Next-generation materials: Comprehensive physical insights into transition metal diborides XB2 (X = Ti, Zr, Hf) via first-principles study for applications in extreme conditions

Transition metal diborides XB₂ (X = Ti, Zr, Hf) have emerged as promising candidates for next-generation ultra-high temperature ceramics (UHTCs) due to their exceptional thermal stability, mechanical hardness, electrical conductivity, and oxidation resistance. In this study, a comprehensive first-principles investigation based on density functional theory (DFT) is conducted to explore the fundamental physical properties of TiB₂, ZrB₂, and HfB₂, targeting their potential application under extreme environments. Most of the properties investigated in this comprehensive study are novel, aiming to provide a comparative understanding among the XB₂ materials. The structural, mechanical, electronic, thermodynamic, elastic, thermophysical, and vibrational properties of these compounds are systematically analyzed. The computed lattice parameters of hexagonal XB₂ exhibit strong consistency with previously reported experimental and theoretical results. All compounds exhibit mechanical and dynamical stability with strong covalent-metallic bonding characteristics. The phonon dispersion and density of states confirm vibrational stability and provide insight into lattice dynamics. Electronic band structures and density of states reveal metallic conductivity (E_g = 0 eV) and non-magnetic ground states. The thermodynamic properties, including Debye temperature, heat capacity, free energy, and melting temperature, suggest robust high-temperature performance. The XB₂ metal diborides exhibit elastic anisotropy and mechanical brittleness, with their hardness following the order: TiB₂ > ZrB₂ > HfB₂. Among them, TiB₂ demonstrates the highest melting point at 3025 K, while ZrB₂ and HfB₂ show slightly lower melting temperatures of 2669 K and 2759 K, respectively. The essential thermodynamic and thermophysical properties of hexagonal XB₂ compounds have been thoroughly investigated for the first time. Remarkably, all three materials demonstrate exceptionally high lattice thermal conductivities at 300 K, measured as 453 Wm⁻¹ K⁻¹ for TiB₂, 302 Wm⁻¹ K⁻¹ for ZrB₂, and 189 Wm⁻¹ K⁻¹ for HfB₂, indicating their strong potential for high-temperature heat management applications. This study highlights the intrinsic superiority of these diborides and their tunable physical responses, affirming their applicability in aerospace, nuclear reactors, and thermal protection systems. Overall, this comprehensive investigation predicts the suitability of XB₂ (X = Ti, Zr, Hf) compounds in high-temperature environments and inspires further experimental validation for practical utilizations.