EPR spectra and crystal field of hexamer rare-earth clusters in fluorites

Abstract

Disordered fluorite-related solid solutions ${(A{\mathrm{F}}_{2})}_{1\ensuremath{-}x\ensuremath{-}y}$ ${({\mathrm{Ln}}^{\ensuremath{'}}{\mathrm{F}}_{3})}_{x}$ ${({\mathrm{Ln}}^{\ensuremath{''}}{\mathrm{F}}_{3})}_{y}$, where $A=\mathrm{Ca}$, Sr, Ba; ${\mathrm{Ln}}^{\ensuremath{'}}=\mathrm{Er}$, Tm, Yb; ${\mathrm{Ln}}^{\ensuremath{''}}=\mathrm{Lu}$, Y; and $x⪡y=0.001--0.4$, were studied by both optical detection and conventional electron paramagnetic resonance (EPR) techniques. The EPR spectra of paramagnetic rare-earth ions ${\mathrm{Er}}^{3+}$, ${\mathrm{Tm}}^{3+}$, and ${\mathrm{Yb}}^{3+}$ in clusters of diamagnetic ${\mathrm{Y}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ ions were recorded. It appears that the crystalline electric field at the sites of Ln ions in the clusters is of ``nearly'' tetragonal symmetry and provides for high values of factors ${g}_{\ensuremath{\Vert}}$, approaching the theoretical limits, and small values of factors ${g}_{\ensuremath{\perp}}\ensuremath{\sim}0$ in the ground states of the paramagnetic Ln ions. It was assumed that all the clusters of Ln ions in the solid solutions appear to be similar in structure to the hexameric clusters, which are the basic structural units of the homologous series of fluorite-related superstructures ${(A{\mathrm{F}}_{2})}_{1\ensuremath{-}y}{(\mathrm{Ln}{\mathrm{F}}_{3})}_{y}$ with compositions $y=5∕m$, where $m$ is an integer in the range of 13--19. The structure of ``symmetric'' hexameric clusters in $\mathrm{Ca}{\mathrm{F}}_{2}$, $\mathrm{Sr}{\mathrm{F}}_{2}$, and $\mathrm{Ba}{\mathrm{F}}_{2}$ hosts was established by computer simulation. The crystalline electric field and the spectroscopic ground-state parameters for ${\mathrm{Er}}^{3+}$, ${\mathrm{Tm}}^{3+}$, and ${\mathrm{Yb}}^{3+}$ ions in the hexameric clusters were calculated and found to be in agreement with the experimental data, being totally different from those known for the ``isolated'' simple cubic and tetragonal centers in the fluorite crystals.