[Paper Review] Rhodium based half-Heusler alloys as possible optoelectronic and thermoelectric materials

Dhurba Raj Jaishi, Sujit Bati+4 more|arXiv (Cornell University)|May 5, 2021

Heusler alloys: electronic and magnetic properties4 citations

TL;DR

This study investigates rhodium-based half-Heusler alloys (RhTiP, RhTiAs, RhTiSb, RhTiBi) using density functional theory and Boltzmann transport calculations, revealing their structural stability, semiconducting behavior with indirect band gaps of 0.94–1.01 eV, strong optical absorption in the UV–visible range, and promising thermoelectric performance. The highest ZT value of ~1.0 under p-type doping in RhTiBi positions it as a top candidate for thermoelectric applications among the studied materials.

ABSTRACT

On the basis of density functional theory and semi-classical Boltzmann theory, we have investigated the structural, elastic, electronic, optical and thermoelectric properties of 18--valence electron count rhodium based half-Heusler alloys focusing on RhTiP, RhTiAs, RhTiSb, and RhTiBi. The absence of imaginary frequencies in the phonon dispersion curve for these system verifies that they are structurally stable. RhTiP is ductile in nature, while others are brittle. The alloys are found to be semiconducting with indirect band gaps ranging from 0.94 to 1.01 eV. Our calculations suggest these materials to have high absorption coefficient and optical conductivity in the ultraviolet as well as visible region. While considering thermoelectricity, we found that $p$--type doping is more favorable in improving the thermoelectric properties. The calculated values of power factor with $p$-type doping are comparable to some of the reported half-Heusler materials. The optimum figure of merit \\zt\\ is $\\sim1$ for RhTiBi suggesting it as a promising candidate for thermoelectric applications while RhTiP, RhTiAs, and RhTiSb with optimum \\zt \\ values between 0.38 to 0.67 are possible candidates for use in thermoelectric devices.

Motivation & Objective

• To evaluate the structural, electronic, optical, and thermoelectric properties of RhTiZ (Z = P, As, Sb, Bi) half-Heusler alloys using first-principles calculations.
• To identify promising candidates for optoelectronic and thermoelectric applications among rhodium-based half-Heusler compounds.
• To assess the impact of p-type and n-type doping on power factor and ZT values, particularly at high temperatures.
• To determine the role of lattice thermal conductivity and carrier concentration in optimizing thermoelectric efficiency.
• To explore the potential of these materials as low-cost, non-toxic alternatives to conventional thermoelectrics.

Proposed method

• Density functional theory (DFT) with the PBE functional was used to calculate electronic structure, band gaps, and phonon dispersion for structural stability.
• Phonon dispersion calculations were performed to confirm dynamic stability, with absence of imaginary frequencies indicating stability.
• Elastic properties were analyzed using Pugh’s ratio to classify ductility or brittleness.
• Optical properties such as absorption coefficient and optical conductivity were computed from the dielectric function.
• The semi-classical Boltzmann transport theory was applied to calculate electrical conductivity, Seebeck coefficient, power factor, and thermal conductivity.
• ZT values were calculated using the formula ZT = (σα²T)/κ, with κ split into lattice (κₗ) and electronic (κₑ) contributions, and optimized for various doping levels.

Experimental results

Research questions

• RQ1 Are RhTiP, RhTiAs, RhTiSb, and RhTiBi half-Heusler alloys dynamically stable and structurally robust?
• RQ2 Do these materials exhibit favorable electronic band structures for optoelectronic and thermoelectric applications?
• RQ3 What is the optimal doping type (p-type or n-type) and concentration for maximizing the power factor and ZT in these alloys?
• RQ4 How does lattice thermal conductivity vary across the series, and what impact does it have on ZT performance?
• RQ5 Can these Rh-based half-Heusler compounds serve as viable, non-toxic, and cost-effective alternatives to state-of-the-art thermoelectric materials?

Key findings

• All RhTiZ compounds are dynamically stable, with no imaginary phonon frequencies, confirming structural stability.
• RhTiP is ductile, while RhTiAs, RhTiSb, and RhTiBi are brittle, based on Pugh’s ratio analysis.
• The materials are semiconducting with indirect band gaps ranging from 0.94 eV (RhTiP) to 1.01 eV (RhTiBi).
• High absorption coefficients and optical conductivities in the UV and visible regions indicate strong potential for optoelectronic applications.
• p-type doping yields significantly higher power factors than n-type doping, with optimal doping levels of 0.27–0.30 holes per unit cell.
• The highest ZT value of 0.98 is achieved for p-type RhTiBi at 1200 K, with ZT values of 0.44–0.67 for other p-type doped systems, indicating strong thermoelectric potential.