Piezoelectric K₃Ta₃B₂O₁₂: Crystal structure at room temperature and crystal growth

K₃Ta₃B₂O₁₂ is a new piezoelectric material that crystallizes in the hexagonal system with space group P6̄2m and one formula unit in the unit cell. The lattice constants are a = 8.781 58±0.000 04 and c = 3.899 02±0.000 02 Å at 298 K (λCuKα = 1.540 598 Å). The integrated intensities of a full sphere of reciprocal space, with radius (sin ϑ)/λ?1.15 Å⁻¹, were measured with a CAD‐4 diffractometer and resulted in 1050 independent reliable structure factors. The crystal structure was solved by means of Patterson and Fourier series and was refined by the method of least squares, with final agreement factor R = 0.0133. Corner‐sharing triads of TaO₆ octahedra are stacked along the c axis and are connected by planar BO₃ groups. The resulting large trigonally girdled pentagonal prismatic interstices are occupied by the K⁺ ions. The average Ta–O distance is 1.973 Å in a strongly distorted octahedron. The B–O distance is 1.364±0.004 Å and the average K–O distance is 2.992 Å. The absolute sense of each polar axis has been determined and the piezoelectric d₂₂ coefficient measured as 1.6±0.1 pC N⁻¹. Piezoelectricity has not been detected in the related material K₃Nb₃B₂O₁₂, neither has evidence been found for a quadrupling in the a axis of K₃Ta₃B₂O₁₂ such as is readily observed in K₃Nb₃B₂O₁₂.