X‐ray diffraction and dielectric temperature dependence study of the K2Cd2(SO4)3 paraelastic‐ferroelastic phase transition

摘要

The thermal expansion coefficients of the orthorhombic a, b, and c axes in langbeinite‐type K2Cd2(SO4)3 are α=24(2) ×10-6, 12(1) ×10-6, and 19(2) ×10-6 deg-1, β=5(1) ×10-8, 8(1) ×10-8, and 5(1) ×10-8 deg-2, γ=2(1) ×10-10, -6(1) ×10-10, and 6(1) ×10-10 deg-3, respectively, between 287 and 425 K. Above the first‐order phase transition at Tc=432 K, α=19(1) ×10-6 deg-1. The sulfate tetrahedra angular rotations are less than 5° between 298 and 417.5 K but are as much as 20° between 417.5 and 443.5 K. Over 70% of the rotations are estimated to take place within 2–3 K of Tc. The room‐temperature dielectric constants ϵ1, ϵ2, and ϵ3 are about 8, 8, and 16, respectively, at 1 kHz: a jump of about 5% occurs at Tc. Two spontaneous strains decrease markedly as Tc is approached, but all three remain greater than 2×10-3. The ferroelastic displacements decrease by as much as 0.25 Å between 298 and 417.5 K to values ranging from 0.22 to 0.91 Å. The absolute sense of the piezoelectric shear coefficient d14 has been measured as positive, i.e., a tensile stress along [011] generates positive polarity on (100), negative on (1¯00). The magnitude of d14 is about 7.3(5) ×10-12 C N-1, the electromechanical coupling factor k14 is 0.124(5), and the elastic compliance s11 is 1.72(2) ×10-11 m2 N-1.