Ab initio study of mechanical and functional properties of novel CaZnC and CaZnSi half-Heusler materials

TL;DR

CaZnC and CaZnSi half-Heuslers are investigated as dual photovoltaic and thermoelectric materials using ab initio FP-LAPW+lo DFT with TB-mBJ bandgap corrections and BoltzTraP transport calculations. The study reports a direct bandgap of $\approx$1.186 eV for CaZnC and an indirect gap of $\approx$1.067 eV for CaZnSi, along with favorable optical responses and near-unity $ZT$ values in certain regions at room temperature, supported by lattice-dominated low thermal conductivity from Slack's model. Elastic analyses indicate mechanical stability but brittleness with covalent bonding and notable elastic anisotropy, complemented by quasi-harmonic Debye thermodynamics showing temperature- and pressure-dependent trends in $B$, $\theta_D$, $\alpha$, $S$, $C_p$, and $C_v$. The results underscore the potential of CaZnC and CaZnSi for renewable-energy devices and offer a detailed fundamental understanding to guide experimental synthesis and assessment of these Ca-based HH materials. Overall, the work provides a comprehensive framework linking structure, electronic, optical, TE, mechanical, and thermodynamic properties to practical energy-conversion applications.

Abstract

This research work introduces the DFT through FP-LAPW+lo technique in WIEN2k software to obtain information about structural, thermoelectric, and optoelectronic characteristics of CaZnC and CaZnSi materials. The structural optimization was performed using PBE-GGA functional, while the rest of the characteristics were obtained with the PBE-GGA + TB-mBJ approach. The thermoelectric parameters were evaluated using BoltzTraP software. The elastic constants and other mechanical parameters were computed by utilizing the ELAST code within the WIEN2k software, while the thermodynamic characteristics were evaluated using the Gibbs2 program. The findings show a correlation between atomic composition and lattice dimensions while finding that CaZnC has a direct ($Γ$-$Γ$) band gap of $1.186$ eV, whereas CaZnSi has an indirect ($Γ$-$X$) band gap of $1.067$ eV. The optical studies of the compounds show potential applications for photovoltaics while the thermoelectric results find optimized power factors and figure of merit values for energy conversion performance. The elastic parameters of CaZnC and CaZnSi demonstrate material stability and brittleness. Lastly, the thermodynamic evaluations provide information about the thermal mechanism and disorder of the materials. As a result, this research work provides significant advancements in the understanding of the fundamentals of these compounds and highlights their promising applications in renewable energy technologies.

Figures (21)

• Figure 1: (Colour online) Unit cell and structure of CaZnX ($X =$ C and Si) HH compounds.
• Figure 2: (Colour online) Volume optimization plot for the HH compounds (a) CaZnC and (b) CaZnSi.
• Figure 3: (Colour online) Bandstructure curves of (a) CaZnC and (b) CaZnSi HH materials, calculated using the TB-mBJ potential.
• Figure 4: (Colour online) Total and partial DoS plots of (a) CaZnC and (b) CaZnSi HH materials.
• Figure 5: (Colour online) Change in (a) $\varepsilon_1(\omega)$ and (b) $\varepsilon_2(\omega)$ of CaZnC and CaZnSi HH materials with incident radiation energy.