Density functional study of CoFeCrZ (Z = Al, Si, Ga, Ge) quaternary Heusler alloys for phonon spectra

Density Functional Theory (DFT) is used to investigate the phonon properties of CoFe-CrZ (Z = Al, Si, Ga, Ge) equiatomic Quaternary Heusler Alloys. These alloys crystallize in face centered cubic (FCC) structure and have three crystal structures Y-Type I, Y-Type II and Y-Type III on the basis of their atomic positions. For CoFeCrZ (Z = Al, Si, Ga, Ge), Y-Type I is the most stable structure found in the literature, so phonon dispersion curves for this structure are obtained with the help of norm-conserving pseudo potentials in Quantum ESPRESSO. Absence of negative frequencies in phonon dispersion curves proves the dynamical stability of all these alloys. Phonon dispersion curves are further used to obtain Reststrahlen band, a region where light reflects 100%. The calculated Reststrahlen bands for CoFeCrAl, CoFeCrSi, CoFeCrGa and CoFeCrGe are 4.179 THz (Delta lambda = 71.73 mu m), 4.30 THz (Delta lambda = 69.71 mu m), 3.35 THz (Delta lambda = 89.49 mu m) and 3.05 THz (Delta lambda = 98.99 mu m), respectively. These obtained values of Reststrahlen bands for CoFeCrZ (Z = Al, Si, Ga, Ge) lie within the far infra-red (FIR) region, and can be used in sensing, imaging and optoelectronic devices.