Structural Transition in the Hidden Ordered Phase of CeCoSi

We have performed X-ray diffraction experiments on a single crystalline CeCoSi to investigate the unresolved ordered phase below T0 ??? 12 K. We have discovered that a triclinic lattice distortion takes place below T0, which is further modified in the subsequent antiferromagnetic ordered phase. The structural domains can be selected by applying a magnetic field, indicating that some electronic ordering exists behind and affects the magnetic anisotropy in the hidden ordered phase below T0. The transition at T0, although the order parameter is still unknown, is associated with the maximum in the c-axis lattice parameter. In magnetic fields along [1 0 0], the structural transition temperature, named as Ts1, deviates from T0 and decreases with increasing the field, whereas T0 increases. This shows that the hidden ordered phase without triclinic distortion exists between Ts1 and T0. The results for H ??? [1 1 0] are also reported.