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

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Abstract
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 SO2−4 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.