We investigated double perovskite compounds of the form Sr$_2$XOsO$_6$ (X = Li, Na, Ca) using the fullpotential linearized augmented plane wave (FP-LAPW) method. For the exchange-correlation energy, Wu andCohen generalized gradient approximation (WC-GGA), Perdew, Burke and Ernzerhof GGA (PBE-GGA), Engel and Vosko GGA (EV-GGA), and GGA plus Hubbard U-parameter (GGA $+$ U) were used. The calculated structuralparameters are in good agreement with the existing experimental results. Calculation of different elastic constants and elastic moduli reveals that these compounds are elastically stable and possess ductile nature. The GGA $+$ Uapproach yields quite accurate results of the bandgap as compared with the simple GGA schemes. The density of states plot shows that Sr-4d, Os-5d and O-2p states predominantly contribute to the conduction and valence bands.Further, our results regarding to the magnetic properties of these compounds reveal their ferromagnetic nature. In addition, these compounds seem to possess half-metallic properties, making them useful candidates for applicationsin spintronics devices.

Abstract. The present investigation explores the significance of creeping viscous nanofluids in an axi-symmetric channel influenced by metachronal waves containing magnetohydrodynamics and Hall current. Heat transport analysis is also per-formed to derive the impact of thermal radiation on internal heat source phenomena. The use of mathematical formulation resulted in a set of nonlinear coupled partial differential equations. The governed differential system is transformed into an ordinary differential system by considering similar variables. Exact solutions in the closed form have been derived for the temperature, momentum and pressure gradient. Moreover, entropy generation due to heat transfer, thermal radiation and magnetic effects has been measured. The graphical results have been presented to interpret sundry parameters of interest. Streamlines and isotherms are also plotted against the multi-walled carbon nanotube. For the validation of our results, a comparison table is presented. It is also seen that entropy of the system increases and the Bejan number decreases with an increase in the Brinkman number.