Effect of disorder on the optical response of NiPt and Ni3Pt alloys

Abstract

In this communication we present a detailed study of the effect of chemical disorder on the optical response of Ni1−xPtx (0.1⩽x⩽0.75) and Ni3(1−x)/3Ptx (0.1⩽x⩽0.3). We shall propose a formalism which will combine a Kubo-Greenwood approach with a DFT based tight-binding linear muffin-tin orbitals (TB-LMTO) basis and augmented space recursion (ASR) technique to explicitly incorporate the effect of disorder. We show that chemical disorder has a large impact on optical response of Ni-Pt systems. In ordered Ni-Pt alloys, the optical conductivity peaks are sharp. But as we switch on chemical disorder, the UV peak becomes broadened and its position as a function of composition and disorder carries the signature of a phase transition from NiPt to Ni3Pt with decreasing Pt concentration. Quantitatively this agrees well with Massalski’s Ni-Pt phase diagram (Massalski, 1990). Both ordered NiPt and Ni3Pt have an optical conductivity transition at 4.12eV. But disordered NiPt has an optical conductivity transition at 3.93eV. If we decrease the Pt content, it results a chemical phase transition from NiPt to Ni3Pt and shifts the peak position by 1.67eV to the ultraviolet range at 5.6eV. There is a significant broadening of UV peak with increasing Pt content due to enhancement of 3d(Ni)-5d(Pt) bonding. Chemical disorder enhances the optical response of NiPt alloys nearly one order of magnitude. Our study also shows the fragile magnetic effect on optical response of disordered Ni1−xPtx (0.4<x<0.6) binary alloys. Our theoretical predictions agree reasonably well with both previous experimental findings as well as theoretical investigations.