Thermal effects in the x-ray spectra of La1-xCaxMnO3

Complementary x-ray emission (XES) and x-ray-absorption (XAS) measurements of La1-xCaxMnO3 as a function of temperature have been performed. For 0.5 l ess than or equal tox less than or equal to0.8 we find changes in the XAS a nd XES spectra consistent with a small increase in the average Mn valence a ccompanying the passage to the low-temperature state. It is suggested that this small Mn-configuration shift could be due to the competition between t he energies associated with ferromagnetic (FM) and charge/orbital (CO) corr elations. In the region 0.0 less than or equal tox less than or equal to0.4 we observe thermal spectral changes quite different from those found in th e x greater than or equal to0.5 materials. Modest thermal changes in the Mn XAS pre-edge (K-edge), for 0.0 less than or equal ton less than or equal t o0.4, were observed to be coupled to the robustness of the;FM-metallic grou nd state. The dearest pre-edge feature variation appears to be related to M n-e(g)-majority-spin state changes. In contrast the thermal XES spectral ch anges appear similar in character in the antiferromagnetic (AF)-insulating x=0.0 and FM-metallic x=0.3 materials. An enhancement of the effective loca l moment via a coupling to the internal exchange held, in the magnetically ordered states, is Proposed to explain these XES results. The x=1.0 end mem ber (CaMnO3) was found to exhibit significant temperature dependence of the absorption and emission spectra. These XAS and XES results, for CaMnO3, ar e discussed in terms of thermal perturbations on the degree of covalency of this material.