Relations between anisotropic defects, structural evolution, and van der Waals bonding in 2H-NbSe2

Correlations between anisotropic defects and van der Waals interactions have been established for the layer compound 2H-NbSe2 which is investigated by low temperature X-ray diffraction techniques. Thermal expansion coefficients and anisotropic Debye temperatures are determined. A diffraction profile analysis reveals the existence of lattice distortions independent of the temperature. They are due to layer defects. To interpret the structural evolution data, the thermal expansion functions, αa(T) and αc(T) are simulated in the low temperature range which yield the elastic constants and the Grueneisen parameters. Using bond energy models, the Van der Waals nature of interlayer Se-Se interactions is confirmed by a model of thermal expansion of bonds and connected with the C13 component of the elastic tensor. Such interactions can explain the presence of some layer defects that can be 4H-NbSe2 nuclei in the 2H host lattice. In addition, no strong change in the Grueneisen parameters is clearly shown to occur at the 35 K transition of 2H-NbSe2. (author)