THERMODYNAMIC PROPERTIES OF TUNGSTEN DISULFIDE FROM FIRST PRINCIPLES IN QUASI-HARMONIC APPROXIMATION

Abstract

The standard values (T = 298.15 K) of thermodynamic properties of hexagonal tungsten disulfide 2H-WS₂ were calculated from first principles using the Density Functional Theory and quasi-harmonic approximation with the finite displacements in supercells. Local density approximation was chosen as the exchange-correlation functional and the calculation was performed without van der Waals corrections. The calculated data are in good agreement, except for the enthalpy of formation, with reliable experimental results, eliminating the discrepancies between the experimental results from different sources and increasing the overall reliability of thermodynamic data for tungsten disulfide. Additionally, the data agreement suggests that more attention should be paid to experimental studies of the phonon spectrum of 2H-WS₂, particularly in the Γ → Κ region of the low-energy fraction of the dispersion, where the deviation between the calculation and inelastic neutron scattering is significant. The values of the heat capacity, entropy, and enthalpy of molybdenum disulfide are recommended for incorporation into thermodynamic databases and practical application and the parameters used to calculate these quantities are recommended as initial parameters in first-principles studies of the 2H-WS₂ properties related to the vibrational spectrum. The calculated value of Δ_fH°(2H-WS₂, 298.15 K) = –275 ± 0.5 kJ ⋅ mole^–1 is about 10% below the median of the existing literature data and about 15 % below the experimental result accepted as the most reliable. To improve the result, the calculation requires additional attention, for example, by the application of van der Waals corrections or hybrid approximations to the exchange-correlation functional.