Crystal structure and specific heat of calcium lanthanide oxyborates Ca4 LnO(BO3)3

Abstract

Calcium lanthanide oxyborates Ca₄ LnO(BO₃)₃ are of interest for their optical and electromechanical properties. Their crystal structure has been well characterised using powder and single-crystal X-ray diffraction but there remains some disagreement regarding cation ordering in these compounds. In this study, combined X-ray and neutron powder diffraction was employed to study the cation distribution and obtain accurate boron and oxygen atomic coordinates for six Ca₄ LnO(BO₃)₃ compounds (Ln = Pr, Nd, Tb, Ho, Er, Yb) at room temperature and one (Ln = Tb) at 50 and 1.5 K. All compounds adopt the previously reported monoclinic structure with space group Cm. The Ln ³⁺ ions are disordered over two of the three metal sites, with the extent of disorder increasing across the lanthanide series with decreasing ionic radius. Low-temperature neutron data for Ca₄TbO(BO₃)₃ showed a decrease in paramagnetic scattering on cooling but no obvious magnetic Bragg or diffuse scattering at the lowest temperature of 1.5 K. We report specific heat data at cryogenic temperatures for eight Ca₄ LnO(BO₃)₃ compounds and relate the magnetic properties of these compounds to their structural behaviour.