INVESTIGATION OF THE ELECTRIC-FIELD GRADIENT AT THE Y-SITE IN YH3

The electric-field gradient (EFG) at the Y site in YH3 has been invest igated as a function of temperature by Ta-181 perturbed angular correl ation (PAC) measurements. At T less-than-or-equal-to 290 K the EFG sho ws a strong axial asymmetry, consistent with the HoD3 structure propos ed for the heavy-rare-earth trihydrides. At higher temperatures the PA C spectra reveal the existence of a second phase, which is characteriz ed by a much stronger EFG and a higher symmetry. The two phases were f ound to transform reversibly into each other between 300 and 400 K.