Elastic Properties of the Triclinic Pseudo-Trigonal Room-Temperature Phase of KIO3

Weakly twinned triclinic crystals of the room temperature γ-phase of KIO₃, grown from aqueous solutions, exhibit a quasi-trigonal behaviour of point symmetry group 3m in respect to thermal expansion, piezoelectric effects, and elastic properties. The elastic tensor and its temperature derivative were determined from ultrasonic resonance frequencies of thick plane-parallel plates and their shift upon variation of temperature, respectively. The static piezoelectric constants d₂₂₂ and d₂₁₁ are about fifty times larger than d₁₁₁ of α-quartz resulting in strong piezoelectric coupling coefficients of elastic waves (k₁ = 0.42; k₆₆ = 0.60). Approaching the γ-β transition at 345.6 K from lower temperatures, the shear stiffness c^E₆₆ shows a drastic softening. The transition appears to be of second-order ferroelastic.