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Theory of elastic anomalies at the gamma--alpha phase transition in solid Ce

Starting from a model of 4f-electron generated quadrupolar densities on a compressible fcc lattice, the elastic anomalies at the Fm3m -> Pa3 phase transition are studied by means of analytical theory. The model is taken as representative for the gamma-alpha phase transition in Ce. The coupling of the (linear) lattice displacements to the square of the quadrupolar orientational density fluctuations renormalizes the elastic constants. The condensation of the quadrupolar densities into the orientationally ordered Pa3 structure is studied as function of temperature and pressure. Precursor effects of the transition lead to an anomalous softening of the elastic constant c11 while c44 exhibits no such softening. The theoretical results are in excellent qualitative agreement with pressure experiments on the elastic constants (equivalently on sound velocities) at the $gamma-alpha transition in Ce. Lattice dynamical analogies in theory and striking similarities in experimental results with the Fm3m -> Pa3 transition in C60 fullerite are discussed.