High-Pressure Low-Temperature Study of LnCa₄O(BO₃)₃ (Ln = Pr, Nd, Gd, Er, Tm)

We investigated the site-disorder and thermal expansion of LnCa O(BO₃)₃ (Ln = Pr, Nd, Gd, Er, Tm) through a single crystal structural study conducted between 100 and 300 K. Additionally, a high-pressure synchrotron X-ray diffraction study at low temperatures was conducted to determine the compressibility of both ordered and disordered prototype oxyborate lanthanides, specifically NdCa₄O(BO₃)₃ and ErCa₄O(BO₃)₃. The study revealed distinct behaviors consistent with their respective ionic radii. Furthermore, satellite reflections at 50 K, along with the onset of a triple a-parameter in the Nd crystal, indicate that the ground state of this extensively studied family may be different from what was previously reported.