Vacancies and electron localization in the incommensurate intergrowth compound (La_0.95Se)_1.21VSe₂

The structure of the inorganic misfit layer compound (La_0.95Se)_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 and its basic structure unit-cell dimensions are a₁₁ = 3.576 (3), a₁₂ = 6.100 (2), a13 = 11.690 (2) Å, α₁ = 95.12 (2), β₁ = 85.96 (2) and γ = 89.91 (2)°. The second subsystem LaSe has a distorted rock-salt structure with space-group symmetry C and a basic structure unit cell given by a₂₁ = 5.911 (2), a₂₂ = 6.101 (2), a₂₃ = 11.684 (2) Å, α₂ = 95.07 (2), β₂ = 85.76 (2), γ₂ = 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 G_s = C [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.