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In order to search for good hydrogen storage materials, the structural, optoelectronic and hydrogen storage properties of double perovskites KNaMg 2 F 6-x H x and KNaAe 2 H 6 (Ae = Be, Mg, Ca) have been studied by first-principles calculations based on density functional theory in this paper. All the negative formation energies demonstrate the thermodynamic stability of these materials. Except for KNaBe 2 H 6 , the rest of the materials are wide-band gap semiconductors or insulators. The gravimetric storage capacity of KNaMg 2 F 6-x H x increases with the introduction of hydrogen and reaches a maximum of 5.19 wt% at x = 6. Meanwhile, the KNaBe 2 H 6 exhibits the best storage capacity of 8.57%. The favorable desorption temperature for stable KNaMg 2 H 6 material is 470.4 K, which is feasible in actual application. As the concentration of hydrogen increases, the electronic bandgap of KNaMg 2 F 6-x H x decreases gradually due to its electron conduction band moving to a lower energy range. The bond population analysis indicates that there is a mixture of ionic and covalent bonds in the studied materials. These materials are all ultraviolet absorber, and a red-shift can be observed in the absorption edge due to the variation of bandgap. • Double perovskites are introduced as the novel hydrogen storage materials. • Hydrogen substitution improves the stability and optoelectronic performance of double perovskites. • All materials exhibit wide band gaps as well as high ultraviolet absorption. • Stable KNaMg 2 H 6 material possesses ideal desorption temperature of 470.4 K and gravimetric storage capacity of 5.19 wt%. [ABSTRACT FROM AUTHOR]

The physical properties of three potential colbat-containing perovskites are studied through first principles calculation. The structural analysis reveals that the Cs 2 InCoI 6 and Cs 2 AgCoI 6 are ductile and potential in flexible devices. The negative formation energies indicate that all these perovskite syntheses are practicable in experiment. However, the results of tolerance factors and phonon spectra are presented against the structural dynamic stability of pure cubic phase of Cs 2 TlCoI 6 and Cs 2 AgCoI 6 and there may be lattice distortion. The band gaps of Cs 2 InCoI 6 , Cs 2 TlCoI 6 and Cs 2 AgCoI 6 are 0.833 eV, 1.713 eV and 0.783 eV, respectively, all values are within the ideal range for semiconductors that applied in optoelectronic devices. They all exhibit broad-spectrum light absorption in ultraviolet and visible region, and the absorption coefficient of Cs 2 InCoI 6 even maintains a value above 10000 cm−1 at wavelength of 1400 nm. Among them, Cs 2 InCoI 6 perovskite has direct band structure, smaller carrier effective mass than that of CsPbI 3 and broad solar absorption characteristics, which is considered as a promising candidate material for optoelectronic devices. • The physical properties of three novel cobalt perovskites are studied. • Cs 2 TlCoI 6 is found to be stable in tetragonal rather than cubic phase thus a phase transition may take place. • Cubic Cs 2 InCoI 6 is found to be a stable perovskite with broadband light absorption. • Pressure-induced optical properties and structural stabilities of Cs2InCoI6 are studied. [ABSTRACT FROM AUTHOR]

[Display omitted] • The materials are stable double perovskites by tolerance factor and formation energy. • The studied materials are all direct and quasi-direct semiconductors. • Tl 2 PdBr 6 is regarded as a potential candidate for single junction solar absorber. The structural stability, energy band and photoelectric properties of vacancy- ordered double halide perovskites A 2 BX 6 (A = In, Tl; B = Pd, Pt; X = Cl, Br, I) are studied in detail by first principles in this paper. The results show that these materials have suitable tolerance factor, negative formation energy and good phonon spectrum, which indicates that they have structural stability. These materials are all semiconductors with band gaps varying from 0.328 eV and 3.238 eV via HSE06 hybrid functional. All the materials have direct or quasi-direct band structures, which are beneficial for carrier generation. These materials have small reflectance and large absorption coefficients for visible - infrared light, especially in the ultraviolet region where the absorption coefficient is very large. The absorption coefficient of the material is obviously improved when the anion X changes from Cl to Br to I. Making a comprehensive analysis of absorption coefficient, transmittance and bandgap values, Tl 2 PdBr 6 is regarded as a potential candidate for solar absorber in single junction solar cells, while In 2 PtCl 6 and Tl 2 PtCl 6 may be applied in transparent conductor films. [ABSTRACT FROM AUTHOR]