Order/disorder processes and electromechanical properties of monoclinic GdCa${}_4$O(BO${}_3$)${}_3$

Large single crystals of GdCa${}_4$O(BO${}_3$)${}_3$ (space group Cm) were grown by the Czochralski method. Dielectric, piezoelectric and elastic coefficients at room temperature as well as specific heat capacity, thermal expansion and cation disorder were studied employing a variety of methods including resonant ultrasound spectroscopy, differential scanning calorimetry, dilatometry and X-ray diffraction techniques. The electromechanical parameters (4 dielectric, 10 piezoelectric and 13 elastic stiffness coefficients) obtained on different samples are in excellent agreement indicating high internal consistency of our approach, whereas the values reported in literature differ significantly. The elastic behaviour of GdCa${}_4$O(BO${}_3$)${}_3$ resembles the one of structurally related fluorapatite, i.e. the elastic anisotropy is relatively small and the longitudinal effect of the deviations from Cauchy-relations exhibit a pronounced minimum along the direction of the dominating chains of cation polyhedra. GdCa${}_4$O(BO${}_3$)${}_3$ exhibits a maximum longitudinal piezoelectric effect of 7.67 × 10${}^{-12}$ CN${}^{-10}$, a value in the order of that of langasite-type materials. Significant changes of the calcium/gadolinium distribution on the 3 independent cation sites accompanied by characteristic anomalies of heat capacity and thermal expansion suggest processes of nonconvergent cation ordering above about 900 K in GdCa${}_4$O(BO${}_3$)${}_3$.