Anomalies in the Compressional and Shear Properties of Hematite in the Region of the Morin Transition

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Abstract
Compressional (vp) and shear (vs) wave velocities have been measured in a hot‐pressed (ρ=5.254 g/cc) polycrystalline hematite (α‐Fe2O3) as a function of temperature from 200° to 300°K using the pulse superposition method of McSkimin. Both vp and vs exhibit anomalous behavior in the region of the Morin transition (TM≅253°K), but the effect is much more pronounced for vs than for vp; vs at TM is changed by less than 0.02% by performing the experiment in an applied field of 650 Oe. In order to test whether the observed anomalies in vp and vs have their origin in domain wall‐stress interaction, vp was measured along the trigonal (c) axis of a high‐purity single crystal kindly lent by Y. Shapira. A sharp increase (Δvp=0.2%) was observed as the crystal was cooled through TM. In the absence of an applied field, TM increases by 3.6°K/kbar; this acoustic determination of (∂TM/∂P) is in agreement with that determined from neutron diffraction and NMR studies but differs from that measured by Kawai and Ôno. The vp jump at TM did not vanish when the experiment was repeated with a field of 650 Oe applied in the c plane; this is in marked contrast with the behavior of the Young's modulus (E) anomaly across TM observed by Makkay, Geiger, and Fine. We conclude that the vp anomaly is intrinsic to the saturated crystal.