The interface-modulated structures of the MA xTe2 phases (M=Nb, Ta; A=Si, Ge; 1/3≤x≤1/2)

摘要

The A/i4xTe2 phases (A/=Nb, Ta; /i=Si,Ge; x rational: 1/3, 2/5, 3/7,...,(l+n)/(3+2n),..,l/2 or irrational 0.360,0.414...) all derive from a stacking of trigonal prismatic [AfTe2] sandwiches. Each basic unit cell contains two sandwiches in either an AA/BB or an AA/BB/CC stacking mode of the Te sheets. The space between two successive sandwiches is empty, giving the structures a quasi two-dimensional character. A atoms are in a very unusual Te square coordination, in the common face of two otherwise empty trigonal prisms. The other trigonal prisms constituting each sandwich are partially occupied by M atoms. Some of the M atoms are linked in pairs through the rectangular face shared by two adjacent trigonal prisms. The remaining isolated M atoms form interfaces that separate the domains in which the M-M pair formation takes place. The repetition of the interfaces throughout the structure determines whether the modulation is commensurate or incommensurate with the basic lattice. The relative magnitude of the wave vector, y, is found to exactly match the relative A content x of the title formula. Superspace group theory is used to unify the crystal structures of the different Afi4xTe2 compounds. It is shown that the setting-up of an (in)commensurate array of interfaces is closely linked to the electronic stabilization of the compounds, more precisely to the charge transfer processes between Te pz orbitals and M d-block bands. Stacking (AA/BB or AA/BB/CC) and symmetry (orthorhombic or monoclinic) are the result of a subtle interplay between the charge transfer processes and the modulation wavelength.