Electron Nuclear Double Resonance of Polarons in α-LixV2O5

A single crystal of α-Li x V 2 O 5 ( x = 0.005) is studied by electron nuclear double resonance spectroscopy (ENDOR). It is shown that the ESR spectrum is composed of two overlapping signals. The first one (A-center) is the well-known 29-line spectrum due to polarons trapped by the Coulomb field of Li + ions. The second (B-center) is unknown so far and is studied here for the first time. It consists of a broad unresolved line hidden inside the 29-line spectrum. The B-center represents a free polaron localized on a single vanadium site of one [V 2 O 5 ] n layer and is responsible for the compound's electronic conductivity. The ENDOR lines of this center are assigned to two nearest neighbor vanadium nuclei, one being located in the same [V 2 O 5 ] n layer as the polaron while the other belonging to the adjacent layer. ENDOR spectroscopy shows the existence of a significant covalent interaction between layers, in contrast with the usual assumption of Van der Waals interactions. This covalency manifests itself by the transfer of about 10 -2 unpaired polaron spin density into 4s and d z 2 vanadium orbitals of the closest vanadium ion in the adjacent layer. Bound polarons (A-centers) do not give an ENDOR response. This feature is attributed to an inhomogeneous distribution of Li + ions in the matrix (in the form of shallow clusters), which provokes a concentration quenching of the ENDOR enhancement.