Site-preferences in the mixed-valent series FeMo1−nVnO4 : controlling the formation of the α-polymorph and the charge localization

Abstract

The mixed-valent, solid solution series FeMo1−nVnO4 (0≤n≤0.3) was synthesized and characterized by x-ray diffraction, and thermal analysis. Assignments of individual Fe-species stem from 57Fe-Mössbauer spectroscopy and DFT-calculations of the electric field gradients. The incorporation of smaller ions (FeIII, VV) represents an internal “chemical pressure” effect stabilizing the low-temperature α-polymorph with increasing n. The site-preference and distribution of vanadium on individual Mo-sites impacts the α→β transition temperatures around 650 K, and causes distinct differences in charge mobility in this hole-doped semiconductor around 225 K. We assign the charge localization in α-FeMo1−nVnO4 to enhanced electron-phonon coupling within a polaronic scenario.