Radiative and optical properties of la_1-xSr_xMnO₃ (0 ≤ x ≤ 0.4) in the vicinity of metal-insulator transition temperatures from 173 to 413 K

Radiative and optical properties of polycrystalline La_1-xSr_xMnO₃ (0≤ x ≤ 0.4) in the vicinity of the metal-insulator transition are presented. The temperature dependence of the total hemispherical emittance ε_H of La_1-xSr_xMnO₃ was measured by the calorimetric method in the temperature range from 173 to 413 K. It was confirmed that ε_H showed unexpected variation as a result of changes in the hole concentration (x). Especially in the case of La_0.825Sr_0.175MnO₃, ε_H remains high above the transition temperature T_C due to insulator-like behavior; on the other hand, it decreases sharply below T_C because of metallic behavior. The spectral reflectance was measured by FT-IR in the wavelength range of 0.25 to 100 μm at room temperature. The optical constants were calculated by Kramers-Kronig analysis of the spectral reflectance data. An insulator-like character of the optical properties appears at lower Sr²⁺ doping levels while a metallic one exists at higher Sr²⁺ doping levels.