Temperature-dependent 27Al quadrupole splitting in YbCuAl

Abstract

The quadrupole splitting of the 27Al NMR spectrum in the intermediate-valent (IV) compound YbCuAl has been measured as a function of temperature between 2 and 300 K. The splitting HQ between maxima of ±3/2⇄±1/2 powder-pattern satellites varies from (222±2) Oe at 10 K to (245±2) Oe above 80 K, in contrast to the nearly constant value (268±3) Oe observed in the nonmagnetic reference compound YCuAl over the same temperature range. Measurements at 4.2 K for applied magnetic fields between 2.3 and 8.2 kOe confirm that the splitting is strictly first-order quadrupolar in origin, i.e., that the temperature variation is not an artifact of the large 27Al magnetic line broadening in this system at low temperatures. The observed variation of HQ is two orders of magnitude larger than the corresponding anomalous lattice constant variation over approximately the same temperature range. This suggests considerable temperature dependence of the Yb elecronic structure, which is apparently an important contributor to the electric field gradient (EFG) at Al sites in YbCuAl. A similar temperature variation of the EFG at Yb sites has been attributed to crystal-line electric field (CEF) splitting of the Yb ionic states, but there is no indication of a CEF splitting in inelastic neutron scattering. The EFG therefore appears to reflect the breakup of the IV ground state with increasing temperature.