Evidence of rotational and tilting disorder of ReO 6 octahedra in single crystals of a 5d 1 double perovskite Ba 2 CaReO 6

We present results of an experimental study on single crystals of a 5 d 1 double perovskite Ba 2 CaReO 6 . Magnetization measurements reveal a weak splitting between zero-field-cooled and field-cooled protocols below 12 K. At magnetic fields above 1 T the splitting is absent and the magnetic susceptibility is featureless. A detailed specific heat study in a wide temperature range and comprising different heat pulses did not reveal any indication of a thermodynamic phase transition. At low temperatures we do observe specific heat deviating from a phonon background, leading to a total electronic entropy release of ∼ R l n 2 . Resonant and non-resonant x-ray diffraction of characteristic Bragg peaks indicates a significant presence of disorder, potentially related to random tilts and rotations of rigid ReO 6 octahedra. • Single crystals of Ba2CaReO6 seem to be dominated by structural disorder. • Thermodynamic and scattering experiments do not reveal any features associated with long-range order. • Non-phonon entropy still amounts to Rln2 at low temperatures. • Due to ionic size mismatch, tilts and rotations of rigid ReO6 octahedra could be the source of disorder.