Jahn-Teller instability in a ternary uranium compound

The hexagonal compound UCu2 Sn has been believed to undergo an antiferromag netic transition at 16.6K. However, no magnetic reflection has been detecte d in the ordered phase by neutron diffraction experiments. In order to eluc idate an origin of the anomalous phase transition, we performed an elastic measurement on a single crystal of this compound. A remarkable softening co rresponding to a crystal-electric-field (CEF) splitting of Sf-electronic le vels was found in the temperature dependence of the elastic modulus. Data a nalysis taking account of a strain-quadrupole interaction strongly suggests that the electronic ground state of UCu2Sn is the non-magnetic doublet TS which has a Jahn-Teller-type instability. (C) 2000 Elsevier Science B.V. Al l rights reserved.