Acta Cryst. (2008). B64, 534-549 [ doi:10.1107/S0108768108026542 ]
Abstract: X-ray atomic orbital (XAO) analysis revealed that at both temperatures the electrons are transferred from B 2px(= py) to Ce 5d and 4f orbitals. At 340 K 5d(j = 5/2)8 orbitals are occupied partially, but 4f(j = 5/2)8 orbitals are more populated than 4f(j = 5/2)7 orbitals, in contrast to our observation at 430 K [Makita et al. (2007). Acta Cryst. B63, 683-692]. At 535 K the XAO analysis revealed clearly that the order of the energy levels of 4f(j = 5/2)8 and 7 states reversed again and is the same as that at room temperature. It also limited the possible 5d configurations to three models among the nine possible ones. However, the XAO analysis could not decide which of the three models was the best with the present accuracy of the measurement. Two of them have partially and fully occupied 5d(j = 5/2)7 orbitals and the remaining one has a fully occupied 5d(j = 3/2)8 orbital. Since the lobes of 5d(j = 3/2)8 or 5d(j = 5/2)7 orbitals do not overlap with the 4f(j = 5/2)8 orbitals as well as the 5d(j = 5/2)8 orbitals, the order of the energy levels of the 4f(j = 5/2) orbitals became the same as that at room temperature. These results indicate that the crystal field varies with temperature due to the electron transfer from B 2p to Ce 5d orbitals. The difference densities after the spherical-atom refinement at the three temperatures clearly revealed the different combinations of 4f and 5d orbitals which are occupied. In the present study positive peaks due to the 4f electrons appear near the Ce nucleus and those due to 5d orbitals are found in the area outside the 4f peaks. Between the two areas there is a negative area distributed spherically at 340 K. The negative area produced by the contraction of 4f(j = 5/2)8 orbitals seems to reduce the electron repulsion of the 5d(j = 5/2)8 orbitals and helps the 4f(j = 5/2)8 orbitals to remain as the ground state.
Keywords: 4f electron state; 5d orbitals; electron-density distribution.
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