When Ce³⁺ ions substitute for La³⁺ in a LaCl₃ host, the lowest-energy peak in the photoluminescence excitation spectrum falls at 281 nm, whereas a similar substitution in the double-perovskite host Cs₂NaLaCl₆ shifts the lowest-energy peak in the excitation spectrum to 342 nm. The La³⁺ ion in LaCl₃ sits in a nine-coordinate tricapped trigonal-prismatic site, while in Cs₂NaLaCl₆ the La³⁺ ion is octahedrally coordinated.

(a) Would you predict a larger, smaller, or similar centroid shift in Cs₂NaLaCl₆?

(b) How will differences in the coordination environment shift the lowest-energy Ce³⁺ excitation peak in Cs₂NaLaCl₆?

(c) Is the observed shift in the Ce³⁺ excitation spectrum due to the centroid shift, the change in coordination, or both?