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)_1.21 VSe₂ 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 C1 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 γ₁ = 89.91(2)°. The second subsystem LaSe has a distorted rock-salt structure with space-group symmetry C1 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*_v2,a*_v3) plane and have a displacive modulation according to the two mutually incommensurate periodicities along the a_v1 axes. The symmetry of the complete system is described by the superspace group G_s = C1 [0.6050(7), 0.0020(7), -0.007(1)] with C-centring (1/2, 1/2, 0, 1/2). Reciprocal lattice parameters for this superstructure embedding are (a*₁,a₂,a*₃,3*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 Se^2-. 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.