Study of energy gaps and their temperature-dependent modulation in LaCrO₃: A theoretical and experimental approach

In the present study, the origin of three energy gaps of lanthanum chromium oxide, one arises due to the charge-transfer gap between O-2p and Cr-3d and two arises from the d-d transitions, is analyzed in detail using the diffuse reflectance spectroscopy technique. The spin allowed transitions, such as ⁴T₁(P)→⁴A₂⁴T₁(F)→⁴A₂, and ⁴T₂→⁴A₂, and the spin and parity forbidden ²E →⁴A₂ transitions are depicted using a Tanabe-Sugano (T-S) diagram and an absorption spectrum. The high crystal field strength of 3.27 obtained from the T-S diagram provides high directional emission and makes the system suitable for near infrared lasing applications. Moreover, the investigations into the variation of a charge-transfer gap with temperature will provide insights into the modification of numerous optical properties toward development of optoelectronic devices. Using this temperature-dependent diffuse reflectance spectroscopy studies, we have obtained the important optical parameters, such as Urbach energy (E_u) and Urbach focus. Furthermore, first-principles calculations are carried out in order to validate the experimental findings on LaCrO₃. The experimental results are in consonance with the charge-transfer gap obtained from the theoretical calculations. Furthermore, the bandgap variation with temperature is fitted using Varshni's relation, and the Debye temperature is calculated.