This study investigates the variable-temperature R-lines luminescence spectra of Cr3+ ions in a composite solid comprising alpha-Al2O3 and borate fluorescent glass. A divergence in the thermal broadening (Delta Gamma) of the R1 and R2 lines is observed at temperatures above 140 K. The experimental data were analyzed using McCumber-Sturge theory, incorporating the significant effects of acoustic phonon scattering on the effective Debye temperatures. The thermal broadening of both R-lines was found to strongly depend on excitation energy over a broad range. Furthermore, the excitations at the low-energy side of 4A2 -> 4T2 band exhibited pronounced negative thermal quenching behaviors in both R-lines. These findings provide new insights into the complex many-body interactions underlying the luminescence of functional ions in solids and offer promising opportunities for regulating the thermal behavior of Cr3+ ions' R-lines.