Impact of Phase Transitions on Temperature Evolution of Absorption Spectra and Electron–Phonon Interactions in [N(C2H5)4]2CoCl2Br2 Crystals

Absorption spectra and structural parameters of [N(C2H5)4]2CoCl2Br2 (TEACCB-2) crystals were investigated. Absorption bands corresponding to internal electronic transitions of Co2+ were identified. The impact of phase transitions on the temperature evolution of absorption spectra was studied. The low-energy tail of the absorption edge formed by ligand–metal charge-transfer electronic transitions was shown to be exponential and obeyed the empirical Urbach rule in the two high-temperature crystal phases. The temperatures and types of phase transitions at T1 = 244 K and T2 = 228 K (in cooling mode) were refined by analyzing the temperature changes of the main characteristics of the obtained spectra. An anomaly at T3 = 200 K was detected for the first time in TEACCB-2 bulk crystals and appeared earlier in the absorption spectrum of TEACCB-2 nanocrystals. It could be attributed to a phase transition.