Local order of V5+ ions in borate glasses investigated by EXAFS

Glasses in the systems Li2O-B2O3-V2O5 and ZnO-B2O3-V2O5 as Well as crystall ine reference substances Ir ere investigated by V-K-EXAFS. The glasses have a high modifier content ([Li2O, ZnO] / [B2O3] ratio > 1) and contain 10-50 mol% V2O5. The V-O interatomic distance, coordination number and Debye-Wal ler factor were termined from the analysis of the measured absorption spect ra. In the glasses investigated (and in binary; lithium or zinc vanadate gl asses too) VO4, tetrahedra are the predominant coordination polyhedra for t he vanadium ions. The symmetry of the tetrahedra is more or less distorted in comparison to crystalline substances. The VO4 tetrahedra can be either d irectly connected to each other or incorporated into berate BO3/BO4 groupin gs. Structural elements which are comparable to those in clystnlline substa nces can be discussed In glasses of the metaborate composition, vanadium ca n be incorporated in chain or ring [B phi O-2(-)](n) metaborate groupings. With increasing V2O5 content the vanadium ions can extensively, replace bor on ions and form distorted tetrahedra in V phi O-2(2)- metavanadate groups In glasses of high modifier content and low vanadium amount the vanadium io ns create nearly regular tetrahedra of isolated VO43- ortho- or' V2 phiO(6) (4-) pyrovanadate groups in the presence of B2 phiO(4)(4-) pyroborate, BO33 - orthoborate and B phi (-)(4) groups.