Exploring the structural, elastic, phonon, optoelectronics, and thermoelectric properties of tetragonal complex metal hydride X2MgH4 (X=K, Rb, and Cs) compounds for hydrogen storage applications

Hydrogen storage is of great interest to researchers due to its renewable and clean energy properties. Therefore, there is a continuous search for innovative compounds that can provide hydrogen storage conditions. A first-principles study of X2MgH4 (X = K, Rb, and Cs) compounds has been carried out to investigate their physical properties using the Generalized Gradient Approximation (GGA) with the SIESTA package. The elastic, optical, structural, electronic, phonon, and thermoelectric properties of X2MgH4 (X = K, Rb, and Cs) compounds with tetragonal crystal structures have been investigated. The electronic properties of X2MgH4 (X = K, Rb, and Cs) compounds are insulators. All three compounds fulfil the Born stability condition. The gravimetric hydrogen storage capacities of X2MgH4 (X = K, Rb, and Cs) compounds are 3.8, 2.02, and 1.37 wt%, respectively. X2MgH4 (X = K, Rb, and Cs) crystals are dynamically stable. The thermoelectric and optical properties of X2MgH4 (X = K, Rb, and Cs) compounds are also discussed.