Since the 1985 discovery of the phase transition at T HO ¼ 17.5 K in the heavy-fermion metal URu 2 Si 2 , neither symmetry change in the crystal structure nor large magnetic moment that can account for the entropy change has been observed, which makes this hidden order enigmatic. Recent high-field experiments have suggested electronic nematicity that breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state in the absence of magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at T HO with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure samples, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.

In Ba(Ti1- x Sn x )O3 solid solution, for Sn concentration x≤0.1, the permittivity follows the Curie-Weiss law above the Curie point T c. The increase in the peak value of the permittivity at T c is due to the change in ferroelectric transition from first order to second order with increasing x. For x>0.1, some deviations from the Curie-Weiss law are observed in a narrow temperature region above T c. The diffuse phase transition was observed. For x>0.2, the Curie-Weiss law does not hold over a wide temperature range. The dielectric dispersion was observed in the radio-frequency range (10 kHz–10 MHz). With increasing Sn concentration, the change in the phase transition from a normal ferroelectric transition to relaxor through the diffuse phase transition was observed.

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