Measurement and crystal-field analysis of Er3+ energy levels in crystals of NaBi(MoO4)2 and NaBi(WO4)2 with local disorder Original Research Article

The polarised optical absorption and photoluminescence of Er3+-doped tetragonal I4 NaBi(MoO4)2 (NBMo) and NaBi(WO4)2 (NBW) single crystals have been measured at 10 K. In these hosts Na+ and Bi3+ cations are statistically distributed over 2b and 2d crystallographic sites both with S4 symmetry. Er3+ is expected to replace Bi3+, therefore inhomogeneous broadening of its spectral bands may arise simultaneously from the occupancy of the above two sites and from the different short-range Na and Bi distributions around them. Despite the broad bands observed, well-defined polarisation rules have been determined, indicating that no reduction of host site symmetry occurs. The positions of 52 and 66 observed Kramers doublets in NBMo and NBW crystals have been established and labelled with their corresponding irreducible representations. Detailed single-electron Hamiltonians combining together free-ion and crystal-field interactions have been used to reproduce the energy levels and associated wavefunctions of the 4f11 configuration of Er3+ in both hosts. Very satisfactory correlations were obtained between experimental and calculated crystal-field levels, with rms deviations σ=9.5 and 13.3 cm−1 for NBMo and NBW, respectively. In an attempt to account for the linewidths observed, the energy separation due to the two non-equivalent 2b and 2d sites where Er3+ can be found has been calculated taking into account only the first oxygen shell (i.e., neglecting the Na and Bi disorder). The site-associated calculated energy separation is generally lower but close to the experimental 10 K linewidths.