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

Abstract

The structure of the inorganic misfit layer compound (La 0.95 Se) 1.21 VSe 2 has been determined on the basis of a four-dimensional superspace group. The crystal is composed of an alternate stacking of VSe 2 sandwiches and two-atom-thick LaSe layers. The first subsystem VSe 2 has a distorted CdI 2 -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), a 13 = 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 v 1 axes. The symmetry of the complete system is described by the superspace group G s = 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 > 2.5 σ ( I ), measured using Mo K α 1 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 La 3+ , V 3+ 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 VSe 2 .