Electric field gradients in the rare earth–aluminium compounds RAl₂ and RAl₃ studied by ^(111)Cd perturbed angular correlations

Abstract

Perturbed angular correlation (PAC) spectroscopy has been used to investigate the electric field gradient (EFG) at the probe nucleus ^(111)In/^(111)Cd in the paramagnetic phase of the rare earth (R)-aluminium compounds RAl₂ for all R elements and Y and in RAl₃ for R=Gd,Tm,Yb,Lu. The nuclear electric quadrupole interaction (QI) between the EFG and the ^(111)Cd quadrupole moment was measured as a function of temperature in the range T_c < T ≤ 1200 K. In the second half of the RAl₂ series and in the RAl₃ compounds, except for YbAl₃, the quadrupole frequency v_q shows the monotonous decrease with increasing temperature normally observed with closed-shell probe nuclei in metallic systems. In the early members of the RAl₂ series, however, v_q (T) passes through a maximum at T similar to 300 K. It is proposed that this unusual behavior reflects a contribution of the 4f shell of the R constituents to the EFG at the Al site which is quenched at higher temperatures by thermal averaging of the 4f quadrupole moment. In the intermediate-valence compound YbAl₃ the temperature dependence of the QI exhibits a shallow maximum which can be related to the temperature variation of the 4f hole occupation. Furthermore the PAC spectra provide information on the site preference of the ^111In solutes in RAl₂ for different R constituents and temperatures. In two-phase samples containing RAl₂ and RAl₃ with AuCu₃ structure, at T < 900 K the solutes show a very pronounced preference for the Al site of RAl₃, but at higher temperatures they migrate to the Al site of RAl₂. Jumps of the ^(111)In/ ^(111)Cd probes on the Al sublattice of RAl₃ compounds with AuCu₃ structure (R=Tm,Yb,Lu) lead to nuclear spin relaxation of ^(111)Cd. The temperature dependence of the relaxation rates shows an Arrhenius behavior with jump activation enthalpies E_(A)=1.6(1) eV for R=Tm, Lu and E_(A)=1.2(1) eV for R=Yb.