First-principles insights into the multifunctional properties of the nitridogermanate Sr4GeN4

Abstract

We present a detailed investigation of the ternary nitride Sr₄GeN₄, combining first-principles calculations with thermodynamic analysis using the quasi-harmonic Debye (QHD) model. Thermodynamic, dynamic, and mechanical stability are demonstrated by the large negative formation and cohesive energies, the absence of imaginary phonon modes, and elastic constants satisfying the Born criteria. The compound shows pronounced elastic anisotropy, moderate compressibility (B ≈ 71 GPa), and ductile behavior (B/G ≈ 2.02). QHD calculations at 300 K and 0 GPa yield a Debye temperature of 318 K, near-saturation heat capacities (C_V = 212.36, C_P = 225.23 J mol⁻¹ K⁻¹), and modest anharmonicity, with phase stability retained up to 800 K. Electronic structure results identify Sr₄GeN₄ as an indirect band-gap semiconductor (E_g = 1.89 eV) exhibiting highly anisotropic charge transport, with very light electrons (0.030–0.037 m₀) and heavier holes (0.222–0.38 m₀), favoring n-type conductivity. The bonding has mixed ionic–covalent character, and the optical response displays strong anisotropy with pronounced UV absorption and a deep-UV plasmon. Band-edge alignment shows a conduction band minimum at −0.886 V and a valence band maximum at +1.00 V vs. Normal Hydrogen Electrode, yielding a 0.226 V shortfall for overall water splitting and indicating the need for band engineering or heterojunction design. These results establish Sr₄GeN₄ as a thermally robust, anisotropic semiconductor with strong prospects for n-type optoelectronics and ultraviolet photonics.