  Lattice dynamics in a sigma-Fe60V40 compound, which shows a re-entrant
magnetism and orders ferromagnetic ally at ca. 170K, was investigated with the
Mossbauer spectroscopy in the temperature interval of 5-300 K. Two relevant
spectral parameters viz. the average center shift, <CS>, and the relative
recoil-free fraction, f/fo, were explored. The former yielded the Debye
temperature, T_D1, and the mean-square velocity of vibration, <v^2>, while the
latter T_D2 and the mean-square amplitude of vibrations, <x^2>. Significant
differences in the lattice-dynamical behaviors in the magnetic and paramagnetic
phases were revealed. In particular, the values of T_D were notably lower and
those of f/fo greatly higher in the former. This anomalous result has likely
its origin in a remarkably high inharmonic contribution to the vibrations found
for the ground magnetic state (spin-glass). Especially anomalous behavior vs.
temperature exhibits <x^2> where four well defined ranges could have been
identified and ascribed to the paramagnetic, ferromagnetic and two spin-glass
phases. Linear correlations between <v^2>-<x^2> were found within each of the
four ranges. They enabled determination of force constants, hence a change of
the potential energy, Ep, in each of the ranges. The total change of Ep=30 meV
while the corresponding one of the kinetic energy, determined from the
knowledge of <v^2>, was Ek=21 meV. The lack of balance between Ep and Ek
follows from the anharmonic lattice-dynamical behavior observed in the
spin-glass state. The results give a strong evidence that magnetism can
significantly affect the lattice dynamics.
