[Paper Review] Theoretical investigations of superconducting MAX phases Ti2InX (X = C, N)

M. Roknuzzaman, A.K.M.A. Islam|arXiv (Cornell University)|Jun 20, 2012

MXene and MAX Phase Materials1 references6 citations

TL;DR

This study employs density functional theory (DFT) to investigate the structural, electronic, mechanical, thermal, and optical properties of superconducting MAX phases Ti2InC and Ti2InN. It reveals that both compounds are moderately coupled superconductors with high reflectivity in the UV-IR range, and predicts thermodynamic properties via the quasi-harmonic Debye model, showing good agreement with experimental data for Ti2InC's thermal expansion coefficient.

ABSTRACT

The structural, elastic, electronic, thermal and optical properties of superconducting MAX phases Ti2InX (X = C, N) are investigated by density functional theory (DFT). The results obtained from the least studied nitride phase are discussed in comparison with those of carbide phase having Tc-value half as that of the former. The band structure and density of states show that these phases are conductors, with contribution predominantly from the Ti 3d states. The bulk modulus, Debye temperature, specific heats, thermal expansion coefficient are all obtained as a function of temperature and pressure for the first time through the quasi-harmonic Debye model with phononic effects. Ti2InC and Ti2InN are indicated to be moderately coupled superconductors. The thermal expansion coefficients for both the phases are calculated, and the calculation is in fair agreement with the only available measured value for Ti2InC. Further the first time calculated optical functions reveal that the reflectivity is high in the IR-visible-UV region up to ~ 10 eV and 12.8 eV for Ti2InC and Ti2InN, respectively showing these to be promising coating materials. Keywords: Ti2InX superconductors; First-principles; Mechanical properties; Band structure; Thermodynamic properties; Optical properties

Motivation & Objective

• To investigate the structural and electronic properties of superconducting MAX phases Ti2InC and Ti2InN.
• To evaluate their mechanical and thermodynamic properties under varying temperature and pressure.
• To analyze optical response and assess potential for coating applications.
• To compare the superconducting transition temperatures and coupling strengths between the carbide and nitride phases.
• To provide first-time theoretical predictions of thermal expansion, specific heat, and Debye temperature using the quasi-harmonic Debye model.

Proposed method

• Employed density functional theory (DFT) with the generalized gradient approximation (GGA) for electronic structure calculations.
• Used the quasi-harmonic Debye model to compute temperature- and pressure-dependent thermodynamic properties, including bulk modulus, specific heat, and thermal expansion.
• Calculated electronic band structure and density of states to analyze metallic character and orbital contributions.
• Computed optical functions, including reflectivity, to evaluate potential for optical coating applications.
• Applied the McMillan formula with electron-phonon coupling to estimate superconducting transition temperatures.
• Validated theoretical predictions against available experimental data, particularly for Ti2InC's thermal expansion coefficient.

Experimental results

Research questions

• RQ1 How do the electronic band structures and density of states of Ti2InC and Ti2InN compare, and what is the dominant orbital contribution to conductivity?
• RQ2 What are the temperature- and pressure-dependent thermodynamic properties of Ti2InC and Ti2InN, and how do they compare to experimental measurements?
• RQ3 What is the predicted superconducting transition temperature and coupling strength for Ti2InC and Ti2InN, and how do they differ?
• RQ4 How do the optical reflectivity and dielectric functions of these MAX phases vary across the UV-IR spectrum, and what does this imply for their use as coatings?
• RQ5 To what extent do the theoretical predictions of thermal expansion and specific heat align with the only available experimental data for Ti2InC?

Key findings

• Ti2InC and Ti2InN exhibit metallic behavior with dominant contributions to the density of states from Ti 3d orbitals.
• The calculated thermal expansion coefficient for Ti2InC shows fair agreement with the only available experimental measurement.
• Both phases are predicted to be moderately coupled superconductors, with Ti2InN having a higher Tc than Ti2InC.
• Reflectivity is high in the UV-IR region, reaching ~10 eV for Ti2InC and ~12.8 eV for Ti2InN, indicating strong potential for optical coating applications.
• The quasi-harmonic Debye model successfully predicts temperature- and pressure-dependent bulk modulus, specific heat, and Debye temperature for both phases.
• The study provides the first theoretical estimates of specific heat and thermal expansion coefficient for Ti2InN, and refined predictions for Ti2InC.