We consider the Lamb-Mössbauer factors corresponding to the structure factors of a material (ethyl alcohol) showing two glassy phases, one of those being an orientationally disordered crystal. The deviations from the idealized Gaussian behavior expected for an isotropic-harmonic vibrator within the amorphous phase and disordered crystal are found to be remarkably close. Such proximity enables us to take advantage of the crystal symmetry to gain access to specific details of the effective interparticle potential. Once this is done the treatment is extended to include the fully amorphous material. The results depict atomic motions within the glassy matrices as significantly anisotropic and highly anharmonic and thus provide a way to understand the microscopic origin of phenomena considered as fingerprints of glassy dynamics. ©2004 American Physical Society

This work was supported in part by DGICYT (Spain) Grant No. MAT2002-04540-C05 and by the Division of Material Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. W-31-109-ENG-38. F.J.B. wishes to acknowledge IPNS Division of ANL for financial support.

10 págs.; 7 figs. ; PACS number(s): 63.50.1x, 61.43.2j

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