MOESSBAUER DIFFRACTION STUDIES OF ANISOTROPIC CRYSTALS

JAMES ROBERT STEVENSON, Purdue University

Abstract

We report the results of three Mossbauer diffraction experiments done with the MICE detector (Mullen and Stevenson, 1978), a conversion electron detector, which uses microfoils as resonant detectors and which gives significantly increased signal-to-noise ratio with net signal comparable to that obtained with conventional detectors. This detector is useful in Mossbauer diffraction experiments and in high temperature diffusion measurements using the Mossbauer effect (Mullen, 1980). We show that the inelastic contribution to the (200) peak of LiF are small and use this result to determine the calibration constant for the MICE detector. This permits the absolute separation of the scattered radiation into elastic and inelastic components. Although the (00.2) diffuse peak in Zr-20%Nb shows a large inelastic component, we find that the modes responsible for this scattering show no visible structure in the energy range from < 10('-8) eV to 10('-6) eV. This is indicated in two ways: the elastic peak shows no broadening of the Mossbauer resonance and there are no auxilary peaks in velocity scans of the scattered radiation. These results though more extensive are consistent with the earlier experiment of Anderson and Batterman (1978). Our results augment their conclusions as to the origin of the inelastic scattering. Measurements of the temperature dependence of the elastic and inelastic scattering at the (100) peak of lT-TaS(,2) reveals two anomalies. First, the elastic scattering has a discontinuity at 79(DEGREES)C as the CDW changes from the quasicommensurate lT(,2) phase to the incommensurate lT(,1) phase. This is due to the change in the geometrical structure factor at the CDW transition. Second, the inelastic scattering increases significantly near the lT(,2) to lT(,1) transition. This peak is interpreted as inelastic critical scattering, possibly due to a softening of the phonon modes at this transition. This behavior is unusual because critical scattering is not expected to accompany a first order phase transition. Measurements of the temperature dependence of the (200) and (300) reflections of zinc give a room-temperature x-ray Debye temperature of 255 (+OR-) 7 K for reflections in the basal plane, in agreement with the literature values for this parameter. The gaussian anharmonic term in the Debye-Waller (D-W) factor for these reflections is a factor of five larger than that reported by Albanese, et al. (1976) for reflections with the scattering vector (k) along the(' ) c axis {M(,1,(PERP)) = (8.1 (+OR-) 2.5) x 10('-8) A('2)/K('2)}. The non-gaussian term is of the same magnitude as and of opposite sign to that for reflections with the k along c{M(,2,(PERP)) = (9.3 (+OR-) 1.4) x 10('-13) A('4)/K('3)}. For k in the(' ) basal plane, the non-gaussian term is non-zero at all temperatures. In contrast, Albanese, et al. (1976) reports the non-gaussian term for k along c is non-zero only above 400 K. Both the gaussian and non-(' ) gaussian anharmonic terms indicate a softening of the lattice potential with increasing temperature. The anharmonic terms in the D-W factor reported here and by Albanese, et al. (1976) do not agree with those determined by Merisalo and Larsen (1977) using room temperature neutron diffraction data but are consistent with elasticity and thermal expansion measurements.

Degree

Ph.D.

Subject Area

Condensation

COinS