Abstract
Analysis of lattice-spacing relationships in thorium-rich Th-Ce-Zr alloys with zirconium contents not exceeding 6 at.% shows that the large contraction for cerium atoms in dilute solution in thorium is increased by the substitution for thorium of the smaller zirconium atom. This supports the hypothesis that cerium atoms in solid solution adopt an apparent atomic diameter, by further ionisation of 4f electrons, which is strongly influenced by the magnitude of the compressive strain energy which would result from the solution in thorium of cerium in its γ-form. This strain energy is enhanced in the ternary alloys by the substitution of zirconium for thorium. The apparent atomic diameter is also decreased by a decrease in temperature to an extent greater than that expected from normal thermal contraction of the alloy, suggesting that 4f → conduction band transitions are encouraged by decrease in temperature. Such transitions occur at the expense of a small strain energy increase, and emphasise a tendency, previously noted, for cerium to change its effective valency towards that of the solvent in which it is dissolved.
Published Version
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