Efficient phonon-assisted long-lifetime<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Nd</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>fluorescence in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cs</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>NaNd<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cl</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> (1974)

Spectroscopic measurements are reported on single crystals of the high-${\mathrm{Nd}}^{3+}$-concentration perovskite ${\mathrm{Cs}}_{2}$NaNd${\mathrm{Cl}}_{6}$ [$\ensuremath{\rho}(\mathrm{Nd})=3.2\ifmmode\times\else\texttimes\fi{}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$] and on ${\mathrm{Cs}}_{2}$Na${\mathrm{Nd}}_{0.01}$${\mathrm{Y}}_{0.99}$ ${\mathrm{Cl}}_{6}$.The room-temperature fluorescence lifetimes are 1.23 and 4.1 msec, respectively, which are the longest reported so far for ${\mathrm{Nd}}^{3+}$ in any environment. This compound shows weak concentration quenching of fluorescence comparable to that observed in Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ which has $\ensuremath{\rho}(\mathrm{Nd})=4\ifmmode\times\else\texttimes\fi{}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. This is related to the isolation of the ${\mathrm{Nd}}^{3+}$ ions which are located in discrete Nd$\mathrm{Cl}_{}^{3\ensuremath{-}}{}_{6}{}^{}$ octahedra and have a minimum separation of 0.77 nm. The long fluorescence lifetime results from the strict octahedral coordination of ${\mathrm{Nd}}^{3+}$ which discourages electric-dipole electronic transitions. Absorption and emission of the $^{4}\mathrm{F}_{3/2}$ - $^{4}\mathrm{I}_{9/2}$ and $^{4}\mathrm{F}_{3/2}$ - $^{4}\mathrm{I}_{11/2}$ levels are found to be dominated by phonon-assisted vibronic transitions. The ground state to the $^{4}\mathrm{F}_{3/2}$ no-phonon line is observed in absorption at 1.8 \ifmmode^\circ\else\textdegree\fi{}K, and the phonon-assisted spectra at shorter wavelengths are used to interpret the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{9/2}$ emission spectra at 1.8 \ifmmode^\circ\else\textdegree\fi{}K. The phononassisted spectra associated with the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{11/2}$ emission are not the same as observed for the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{9/2}$ emission but a tentative assignment has been made. Due to the reduction of the phonon populations at lower temperatures the emission cross sections are reduced and the lifetimes of the concentrated and dilute crystals increase to 5 and 11 msec, respectively, at 77 \ifmmode^\circ\else\textdegree\fi{}K. A comparison with YAG: Nd and Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ indicates that \ensuremath{\sim} 5% of absorbed photons are emitted into the $^{4}\mathrm{I}_{11/2}$ levels in ${\mathrm{Cs}}_{2}$NaNd${\mathrm{Cl}}_{6}$ at room temperature, and an approximate peak emission cross section of 1.2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$ is calculated for the $^{4}\mathrm{F}_{3/2}$ to $^{4}\mathrm{I}_{11/2}$ emission. This is \ensuremath{\sim} 100 times lower than found for Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ and indicates a low gain in pulsed laser operation. Because of the far longer lifetime, however, the gain per ion under cw operation is predicted to be \ensuremath{\sim} 10 times lower than for Nd${\mathrm{P}}_{5}$${\mathrm{O}}_{14}$ and \ensuremath{\sim} 7 times lower if diluted samples of the two compounds are compared with each other.