Structure–property relations of orthorhombic [(CH3)3NCH2COO]2(CuCl2)3·2H2O[(CH3)3NCH2COO]2(CuCl2)3·2H2O

•Large single crystals (40 ×40 ×30 mm3) of [(CH3)3NCH2COO]2(CuCl2)3·2H2O[(CH3)3NCH2COO]2(CuCl2)3·2H2O were grown.•The elastic and piezoelastic coefficients were derived from ultrasonic resonance frequencies.•Thermal expansion (95 K–305 K) and heat capacity (113 K–323 K) were determined.•The orthorhombic structure shows pseudo-tetragonal elastic anisotropy at ambient conditions.•The crystal structure is stable in the investigated range (1–1600 bar, 95–303 K).

Large single crystals of orthorhombic [(CH3)3NCH2COO]2(CuCl2)3·2H2O[(CH3)3NCH2COO]2(CuCl2)3·2H2O with dimensions up to 40×40×30 mm3 were grown from aqueous solutions. The elastic and piezoelastic coefficients were derived from ultrasonic resonance frequencies and their shifts upon variation of pressure, respectively, using the plate-resonance technique. Additionally, the coefficients of thermal expansion were determined between 95 K and 305 K by dilatometry. The elastic behaviour at ambient conditions is dominated by the 2-dimensional network of strong hydrogen bonds within the (001) plane leading to a corresponding pseudo-tetragonal anisotropy of the longitudinal elastic stiffness. The variation of elastic properties with pressure, however, as well as the thermal expansion shows strong deviations from the pseudo-tetragonal symmetry. These deviations are probably correlated with tilts of the elongated tri-nuclear betaine–CuCl2–water complexes. Neither the thermal expansion nor the specific heat capacity gives any hint on a phase transition in the investigated temperature range.

Graphical abstractSingle crystal of orthorhombic [(CH3)3NCH2COO]2(CuCl2)3·2H2O[(CH3)3NCH2COO]2(CuCl2)3·2H2O.Figure optionsDownload full-size imageDownload as PowerPoint slide