Crystal Growth and Elastic Properties of Hopeite, Zn 3 (PO 4 ) · 4 H 2 O

Single crystals of orthorhombic Zn 3 (PO 4 ) 3 · 4 H 2 O of optical quality with dimensions up to 12 × 10 × 8 mm were grown from aqueous solutions of ZnSO 4 and NaH 2 PO 4 by controlled diffusion of NH 3 into the solution. The elastic constants c ij and the thermoelastic constants T ij = d c ij /d T , T temperature, were determined from ultrasonic resonance frequencies of thick plates in the range between 5 and 40 MHz and between 250 and 320 K. In respect to the longitudinal elastic stiffness hopeite behaves quasi isotropically, however, the elastic shear stiffness shows a large anisotropy of about a factor 2.5. Above 260 K the shear stiffness c 66 possesses a quite anomalous temperature dependence ( T 66 > 0). Further, the linear thermal expansion reveals a strong anisotropy (α 11 = −3.3, α 22 = 3.4, α 33 = 33 · 10 −6 /K).