Hexagonal b-NaYF 4 co-doped with Yb 3+ and Er 3+ , which was first studied more than 30 years ago, [1][2][3] is extremely efficient in converting near infrared (NIR) radiation into visible (VIS) light in a so-called upconversion (UC) process. [4][5][6] Such UC phosphors can be applied in display devices [7] and as labels in immunoassays. [8] Efficient UC has also been reported for b-NaYF 4 doped with Pr 3+ . [9] Intense light emission was found in b-NaEuF 4
[10] and b-NaGdF 4 doped with Ce 3+ and Eu 3+ . [11] Evidence of a multiple-site character of the emission was found by site-selective spectroscopy, and the high efficiency of the light emission was vaguely ascribed to it. [9][10][11][12][13] To date, all reported spectroscopic studies on doped b-NaLnF 4 (Ln = Y, La-Lu) have been done on powder samples. [6,[9][10][11][12][13] The number of crystallographic sites occupied by the dopant Ln 3+ is controversially given as two [11,13] or three. [10,12] Burns has found that crystals of hexagonal b-NaNdF 4 are disordered and has reported an average structure in space group P6 ¯.[14] More recent X-ray powder data of the related phase b-Na 1.34(1) (Y 0.747 Yb 0.25 Tm 0.003 ) 1.555(4) F 6 have been interpreted in terms of space group P6 3 /m. [5] An unambiguous structural characterization of the spectroscopic sites, one of the prerequisites for understanding the energy-transfer processes underlying the light emission, has not been provided to date. Herein we report the first polarized absorption spectra from single crystals of b-NaGdF 4 doped with Er 3+ , interpret the