TIME AND COMPOSITION DEPENDENCE OF THE SPECTRAL SHAPE OF THE X-RAY CEL(111) ABSORPTION IN THE R2-XCEXCUO4-DELTA SUPERCONDUCTORS

X-ray absorption Ce L(III) spectra in electron superconductors Nd2-xCe xCuO4-delta and Pr2-xCexCuO4-delta with perovskite-like T' structure h ave been obtained. The spectral shape of freshly prepared samples is s imilar to that of Ce L(III), in cubic perovskite BaCeO3, but not to Ce L(III) in CeO2, in contrast to what has previously been published. Th e changes of the spectral shape with time have been detected and thoug ht to be due to the degradation of the samples. The spectrum of the mo st degraded sample is similar to the spectra of CeO2 and Ce(OH)4. It i s found that the compositions most stable towards degradation are thos e which exhibit maximal T(c) values. It is supposed that at values of x corresponding to 0.15 in Nd-Ce-Cu-O and 0.14 in Pr-Ce-Cu-O, Ce atoms are ordered, and perovskite-like structure acquires the highest therm odynamical stability. LMTO band structure calculations of fictitious o rdered compound Nd3CeCu2O8 in the T' structure were performed, and Ce 5d partial density of states compared to the Ce L(III) spectrum. All t he main features of the Ce L(III) spectrum are explained. The valence state of Ce is close to Ce4+, similarly to that in CeO2 and BaCeO3.