Investigation of the electric-field gradient at the Y site inYH3

The electric-field gradient (EFG) at the Y site in ${\mathrm{YH}}_3$ has been investigated as a function of temperature by ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ perturbed angular correlation (PAC) measurements. At T≤290 K the EFG shows a strong axial asymmetry, consistent with the ${\mathrm{HoD}}_3$ structure proposed for the heavy-rare-earth trihydrides. At higher temperatures the PAC spectra reveal the existence of a second phase, which is characterized by a much stronger EFG and a higher symmetry. The two phases were found to transform reversibly into each other between 300 and 400 K.