Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures
Journal of Physics and Chemistry of Solids 67(8): 1754-1768
Combined neutron diffraction and high-resolution synchrotron X-ray powder diffraction methods have been used to examine the crystal structures of two sample sets of Sr/Mg-doped Lanthanum gallate with the compositions La0.9Sr0.1Ga1-yMgyO3-0.5(0.1+y) (Y = 0, 0.1, 0.2) and La0.8Sr0.2Ga1-yMgyO3-0.5(0.2+y) (y = 0.15, 0.2) up to 900 degrees C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La0.9Sr0.1GaO2.95: a=5.4904(1)angstrom, b=7.7757(1)angstrom, c=5.5229(l)angstrom; La0.9Sr0.1 Ga0.9Mg0.1O2.9: a=5.5100(1)angstrom, b=7.8080(l)angstrom, c=5.5411(1)angstrom; La0.9Sr0.1Ga0.8Mg0.2O2.85: a=5.5269(1)angstrom, b=7.8318(2)angstrom, c = 5.5459(1) angstrom. The samples of the second series have the cubic perovskite structure with space group Pm3m at room temperature: La0.8Sr0.2Ga0.85Mg0.15O2.825: a = 3.9160(1) angstrom; La(0.8)sr(0.2)Ga(0.8)Mg(0.20)O(2.80): a = 3.9195(1)angstrom. Samples of the first series transform from the orthorhombic to a rhombohedral (Imma -> R3c) structure at similar to 170 degrees C for La0.9Sr0.1GaO2.95, at similar to 430 degrees C for La0.9Sr0.1Ga0.9Mg0.1O2.9, and between 600 and 700 C for La0.9Sr0.1Ga0.8Mg0.2O2.85. Both La0.8Sr0.2Ga0.85Mg0.15O2.825 and La0.8Sr0.2Ga0.8Mg0.2 show no structural deviations from the cubic aristotype over the whole temperature range. The room temperature Imma structures of the first series are justified by a domain model and are rationalized in terms of static disorder increasing with Mg content, thus driving the phase transition temperatures to higher values in agreement with tolerance factor considerations. The distortion of the rhombohedral high-temperature phases (octahedra tilting and compression) and the effect of phase transitions on the ionic conductivity are discussed. (c) 2006 Elsevier Ltd. All rights reserved.