MIXED LAYERS IN M(X)TA(1-X)SE(2) (M=RE, OS 0-LESS-THAN-X-LESS-THAN-1)

The layer stackings of the transition metal dichalcogenides were investigated in terms of the number of valence electrons per chemical formula. It is known that the isoelectronic compounds Re1/2Ta1/2Se2, Os1/3Ta2/3Se2, and WSe2 crystallize into the 2Hb-MoS2 structure, and TaSe2 crystallizes into the 2Ha-NbS2 structure or 1T-TaS2 structure, depending on the temperature. The compounds outside these critical valence electron counts crystallize into so-called mixed-layer structures. The mixed-layer structures were simulated by stacking 300 random layers of 2Hb layers, 3R layers and 1T layers. The randomness and the mixing ratio of the prototype layers depend on the number of valence electron per chemical formula. There exists a two-phase region between the 1T-TaSe2 phase and the mixed-layer phase. Hence there is no two-phase region between the mixed-layer phase and the M(x)Ta(1-x)Se(2) critical phase. Therefore the mixed-layer phases of these systems are equilibrium phases and the 2Hb structure is the special structure of the M(x)Ta(1-x)Se(2) mixed-layer structures. The Fermi level of the compounds is tuned by mixing the prototype layers to produce compounds with different numbers of valence electrons per chemical formula.