Piezoelectric langbeinite-type K2Cd2(SO4)3: Room temperature crystal structure and ferroelastic transformation

Langbeinite-type dipotassium dicadmium trisulfate K₂Cd₂(SO₄)₃ is piezoelectric at room temperature and can be ferroelastically reoriented at higher temperatures. Crystallizing in space group P2₁2₁2₁ with four formulas in the unit cell, the lattice constants at 298.2 °K are a=10.2082(2), b=10.2837(3), c=10.1661(1) Å for CuKα₁=1.540562 Å. The integrated intensities of 13 525 structure factors, within a complete sphere of reflection of radius (sinϑ)/λ=1.20 Å^-1, were measured on a CAD-4 diffractometer with Nb-filtered MoKα radiation. The crystal structure was solved by use of Patterson and Fourier series, and refined by the method of least squares based on 3554 symmetry-independent Fmeas. The final agreement factor R=0.023 and the δR normal probability plot is slightly nonlinear. All atomic positions are within 0.8 Å of those reported in the archetype cubic mineral langbeinite K₂Mg₂(SO₄)₃ following a simple transformation. The S-O distances in the three independent SO^2-₄ tetrahedra range from 1.460(3) to 1.484(3) Å, with an average value of 1.470(8) Å. The two independent Cd²⁺ ions occupy irregular six-coordinated polyhedra with average Cd-O distances of 2.278 Å. One of the independent K⁺ ions has eight and the other ten nearest-neighbor O atoms less than 3.25 Å distant, with respective average K-O distances of 2.954 and 2.956 Å. Application of 5 MNm^-2 normal to (010), followed by heating to 435 °K and then cooling to room temperature still under applied stress, results in reorientation from abc to bca. The largest displacement required for room temperature ferroelastic reorientation is 1.19 Å for an oxygen atom.