Semiconducting ferromagnetism and thermoelectric performance of Rb2GeMI6 (M = V, Ni, Mn): a computational perspective

Abstract

We present a comprehensive first-principles investigation into the structural, electronic, magnetic, and transport properties of halide double perovskites Rb₂GeMI₆ (M = V, Mn, Ni) utilizing density functional theory (DFT). Structural stability is rigorously validated through geometric optimization, mechanical stability criteria, and tolerance factor analysis, confirming the feasibility of these compounds in the cubic Fmm phase. Total energy calculations based on the Birch–Murnaghan equation of state establish a robust ferromagnetic ground state, further corroborated by positive Curie–Weiss constants of 98 K (Rb₂GeVI₆), 90 K (Rb₂GeMnI₆), and 95 K (Rb₂GeNiI₆), underscoring their intrinsic ferromagnetic behavior. Electronic structure analyses performed using both the generalized gradient approximation (GGA) and the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential reveal that these materials exhibit semiconducting ferromagnetism, characterized by a significant spin-splitting gap. The underlying mechanism is traced to the crystal field effects influencing the d-orbitals of the transition metal atoms. Magnetic moment calculations indicate values of 3μ_B, 5μ_B, and 2μ_B for the V-, Mn-, and Ni-based compounds, respectively, underscoring the pivotal role of transition metals in governing their magnetic properties. Furthermore, Curie temperature estimations of 530.39 K (Rb₂GeVI₆), 580.72 K (Rb₂GeMnI₆), and 440.47 K (Rb₂GeNiI₆) significantly exceed room temperature, reinforcing their potential for spintronic applications. A rigorous thermodynamic analysis, incorporating vibrational contributions to internal energy, Helmholtz free energy, entropy, and specific heat, confirms the stability of these materials across a broad temperature range. Finally, an in-depth investigation of transport properties, considering both temperature and chemical potential dependence of the Seebeck coefficient, electrical conductivity, and figure of merit (zT), highlights their exceptional thermoelectric potential. Notably, the materials exhibit remarkably low thermal conductivities of 3.10 W m⁻¹ K⁻¹ (Rb₂GeVI₆), 2.05 W m⁻¹ K⁻¹ (Rb₂GeMnI₆), and 1.57 W m⁻¹ K⁻¹ (Rb₂GeNiI₆), translating into impressive zT values of 1.00, 0.99, and 0.97 at room temperature. Overall, this study demonstrates that Rb₂GeMI₆ halide perovskites exhibit a unique synergy of structural stability, ferromagnetic semiconducting behavior, and high thermoelectric efficiency, positioning them as promising candidates for next-generation spintronic devices, thermoelectric energy harvesting, and sustainable energy technologies.