Strong Quenching of Tb3+ Emission by Tb–V Interaction in YPO4–YVO4

Abstract

Solid solutions in the system YPO₄–YVO₄, with and without 2 mol% of Tb³, were studied to determine the reason for the total inactivity of Tb in YVO₄. The comparison of the diffuse reflectivity of Tb‐containing solutions with that of Tb‐free ones shows a uv absorption at energies below the absorption edge of YVO₄ in materials containing both terbium and vanadium. Selective excitation directly into Tb₃₊ in these wavelength regions produced reduced Tb³⁺ emission when there was vanadium present. The band emission of the VO₄³⁺ groups, and the Tb³⁺ emission under electron beam excitation, were strongly quenched whenever terbium and vanadium were both present. Emission of Dy³⁺, present as a 10‐ppm impurity in all samples, was also quenched by the presence of both terbium and vanadium, but not by either alone. All these observations imply energy loss by Tb–V interaction. A particular loss mechanism—the well‐known intervalence transfer of charge from Tb to V resulting in an abrupt change of bond strength leading to a transfer of electronic excitation into lattice vibrations—is supported by measurements of Tb activity as a function of V concentration. The intensity of Tb emission is proportionate to the probability that none of four metal ion sites about a Tb is occupied by a V. This suggests the intervalence transfer absorption mechanism because only four of the nearest six metal ion sites have Tb–O–M bond angles (about 150°) consistent with good charge transfer from Tb to V; the remaining two sites have a Tb–O–M near 90° and should be relatively inactive in charge transfer.