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.

A Crystal-Chemical Framework for Relaxor versus Normal Ferroelectric Behavior in Tetragonal Tungsten Bronzes

“…TTBs consist of layers of distorted B1O 6 and B2O 6 octahedra sharing corners so that they form three different interstices: pentagonal (A2); square (A1, sometimes referred to as the perovskite site) and trigonal (C) which may be occupied by cations according to a general formula (A2) 4 (A1) 2 (C) 4 (B1) 2 (B2) 8 O 30 , Figure 1. 2 For so-called 'stuffed' TTBs, such as K 6 Li 4 Nb 10 O 30 (KLN), Li is found in the trigonal C site 3 but in 'filled' TTBs the trigonal interstice C is empty and the general formula reverts to (A2) 4 (A1) 2 (B1) 2 (B2) 8 3 For 'unfilled' TTBs, such as Sr x Ba 1-x Nb 2 O 6 , some of the A1/A2 sites remain vacant. 4 Several studies have attempted to classify TTBs using a geometric tolerance-factor (t) approach similar to that defined for perovskites by Goldschmidt 5 .…”

Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze K3Li2Ta5OSmith1, Gardner2, Morrison3 et al. 2018Phys. Rev. Materials2020View full textAdd to dashboardCite

“…The ferroelectric transition in KLN is already known to be frequency dependent, with a dielectric permittivity maximum shifting around 750 K [40]. These two relaxor-type peaks from KLT may be explained in terms of partial occupancies or mixing of the sites in the TTB, since both explanations have been reported in KLN [28,66], alternatively incommensurate transitions may occur, as reported [21] with other TTB compounds. Which of these exist in KLT will be the subject of further (neutron scattering) investigations…”

“…(Note that after a series of orthorhombic phases at intermediate temperatures, Ba 2 NaNb 5 O 15 reverts to a tetragonal P 4nc commensurate ground state at lowest temperatures [23]; this "reverse symmetry" orthorhombic-tetragonal cooling transition involves a unit-cell doubling.) K 3 Li 2 Nb 5 O 15 (KLN) is a possible "stuffed" TTB since the stoichiometric material has K on all the A2 and A1 sites and Li on all the C sites; however, several authors [24][25][26][27][28] suggest that an excess of Nb is required to form single-phase samples. Therefore the structure either has vacancies and/or mixing of the crystallographic site occupancy.…”

Interdependence between electrical and magnetic properties of polycrystalline cobalt-substituted tungsten bronze multiferroic ceramics

“…The bond dissociation energy of Nb-O is 753 kJ/mol which is greater than Co-O (362 kJ/mol) and Ti-O (662 kJ/mol). 24 So, during high temperature sintering Co, atom starts entering at Ti site and Nb due to which the formation of Co-O bond takes place. For lower concentration of Co content, Ba-O and Ca-O bonds are larger and Ti-O and Nb-O bonds (B-site) are stronger than A site bond and it results in the occurrence of phase transition at higher temperature due to which the transition temperature increases.…”

“…For lower concentration of Co content, Ba-O and Ca-O bonds are larger and Ti-O and Nb-O bonds (B-site) are stronger than A site bond and it results in the occurrence of phase transition at higher temperature due to which the transition temperature increases. [24][25][26] With further increase of cobalt concentration, the Co-O bond increases and this bond becomes stronger than Ti-O and Nb-O bonds which lead to weak interaction between BO 6 octahedra. This results in the decrease of transition temperature (T c ).…”