Electron Microscopic and X-Ray Structural Analysis of the Layered Crystals TaReSe4: Structure, Defect Structure, and Microstructure, Including Rotation Twins

A combined X-ray and electron microscopy analysis was used for structure and microstructure characterisation of the newly synthesized layered compound TaReSe4. This compound crystallizes in the hexagonal system with lattice parametersa=b=3.32130(4),c=12.95910(16) Å andα=β=90°,γ=120° (space groupP63/mmc,Z=1,Dx=9.161 g/cm3). X-ray powder structure refinement confirmed TaReSe4as a full structural analogue of 2H-WSe2. The defect structure of TaReSe4is characterized by the presence of a number of rotation twins with twist angles 9.43°, 13.17°, 17.89°, 21.78°, and 27.79°. Apart from these, a high density of 2D-defects was observed in the (0001) plane. In some cases these results in the formation of an almost periodical “superlattice” with a period along thecaxis of about 60 Å. A simple geometrical theory for the rotations- (R-) twins on the hexagonal lattice has been developed as well. Experimental results on R-twins fit very well the predicted ones based on an explicit formulae derived in this work. Along with the main phase a small admixture of a superstructure phase (