Hindered rotations probed by rare earths in crystals: Er3+ and Tm3+ in BaY2F8

The sharpness of absorption lines induced by crystal-field CF transitions of rare earths RE can be exploited to disclose the rotational structure usually hidden under the more common broad electronic absorptions. In the present work the effectiveness of such an approach is proved by the analysis of the fine structure FS accompanying the Er3+ and Tm3+ CF lines in BaY2F8 single crystals. Sequences of weak, very narrow 0.03–0.1 cm−1, closely spaced 0.2–0.8 cm−1 lines were monitored in high-resolution as fine as 0.01 cm−1, low-temperature 9 K absorption spectra in the 2000–24 000 cm−1 range. The FS covers a few cm−1 on both sides of the narrowest among the RE-CF lines and is tightly associated with them, as proved by the amplitude dependence on the RE concentration in the 0.5–20 at. % range and by linear dichroism measurements. The FS lines vanishing at temperatures as low as 40–60 K and the close spacing suggest that they may be ascribed to the simultaneous excitation of both RE-CF electronic transition and hindered rotation or libration mode of RE3+-F− group. The attribution is supported both by the specific structure of the host matrix which allows some F− mobility and by the very small line spacing which is in excellent agreement with the RE3+-F− rotational constant 2B=0.39 cm−1. Complementary specific-heat measurements in the temperature range 1.5–25 K show that Er3+-doped samples display contributions, in addition to the vibrational one of a pure sample, which scale with the Er3+ concentration. The extra specific heat is interpreted in terms of Schottky anomalies; that peaking at 17 K accounts for electronic transitions between the lowest sublevels of the 4I15/2 ground manifold, in agreement with the CF spectroscopy results while those occurring below 3.5 K are consistent with level pairs separated by 0.55 and 0.36 cm−1, in agreement with the FS line spacing.