Changes of valence state of Yb in YbCuAl at high pressure: A M�ssbauer study

Abstract

The 84.3 keV Mossbauer transition in170Yb was used to investigate the intermediate valent intermetallic compound YbCuAl at pressures up to 130 kbar and in the temperature range between 1.6 K and 77 K. The electric quadrupole interaction parameters |eQVzz| and η, as well as their temperature variation are drastically dependent on pressure: |eQVzz| is at 4.2 K larger by almost a factor of three at 130 kbar when compared to ambient pressure. On the other hand, |eQVzz| drops by more than a factor of two at 130 kbar when the temperature is raised from 1.6 K to 77 K. A much smaller temperature dependence of quadrupole coupling is present at ambient pressure. Both these effects give strong evidence for a valence transition towards the 4f13 (Yb3+) configuration under applied pressure. At 4.2 K this transition is completed near ∼50 kbar. No magnetic order could be detected down to 1.6 K over the whole pressure range. Some line broadening is observed at high pressures and low temperatures which most probably is related to paramagnetic relaxation phenomena and supports the claim that the Yb ion is in a magnetic state under these conditions.