The gamma-to-alpha transition in cerium (continued)

In 1982, Jim Allen and Richard Martin developed the Kondo volume collapse model (consistent with the photoemission data) where the residual interaction between the localized magnetic moments in the 4f shell with the spins of the conduction electrons results in conduction electrons being bound to the local moments such that the net moment cancels. For temperatures below a characteristic temperature TK known as the Kondo temperature, the bound states are occupied and the system is nonmagnetic. For temperatures greater than TK, the electrons are thermally excited so that the bound states are unoccupied and the localized magnetic moment is uncompensated. Furthermore, in this model TK is volume dependent and changes from the small value of 70 kelvin (K) in the γ phase to 812 K in the α phase.

Inelastic neutron scattering experiments performed in 1983 at the Institut Laue-Langevin in Grenoble, France, by Amir Murani and co-workers have shown that the energy needed to excite the nonmagnetic local moments in the α phase is about 150 meV (TK ∼ 1740 K), which is two times larger than the value of 812 K initially suggested. Other measurements performed at Los Alamos in 2001 by Robert McQueeney and co-workers on the high temperature γ phase showed a quasi-elastic peak consistent with the assumed small Kondo temperature along with a small and very broad feature, which indicates that the cubic crystalline environment of the atomic 4f magnetic moments produces an orientational anisotropy in the energy of the moments.

Recently, several groups at Los Alamos have investigated the effects of the thermal disorder that appears as vibrations of the atoms in the solid, using inelastic neutron scattering and X-ray diffraction. As predicted by Peter J.W. Debye and later by Ivar Waller, the thermal vibrations of the atoms reduce the intensity of the peaks of the elastically scattered X-rays. From the measured intensities of the X-ray peaks, Il-Kyoung Jeong and co-workers at Los Alamos inferred that there was a signifi cant discrepancy between the change in thermal disorder at the transition and the amount of change needed in the Kondo volume collapse scenario. In 2001, Maxim Dzero, Lev Gor’kov, and Anatolii Zvezdin from the National High Magnetic Field Laboratory in Tallahassee, Fla., suggested that if the transition is due to the competition between thermal disorder of the localized

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