Spontaneous strain due to ferroquadrupolar ordering in UCu2Sn

The ternary uranium compound ${\mathrm{UCu}}_{2}\mathrm{Sn}$ with a hexagonal ${\mathrm{ZrPt}}_{2}\mathrm{Al}$-type structure shows a phase transition at 16 K. We reported previously that huge lattice softening is accompanied by the phase transition, which originates from ferroquadrupolar ordering of the ground-state nonKramers doublet ${\ensuremath{\Gamma}}_{5}.$ A macroscopic strain, which is expected to emerge spontaneously, was not detected by powder x-ray diffraction in the temperature range between 4.2 and 300 K. To search the spontaneous strain, we have carried out thermal expansion measurements on a single-crystalline sample along the a, b, and c axes using a capacitance technique with the resolution of ${10}^{\ensuremath{-}8}.$ In the present experiment, we found the spontaneous ${e}_{\mathrm{xx}}\ensuremath{-}{e}_{\mathrm{yy}}$ strain, which couples to the ground state doublet ${\ensuremath{\Gamma}}_{5}.$ The effect of uniaxial pressure along the a, b, and c axes on the transition temperature is also discussed.