Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures Original Research Article

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−y Mgy O3−0.5(0.1+y ) (y=0y=0, 0.1, 0.2) and La0.8Sr0.2Ga1−y Mgy O3−0.5(0.2+y ) (y=0.15y=0.15, 0.2) up to 900 °C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La0.9Sr0.1GaO2.95: View the MathML sourcea=5.4904(1)Å, View the MathML sourceb=7.7757(1)Å, View the MathML sourcec=5.5229(1)Å; La0.9Sr0.1Ga0.9Mg0.1O2.9: View the MathML sourcea=5.5100(1)Å, View the MathML sourceb=7.8080(1)Å, View the MathML sourcec=5.5411(1)Å; La0.9Sr0.1Ga0.8Mg0.2O2.85: View the MathML sourcea=5.5269(1)Å, View the MathML sourceb=7.8318(2)Å, View the MathML sourcec=5.5459(1)Å. The samples of the second series have the cubic perovskite structure with space group View the MathML sourcePm3¯m at room temperature: La0.8Sr0.2Ga0.85Mg0.15O2.825: View the MathML sourcea=3.9160(1)Å; La0.8Sr0.2Ga0.8Mg0.20O2.80: View the MathML sourcea=3.9195(1)Å. Samples of the first series transform from the orthorhombic to a rhombohedral (Imma→View the MathML sourceR3¯c) structure at ∼170 °C for La0.9Sr0.1GaO2.95, at ∼430 °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.