Effect of pressure and crystal structure on energy levels of Pr3+ in LaCl3.

Abstract

The fluorescence of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{LaCl}}_{3}$ as well as the single-crystal x-ray diffraction of ${\mathrm{LaCl}}_{3}$ under pressure have been studied in order to determine the dependence of the energy levels of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{LaCl}}_{3}$ on the host-lattice structure. An analysis of this dependence is presented up to about 8 GPa. In this pressure range the Slater parameters ${F}_{2}$, ${F}_{4}$, and ${F}_{6}$ show a decrease of 1.2(2)%, 0.6(3)%, and 0.7(9)%, respectively. The spin-orbit coupling parameter ${\ensuremath{\zeta}}_{4f}$ decreases by 0.3(2)%. The various crystal-field parameters show distinctively different behavior: (1) \ensuremath{\Vert}${A}_{60}$〈${r}^{6}$〉\ensuremath{\Vert} and ${A}_{66}$〈${r}^{6}$〉 both increase with pressure, (2) \ensuremath{\Vert}${A}_{40}$〈${r}^{4}$〉\ensuremath{\Vert} decreases with pressure, and (3) ${A}_{20}$〈${r}^{2}$〉 goes through a minimum at about 5(1) GPa.It is shown that the present theoretical models for the variation of the ``free-ion'' parameters in crystals cannot account consistently for the observed shifts. An analysis of the high-pressure behavior of crystal-field parameters using superposition and angular overlap models is shown to be successful for ${A}_{60}$〈${r}^{6}$〉, ${A}_{66}$〈${r}^{6}$〉, and ${A}_{40}$〈${r}^{4}$〉. However, the same analysis for ${A}_{20}$〈${r}^{2}$〉 breaks down, unless one assumes a nonlinear dependence for the intrinsic parameter A${\ifmmode\bar\else\textasciimacron\fi{}}_{2}$(R) on the internuclear distance R with a maximum in the experimental range. The present angular-overlap model analysis of the crystal fields, as well as a comparison with the ab initio calculations for intrinsic parameters A${\ifmmode\bar\else\textasciimacron\fi{}}_{4}$(R) and A${\ifmmode\bar\else\textasciimacron\fi{}}_{6}$(R) indicate that the ligand contributions reproduce fairly well the variations of these two parameters. The peculiar behavior of the ${A}_{20}$〈${r}^{2}$〉 under pressure, however, still needs an explanation. The local distortions around the ${\mathrm{Pr}}^{3+}$ impurity has been estimated from a comparison of the structural and spectroscopic data for both ${\mathrm{Pr}}^{3+}$ in ${\mathrm{LaCl}}_{3}$ under pressure and ${\mathrm{Pr}}^{3+}$ in isostructural lanthanide trichloride host lattices at ambient pressure. It is shown that the largest local distortions of about 2% occur along the local c axis. Some fluorescence anomalies observed beyond 8-GPa-pressure region are also presented.