Thermal effects in the x-ray spectra of La_1-xCa_xMnO₃

Complementary x-ray emission (XES) and x-ray-absorption (XAS) measurements of La_1-xCa_xMnO₃ as a function of temperature have been performed. For 0.5≤x≤0.8 we find changes in the XAS and XES spectra consistent with a small increase in the average Mn valence accompanying the passage to the low-temperature state. It is suggested that this small Mn-configuration shift could be due to the competition between the energies associated with ferromagnetic (FM) and charge/orbital (CO) correlations. In the region 0.0≤x≤0.4 we observe thermal spectral changes quite different from those found in the x≥0.5 materials. Modest thermal changes in the Mn XAS pre-edge (K-edge), for 0.0≤x≤0.4, were observed to be coupled to the robustness of the FM-metallic ground state. The clearest pre-edge feature variation appears to be related to Mn-e_g-majority-spin state changes. In contrast the thermal XES spectral changes appear similar in character in the antiferromagnetic (AF)-insulating x=0.0 and FM-metallic x=0.3 materials. An enhancement of the effective local moment via a coupling to the internal exchange field, in the magnetically ordered states, is proposed to explain these XES results. The x=1.0 end member (CaMnO₃) was found to exhibit significant temperature dependence of the absorption and emission spectra. These XAS and XES results, for CaMnO₃, are discussed in terms of thermal perturbations on the degree of covalency of this material.