Temperature-Induced Valence Transition of EuNi2(Si0.25Ge0.75)2Studied by Eu 4d–4fResonant Photoemission and Optical Conductivity

Abstract

Eu 4 d –4 f resonant photoemission and optical reflectivity measurements of a temperature-induced valence transition material, EuNi 2 (Si 0.25 Ge 0.75 ) 2 , have been performed at several temperatures. The valence transition was observed and the mean valence was evaluated to be 2.37 ±0.02 and 2.75 ±0.25 below and above the valence transition temperature, respectively. The mixing between the Ni 3 d conduction band and the Eu 3+ 4 f hole becomes weak as the temperature increases, showing the valence transition from Eu 3+ to Eu 2+ .