On the electronic nature of silicon and germanium based oxynitrides and their related mechanical, optical and vibrational properties as obtained from DFT and DFPT

► Oxynitrides electronic structure is provided using FP-(L)APW+lo method. ► The present calculation shows a good agreement with experiments and theoretical data. ► PW-PP and DFPT methods are employed to calculate the elastic properties and phonons. ► Si2N2O and Ge2N2O compounds are mechanically and dynamically stables. ► Elastic constants are reporte, the thermodynamic properties are deduced from phonons.