In this paper, we present the results of ab initio model potential embedded-cluster calculations of ${\mathrm{Sm}}^{2+}$ impurities in $\mathrm{Sr}{\mathrm{F}}_2$ in order to study the behavior of the electronic transitions of the dopant ion under high hydrostatic pressure. We find that the impurity-ligand bond length shortens upon $f\to d\left(e_g\right)$ excitation and, as a consequence, the $f\to d\left(e_g\right)$ transition energy decreases with increasing applied pressure. On the other hand, the bond lengths do not appreciably change upon $f\to f$ excitation and the energies of the $f\to f$ transitions are almost constant with pressure. These trends are in agreement with spectroscopic measurements under pressure in the title material, which gives credit to the computed bond length changes upon excitation, in contradiction with the widespread assumption of bond length lengthening upon $f\to d$ excitations. Spectroscopic experiments under high pressure are shown to be able to provide the sign of bond length changes in electronic transitions, constituting a simpler alternative to difficult excited-state x-ray absorption fine structure experiments.

• Received 26 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.104109