Half-metallic ferromagnetism and thermoelectric properties of double perovskites Rb2Z(Cl/Br)6 (Z = Ta, W, Re)

•Above room temperature ferromagnetism, and spintronic applications.•Quantum spin of electrons dominant the charge with multifunctional properties.•Exchange mechanism certifies the ferromagnetism by electron spin instead of magnetic ion clustering.•Half metallic ferromagnetism and 100% spin polarization.•Large value of electrical conductivity and ultralow low values of thermal conductivity. The charge and spin degrees of freedom of electrons employed together introduce a new field of electronics, known as spintronics, which has multifunctional applications in data storage, control, and transfer in supercomputing. In this perspective, we have comprehensively investigated cubic double perovskites Rb2Z(Cl/Br)6 (Z = Ta, W, Re). More precisely, we explored their structural, electronic, magnetic, and thermal transports. The structural stability is addressed by a tolerance factor, enthalpy of formation, and Curie temperature calculations. Half metallic ferromagnetism (HMF), quantum exchange process, spin polarization, and exchange energies are explained in terms of hybridization and double exchange mechanism. Furthermore, spin-polarized thermal transports are revealed by investigating thermal conductivity, Seebeck coefficient, and power factor.