[Paper Review] Ab initio study of structural, electronic, and thermal properties of Ir$_{1-x}$Rh$_{x}$ alloys

S. Ahmed, Muhammad Zafar+2 more|arXiv (Cornell University)|Jun 12, 2015

Advanced Physical and Chemical Molecular Interactions2 references3 citations

TL;DR

This study presents an ab initio investigation of Ir₁₋ₓRhₓ alloys using density functional theory (DFT) and the quasiharmonic approximation to explore structural, electronic, and thermal properties across varying Rh concentrations (x = 0.00–1.00). The key finding is that Ir₀.₅Rh₀.₅ exhibits maximum lattice mismatch, leading to enhanced entropy and specific heat, but reduced Debye temperature and vibration energy, indicating unique thermodynamic behavior at 50 at.% Rh due to strong disorder effects.

ABSTRACT

The structural, electronic, mechanical and thermal properties of Ir$_{1-x}$Rh$_{x}$ alloys were studied systematically using ab initio density functional theory at different concentrations (x = 0.00, 0.25, 0.50, 0.75, 1.00). A Special Quasirandom Structure method was used to make alloys having FCC structure with four atoms per unit cell. The ground state properties such as lattice constant and bulk modulus were calculated to find the equilibrium atomic position for stable alloys. The calculated ground state properties are in good agreement with the experimental and previously presented other theoretical data. The electronic band structure and density of states were calculated to study the electronic properties for these alloys at different concentrations. The electronic properties substantiate the metallic behavior of alloys. The first principle density functional perturbation theory as implemented in quasiharmonic approximation was used for the calculation of thermal properties. We have calculated the thermal properties such as Debye temperatures, vibration energy, entropy, constant-volume specific heat and internal energy. The ab initio linear-response method was used to calculate phonon densities of states.

Motivation & Objective

• To systematically investigate the structural, electronic, and thermal properties of Ir₁₋ₓRhₓ alloys across varying Rh concentrations.
• To model disordered FCC alloys using the Special Quasirandom Structure (SQS) method for accurate DFT simulations.
• To predict thermal properties such as Debye temperature, specific heat, entropy, and internal energy using the quasiharmonic approximation.
• To analyze electronic behavior via band structure and density of states, confirming metallic character across all compositions.
• To provide the first theoretical predictions for Ir–Rh alloy properties, as no experimental or prior theoretical data exist for these systems.

Proposed method

• Ab initio calculations based on density functional theory (DFT) with the ultrasoft pseudopotential and local density approximation (LDA).
• Special Quasirandom Structure (SQS) method applied to model disordered FCC alloys with four atoms per unit cell at different Rh concentrations (x = 0.00, 0.25, 0.50, 0.75, 1.00).
• First-principles linear-response method used to calculate phonon density of states and vibrational contributions.
• Quasiharmonic approximation (QHA) employed to compute temperature-dependent thermal properties, including Helmholtz free energy, internal energy, constant-volume specific heat, entropy, and Debye temperature.
• Ground state properties such as lattice constant and bulk modulus were determined via total energy minimization to find equilibrium atomic positions.
• Electronic band structure and density of states (DOS) were calculated to analyze electronic behavior and metallic character.

Experimental results

Research questions

• RQ1 How do lattice constant and bulk modulus of Ir₁₋ₓRhₓ alloys vary with Rh concentration, and how do they deviate from Vegard’s law?
• RQ2 What is the nature of electronic band structure and Fermi level behavior in Ir₁₋ₓRhₓ alloys, and how does Rh doping affect electronic conductivity?
• RQ3 How do thermal properties such as Debye temperature, specific heat, entropy, and internal energy evolve with temperature and Rh concentration?
• RQ4 Why does Ir₀.₅Rh₀.₅ exhibit anomalous behavior in thermal and vibrational properties compared to other compositions?
• RQ5 To what extent does lattice mismatch at x = 0.50 influence the thermodynamic and electronic response of the alloy?

Key findings

• The lattice constant and bulk modulus of Ir₁₋ₓRhₓ alloys decrease with increasing Rh concentration, showing slight deviations from Vegard’s law.
• Electronic band structures show overlapping states at the Fermi level, with increasing band overlap and decreasing Fermi energy as Rh concentration increases, indicating enhanced d-band hybridization and metallic conductivity.
• The phonon density of states reaches a minimum at x = 0.50, indicating maximum lattice disorder and anharmonicity at this composition.
• Debye temperature peaks at low temperatures (0–20 K) and decreases rapidly before stabilizing at higher temperatures, with the lowest value observed at x = 0.50.
• Entropy and constant-volume specific heat increase with temperature and reach saturation, showing a maximum at x = 0.50 due to lattice mismatch, despite decreasing values at higher Rh concentrations.
• Vibration energy remains nearly constant at low temperatures and decreases with increasing temperature; it is lowest at x = 0.50, while internal energy shows a slight increase with Rh concentration but a dip at x = 0.50.