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Following an investigation proving La 2 Si 4 N 6 C crystallizes in a monoclinic space group, isostructural to Y 2 Si 4 N 6 C, the reportedly hexagonal (La 0.5 Y 0.5 ) 2 Si 4 N 6 C was reinvestigated to examine the apparent crystal structure change across the solid solution. Initially, calculating the electronic structure and phonon density of states of (La 0.5 Y 0.5 ) 2 Si 4 N 6 C in the P6 3 mc space group revealed an imaginary phonon mode, which is indicative of a structural instability. Displacing the atoms along the pathway of the imaginary vibration led to a previously unreported space group for carbonitridosilicates, trigonal P31c. The assignment of the trigonal space group was subsequently confirmed by synthesizing (La 0.5 Y 0.5 ) 2 Si 4 N 6 C using high-temperature, solid state synthesis and analyzing the crystal structure with high-resolution synchrotron X-ray powder diffraction. Preparing the solid solution, (La δ Y 1-δ ) 1.98 Ce 0.02 Si 4 N 6 C (δ = 0-0.5), showed that the crystal structure changes from the monoclinic to the trigonal space group at δ ≈ 0.25. Finally, substituting Ce 3+ in the crystal structure to investigate the optical response via steady-state luminescent and photoluminescent quantum yield measurements reveals severe luminescent quenching with increasing La 3+ content, due to a combination of absorption of luminescence by the host structure and thermal quenching. These results display the virtue of combining computational and experimental techniques to solve inorganic crystal structures and assess potential phosphor hosts.