Spectroscopic properties and configuration interaction assisted crystal field analysis ofNd3+in neatCs2Na

This paper focuses upon the luminescence and absorption optical properties of trivalent neodymium ions in the neat hexachloroelpasolite ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Nd}{\mathrm{Cl}}_{6}$, and the crystal field analysis of the resulting dataset. The luminescence investigation spans the spectral region from $7950\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}27800\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, wherein four metastable emitting states $^{4}F_{3∕2}$, $^{4}F_{5∕2}$, $^{4}G_{7∕2}$, and $^{4}D_{3∕2}$ occur, as well as $^{2}P_{3∕2}$ in ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Y}{\mathrm{Cl}}_{6}:{\mathrm{Nd}}^{3+}$. The zero phonon lines as well as vibronic sidebands in the transitions from these luminescent states to lower terminal terms have been assigned in detail. In particular, for the highest emitting state $^{4}D_{3∕2}$ in ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Nd}{\mathrm{Cl}}_{6}$ all the transitions to terminal crystal levels with energies below $17\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ have been assigned. The high resolution absorption spectrum of ${\mathrm{Cs}}_{2}\mathrm{Na}\mathrm{Nd}{\mathrm{Cl}}_{6}$ has been recorded and analyzed in the range from $400\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}40\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, spanning 34 terminal multiplet manifolds. The comparison with the spectra of ${\mathrm{Cs}}_{2}\mathrm{Na}M{\mathrm{Cl}}_{6}:{\mathrm{Nd}}^{3+}$ $(M=\mathrm{Gd},\mathrm{Y})$ is included. The experimental dataset encompasses all 109 crystal field energy levels below $40\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, comprising 86 firmly assigned levels and a further 12 tentative ones, i.e., from the total 364 states of ${f}^{3}$, 300 of the first 332 states have been assigned or tentatively assigned. It has been analyzed by the conventional ${f}^{3}$ model as well as by a model which includes configuration interaction with an equiparity $p$-electron configuration. The ${f}^{2}p$ model provides a superior fit than the ${f}^{3}$ model, with overall standard deviations of 12.9 and $32.2\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, respectively for 98 levels. The nature of the latter configuration is discussed and rationalized with reference to previous analyses for ${\mathrm{Pr}}^{3+}$, ${\mathrm{Er}}^{3+}$, and ${\mathrm{Tm}}^{3+}$. A comparison is made with other energy level fittings for solid-state ${\mathrm{Nd}}^{3+}$ compounds.