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

摘要: The sharpness of absorption lines induced by crystal-field (CF) transitions rare earths (RE) can be exploited to disclose the rotational structure usually hidden under more common broad electronic absorptions. In present work effectiveness such an approach is proved analysis fine (FS) accompanying ${\text{Er}}^{3+}$ and ${\text{Tm}}^{3+}$ CF in ${\text{BaY}}_{2}{\text{F}}_{8}$ single crystals. Sequences weak, very narrow $(0.03--0.1\text{ }{\text{cm}}^{\ensuremath{-}1})$, closely spaced $(\ensuremath{\sim}0.2--0.8\text{ }{\text{cm}}^{\ensuremath{-}1})$ were monitored high-resolution (as as $0.01\text{ }{\text{cm}}^{\ensuremath{-}1}$), low-temperature (9 K) spectra $2000--24\text{ }000\text{ }{\text{cm}}^{\ensuremath{-}1}$ range. FS covers a few ${\text{cm}}^{\ensuremath{-}1}$ on both sides narrowest among RE-CF tightly associated with them, amplitude dependence RE concentration (in $0.5--20\text{ }\text{at}\text{.}\text{ }%$ range) linear dichroism measurements. vanishing at temperatures low 40--60 K close spacing suggest that they may ascribed simultaneous excitation transition hindered rotation (or libration) mode ${\text{RE}}^{3+}{\text{-F}}^{\ensuremath{-}}$ group. attribution supported specific host matrix which allows some ${\text{F}}^{\ensuremath{-}}$ mobility small line excellent agreement constant $(2B=0.39\text{ }{\text{cm}}^{\ensuremath{-}1})$. Complementary specific-heat measurements temperature range 1.5--25 show ${\text{Er}}^{3+}$-doped samples display contributions, addition vibrational one pure sample, scale concentration. extra heat interpreted terms Schottky anomalies; peaking $\ensuremath{\sim}17\text{ }\text{K}$ accounts for between lowest sublevels ${^{4}I}_{15/2}$ ground manifold, spectroscopy results while those occurring below 3.5 are consistent level pairs separated 0.55 $0.36\text{ }{\text{cm}}^{\ensuremath{-}1}$, spacing.