Vacancies and electron localization in the incommensurate intergrowth compound (La0.95Se)1.21VSe2

Abstract

The structure of the inorganic misfit layer compound (La0.95Se)1.21VSe2 has been determined on the basis of a four-dimensional superspace group. The crystal is composed of an alternate stacking of VSe2 sandwiches and two-atom-thick LaSe layers. The first subsystem VSe2 has a distorted CdI2-type structure with V atoms in trigonal antiprisms of Se atoms. It has space-group symmetry C{\bar 1} and its basic structure unit-cell dimensions are a 11 = 3.576 (3), a 12 = 6.100 (2), a13 = 11.690 (2) Å, α 1 = 95.12 (2), β 1 = 85.96 (2) and γ = 89.91 (2)°. The second subsystem LaSe has a distorted rock-salt structure with space-group symmetry C{\bar 1} and a basic structure unit cell given by a 21 = 5.911 (2), a 22 = 6.101 (2), a 23 = 11.684 (2) Å, α 2 = 95.07 (2), β 2 = 85.76 (2), γ 2 = 90.02 (2)°. The two subsystems have the common (a* ν2, a* ν3) plane and have a displacive modulation according to the two mutually incommensurate periodicities along the v1 axes. The symmetry of the complete system is described by the superspace group Gs = C{\bar 1} [0.6050 (7), 0.0020 (7), −0.007 (1)] with C-centring (½,½, 0, ½). Reciprocal lattice parameters for this superstructure embedding are (a * 1, a * 2, a * 3, a * 4) = (a * 11, a * 12, a * 13, a * 21). For 2125 unique reflections with I> 2.5σ(I), measured using Mo Kα1 radiation, refinement smoothly converged to wR = 0.055 (R = 0.045) on a modulated structure model with 77 parameters including La vacancies. The presence of ~ 5% of La vacancies in the LaSe subsystem leads to an exact charge balance between La3+, V3+ and Se2−. The largest modulation occurs on the V atoms, which results in strong variation in the V—V distances. Thus, the semiconducting behaviour of this compound is interpreted in terms of La vacancies in LaSe and modulation-induced Mott localization in VSe2.