Ferroelectric Tungsten Bronze-Type Crystal Structures. III. Potassium Lithium Niobate K(6−x−y)Li(4+x)Nb(10+y)O30

Abstract

Ferroelectric K(6−x−y)Li(4+x)Nb(10+y)O30, with x ≃ 0.07, y ≃ 0.23, and a Curie temperature of 613°K, crystallizes with a tungsten bronze-type structure in the tetragonal system. The lattice constants are a = 12.5764 ± 0.0002 and c = 4.0149 ± 0.0001 Å at 298°K. The space group is P4bm. There is one formula per unit cell. The integrated intensities of 6578 structure factors, inside a reciprocal hemisphere of radius (sinθ) / λ = 1.02 Å−1, were measured with PEXRAD. There are 998 symmetry-independent Fmeas significantly above background. Isomorphism with BaxSr(5−x)Nb10O30 allowed the metal atom positions to be deduced. Oxygen atom positions were obtained by Fourier series methods. Refinement of structural parameters by the method of least squares resulted in a final agreement factor R = 0.0401. The structure is generally like that of other tungsten bronzes of formula (A1)2(A2)4C4(B1)2(B2)8O30, with the A1 site occupied by 87% K and 13% Li, the A2 site by 99% K and 1% Li, and the C site by 94% Li and 6% Nb. The B sites are fully occupied by Nb. X-ray spectrochemical analysis and the present diffraction study suggest x ≃ 0.07 and y ≃ 0.23 for the nonstoichiometric formula above. Evidence for major positional disorder, excluding occupancy disorder, similar to that observed in BaxSr(5−x)Nb10O30 and in Ba(4+x)Na(2−2x)Nb10O30 is not present for K(6−x−y)Li(4+x)Nb(10+y)O30. The K/Li atom at the A1 site is 12 coordinated with average K/Li–O distance of 2.833 Å. The K/Li atom at the A2 site is nine coordinated with average K/Li–O distance of 3.024 Å. The Li/Nb atom at the C site has three nearest oxygen neighbors at an average distance of 2.153 Å and six additional next-nearest neighbors at an average distance of 2.542 Å. The two independent irregular NbO6 octahedra have an average Nb–O separation of 1.973 Å. The Nb atom displacements, of 0.167 and 0.160 Å from the nearest mean oxygen plane, may be compared with the value of 0.175 Å predicted from the parabolic relation with Curie temperature. All metal atoms are displaced from the mean planes of oxygen in the same sense, making the positive polarization direction parallel to the sense of this displacement.