Hindered rotations probed by rare earths in crystals:Er3+andTm3+inBaY2F8

Abstract

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 ${\text{Er}}^{3+}$ and ${\text{Tm}}^{3+}$ CF lines in ${\text{BaY}}_{2}{\text{F}}_{8}$ single crystals. Sequences of weak, very narrow $(0.03--0.1\text{ }{\text{cm}}^{\ensuremath{-}1})$, closely spaced $(\ensuremath{\sim}0.2--0.8\text{ }{\text{cm}}^{\ensuremath{-}1})$ lines were monitored in high-resolution (as fine as $0.01\text{ }{\text{cm}}^{\ensuremath{-}1}$), low-temperature (9 K) absorption spectra in the $2000--24\text{ }000\text{ }{\text{cm}}^{\ensuremath{-}1}$ range. The FS covers a few ${\text{cm}}^{\ensuremath{-}1}$ 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 $0.5--20\text{ }\text{at}\text{.}\text{ }%$ 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 ${\text{RE}}^{3+}{\text{-F}}^{\ensuremath{-}}$ group. The attribution is supported both by the specific structure of the host matrix which allows some ${\text{F}}^{\ensuremath{-}}$ mobility and by the very small line spacing which is in excellent agreement with the ${\text{RE}}^{3+}{\text{-F}}^{\ensuremath{-}}$ rotational constant $(2B=0.39\text{ }{\text{cm}}^{\ensuremath{-}1})$. Complementary specific-heat measurements in the temperature range 1.5--25 K show that ${\text{Er}}^{3+}$-doped samples display contributions, in addition to the vibrational one of a pure sample, which scale with the ${\text{Er}}^{3+}$ concentration. The extra specific heat is interpreted in terms of Schottky anomalies; that peaking at $\ensuremath{\sim}17\text{ }\text{K}$ accounts for electronic transitions between the lowest sublevels of the ${^{4}I}_{15/2}$ 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 $0.36\text{ }{\text{cm}}^{\ensuremath{-}1}$, in agreement with the FS line spacing.