A temperature-dependent structural investigation of electrical transitions in A₃conb₂o₉ perovskites (A=Ca²⁺, Sr²⁺, Ba²⁺)

Upon heating, the 1:2 triple perovskites A₃CoNb₂O₉ (A=Ca, Sr or Ba) each undergo well-defined insulator to conductor phase transitions at ∼8, ∼126 and ∼325 °C, respectively. As the trend in the transition temperatures for these materials does not correlate with the size of the reported band gaps, neutron powder diffraction has been used to investigate if this change in electrical behaviour of the materials was due to a structural phase change. It was found that in the regions of the suspected phase transitions there were only slight perturbations of the structures, namely thermal expansion of the lattices and an apparent muting of the amplitude of the octahedral rotations in the A=Ca and Sr compound at higher temperatures.