Site‐Preferences in the Mixed‐Valent Series FeMo1−nVnO4: Controlling the Formation of the Α‐polymorph and the Charge Localization

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 Fe-57-Mbssbauer spectroscopy and DFT-calculations of the electric field gradients. The incorporation of smaller ions (Fe-III, V-V) represents an internal "chemical pressure" effect stabilizing the low-temperature alpha-polymorph with increasing n. The site- preference and distribution of vanadium on individual Mo-sites impacts the alpha ->beta 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 alpha-FeMo1-nVnO4 to enhanced electron-phonon coupling within a polaronic scenario.