powder diffraction technique was found to be very powerful to determine precise lattice parameters around a phase transition temperature. This compound exhibited a reversible phase transition between the orthorhombic and tetragonal phases at around 370degreesC. (1) The lattice parameters increased continuously with temperature, while the b/a ratio decreased continuously with temperature and became unity at the orthorhombic-tetragonal transition point. (2) No hysteresis was observed in the lattice parameter values between heating and cooling. These results of (1) and (2) suggest that the orthorhombic-tetragonal phase transition is continuous. (C) 2002 Elsevier Science (USA).