Electronic excitations in PrAlO3

Previous spectroscopic experiments have shown that PrAl${\mathrm{O}}_3$ exhibits structural phase transitions at ∼ 1640, 205, and 151 K from respectively cubic to rhombohedral to orthorhombic to monoclinic symmetry. The 205 and 151 K transitions are now known to be driven by the coupling between the low-lying ${\mathrm{Pr}}^{3+}$ electronic levels and the phonon modes corresponding to staggered rotations of the Al${\mathrm{O}}_6^{3-}$ octahedra. In this paper we report an electronic Raman spectroscopic investigation of the crystal-field excitons originating in transitions from the ground and first excited states of the ${\mathrm{Pr}}^{3+}$ atom to the uppermost ($A_1$) level of the ${\mathrm{Pr}}^{3+}$ ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ ground manifold. The principal results of this study are (a) the over-all splitting of the ${}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}$ manifold varies markedly with temperature, ranging from ∼750 ${\mathrm{cm}}^{-1\mathrm{}}$ at 300 K to 925 ${\mathrm{cm}}^{-1\mathrm{}}$ at 20 K. (b) Thermally induced two-exciton Raman scattering is observed in the orthorhombic and monoclinic phases; the ground-state-first-excited-state splittings so determined are in good agreement with the corresponding fluorescence results. (c) In the orthorhombic phase the Raman selection rules imply that the ground state is $B_1\left(C_{2v}\right)$ rather than $A_1\left(C_{2v}\right)$ as previously supposed. Results (a) and (c) are in substantial numerical disagreement with the recent model calculation of Birgeneau et al. We have extended this calculation to include the effect of the ${}_{}{}^3{}_{}{}^{}H_5^{}{}_{}{}^{}$ manifold. This more quantitative crystal-field analysis yields a good description of all measured levels at all temperatures with (in the notation of Birgeneau et al.) $B_2=825\mathrm{}$, $B_4=-699\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, $B_6=-949\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, and $\alpha =0.4\mathrm{}$. The standard deviation of the fit in the orthorhombic phase and at $T\sim 0$ K is 27 ${\mathrm{cm}}^{-1\mathrm{}}$, which is the limit of the accuracy of the calculation.