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

The structure of the inorganic misfit layer compound (La0.95Se)(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 C (1) over bar and its basic structure unit-cell dimensions are a(11) = 3.576(3), a(12) = 6.100(2), a(13) = 11.690(2)Angstrom, alpha(1) = 95.12(2), beta(1) = 85.96(2) and gamma(1) = 89.91(2)degrees. The second subsystem LaSe has a distorted rock-salt structure with space-group symmetry C (1) over bar and a basic structure unit cell given by a(21) = 5.911(2), a(22) = 6.101(2), a(23) = 11.684(2)Angstrom, a(2) = 95.07 (2), beta(2) = 85.76(2), gamma(2) = 90.02(2)degrees. The two subsystems have the common (a(nu 2)*,a(nu 3)*) plane and have a displacive modulation according to the two mutually incommensurate periodicities along the a(nu 1) axes. The symmetry of the complete system is described by the superspace group G(s) = C (1) over bar [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)*,a(2)*,a(3)*,a(4)*) = (a(11)*,a(12)*,a(13)*,a(21)*). For 2125 unique reflections with I > 2.5 sigma(I), measured using Mo K alpha(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 similar to 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.