ELECTRON-NUCLEAR DOUBLE-RESONANCE OF POLARONS IN ALPHA-LIXV2O5

A single crystal of alpha-LixV(2)O(5) (x = 0.005) is studied by electr on 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 39-line spectrum due to pularons trapped by the Coulomb field of Li+ ions. The second (B-center) is unknown so fa r and is studied here for the first time. It consists of a broad unres olved line hidden inside the 29-line spectrum. The B-center represents a foe polaron localized on a single vanadium site of one [V2O5](n), l ayer and is responsible for the compound's electronic conductivity, Th e ENDOR lines of this center are assigned to two nearest neighbor vana dium nuclei, one being located in the same [V2O5](n) layer as the pola ron while the other belonging to the adjacent layer. ENDOR spectroscop y shows the existence of a significant covalent interaction between la yers, in contrast with the usual assumption of Van der Waals interacti ons. This covalency manifests itself by the transfer of about 10(-2) u npaired polaron spin density into 4s and d(z)2 vanadium orbitals of th e closest vanadium ion in the adjacent layer. Bound polarons (A-center s) do not give an ENDOR response. This feature is attributed to an inh omogeneous distribution of Li+ ions in the matrix (in the form of shal low clusters), which provokes a concentration quenching of the ENDOR e nhancement.