HfP2 monolayer: A material with low lattice thermal conductivity and excellent thermoelectric performance

Density functional theory based first principle calculations are performed to investigate the properties of ${\text{HfP}}_2$ monolayer having hexagonal lattice structure (lattice constant = 3.98 Å). The investigated results show that it is an indirect band gap semiconductor with the energy band gap of 1.26 eV. The projected density of states calculation demonstrates that the band formation in ${\text{HfP}}_2$ monolayer is mainly contributed by the ‘d’ and ‘p’ orbital of Hf and P atoms respectively. The non existence of imaginary curves in the phonon spectra, calculated cohesive energy and elastic constants confirmed that ${\text{HfP}}_2$ monolayer is dynamically, energetically and mechanically stable. In order to assess the thermoelectric performance of ${\text{HfP}}_2$ monolayer, the electronic transport coefficients and lattice thermal conductivity has been calculated. It is found to possess a large Gruneisen parameter and low phonon group velocity, hence a low lattice thermal conductivity of 0.12 Wm⁻¹K⁻¹ at room temperature. At room temperature, the n-type (p-type) monolayer is found to have a ZT value of 1.81 (1.34). The obtained results show that ${\text{HfP}}_2$ monolayer could be used as a thermoelectric material.