Optimization of heat capacities of wurtzite phases as a single system and thermodynamic properties of nihonium nitride

Abstract

In this work, two independent methods were used to describe the heat capacity of reference compounds with the wurtzite structure, as well as their isostructural analogs of fourth group elements in the solid state: 1) a semi-empirical method using a multiparameter function and 2) a method based on the density functional theory. In the first method, to describe a set of isostructural experimental data C_p(T) of substances in the solid state, a description of each individual phase was carried out in the first approximation, and then, after finding a set of its fitting parameters (X), a description of the entire set of data as an integral system, where the set of each individual fitting parameter was described by a polynomial equation of functions included in the multiparameter family with the best description of the set of all data. The parameters of unexplored substances were found by interpolation of the multiparameter function. The fitting parameters (X) are a function of the atomic number of the elements Si, Ge, Sn, Pb, and Fl or the half-sum of the atomic numbers of the binary phases A^IIIB^V: B, Al, Ga, Tl, and Nh with the element germanium having a characteristic point for the set of parameters (X) depending on the atomic number. For each substance, the parameters are found by minimizing the discrepancy between the theoretical dependence C_p(T) and the corresponding experimental data. According to the fine structure constant (α = 1/137) (or Sommerfeld constant), there are no other elements in this group. Therefore, the limiting heat capacity of the wurtzite phases is at element 114 (¹¹⁴Fl) and has a value of C_p = 30.5 ± 0.3 J · mol-at⁻¹ · K⁻¹.

The second method allows us to describe the heat capacities (C_p) for these materials using a new first-principles method based on the density functional theory. This method, called the Beyond Quasi-Harmonic method, includes all anharmonic vibrations - volume changes and phonon-phonon interactions. Our calculated values for III-V compounds with wurtzite and sphalerite structures show good agreement between the two methods.

This work also presents optimized thermodynamic properties, Gibb's energy, enthalpy and entropy of formations, as well as standard entropy and melting point of III-V nitrides, including nihonium nitride.