Low Temperature Crystal Structure Behaviour of Complex Yttrium Aluminium Oxides YAlO3 and Y3Al5O12

Crystal structures of two yttrium aluminium oxides, namely YAlO3 and Y3Al5O12, were investigated in the temperature range 3.4-300 K by high-resolution neutron powder diffraction. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Thermal expansion of yttrium aluminium oxides was evaluated in terms of 1st order Gruneisen approximation, where the Debye temperatures and the Gruneisen parameters have been estimated for both compositions. Anomalies in the thermal expansion of yttrium aluminium perovskite have been observed and modelled using the Einstein oscillator with negative Gruneisen parameter. Extended bond length analysis revealed significant thermally-driven modifications of the aluminium oxygen framework.