Optical behaviour of sodium β-aluminogallate single crystals doped with Cr3+ and Cr3+–Nd3+

Abstract

Single crystals of formula Na_1.6(Al_0.5–xGa_0.5Cr_x)₁₁O_17.3(x= 0.002, 0.01), were grown by the flux evaporation method. The crystal structure study shows a β-alumina type structure with strong delocalisation of Na⁺ ions and an inhomogeneous distribution of Ga³⁺. In addition to a broad and intense emission band, corresponding to the Cr(⁴T₂→⁴A₂) transition, the fluorescence spectra present two other signals attributed to the Cr(²E→⁴A₂) transition of various chromium species. At 700 nm, the emission band corresponds to a D_3d symmetry site and at 709 nm to a CM_s symmetry site occupancy. Cr³⁺ pairs are also identified. Time-resolved fluorescence spectroscopy and lifetime measurements are used to determine the Cr³⁺ site occupancy and to study Cr³⁺–Cr³⁺ interactions.

These single crystals are doped by ionic exchange in molten salts, part of the sodium being replaced by Nd³⁺. Studies of the Cr³⁺ emission and Nd³⁺ excitation spectra indicate that efficient Cr³⁺→ND³⁺ energy transfer takes place in this material at 300 K. From the low-temperature Nd³⁺ decay profile, it is suggested that the transfer occurs mainly from the ⁴T₂ Cr³⁺ state, and that Cr³⁺ pairs do not participate significantly in this transfer.