Site‐Preferences in the Mixed‐Valent Series FeMo _1−n V _n O ₄ : Controlling the Formation of the α‐Polymorph and the Charge Localization

Abstract The mixed‐valent, solid solution series FeMo 1−n V n O 4 (0≤n≤0.3) was synthesized and characterized by x‐ray diffraction, and thermal analysis. Assignments of individual Fe‐species stem from 57 Fe‐Mössbauer 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 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‐FeMo 1−n V n O 4 to enhanced electron‐phonon coupling within a polaronic scenario.