STRUCTURE IN DICHALCOGENIDES AT HIGH PRESSURE

The structural and vibrational properties of TiS₂ with the CdI₂ structure have been studied to high pressures from density functional calculations within the local density approximation (LDA). The calculated axial compressibility of the CdI₂-type phase agrees well with experimental data and is typical of layered transition-metal dichalcogenides. The calculated phonon frequencies also show good correspondence with available experiments. Our LDA study predicts that the CdI₂-type TiS₂, the phase stable at ambient conditions, should transform to the cotunnite phase at 15.1 GPa, in good agreement with the results of Yu et al. [1]. However, we also predict a further transition from the cottunite phase to a tetragonal phase (I 4/mmm) at 45.0 GPa. The tetragonal phase has a structure similar to that of MoSi₂ and consists of layers of NiS₈ tetragonal prisms which share faces within the layers. The possible existence of the MoSi₂ phase at high pressures is consistent with our previous findings in NiS₂ [2]. In addition, recent simulations on ZrSe₂, which also crystallizes with the CdI₂ structure under ambient conditions, suggest that there is a direct transformation from the CdI₂ structure to the MoSi₂ phase at pressures less than 10 GPa.

[1] Yu F, Sun J-X, Zhou Y-H (2010) Solid State Sciences 12: 1786-1790; [2] Yu Y and Ross NL (2010) J. Phys: Cond. Matter 22:235401.