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 $T_0 \sim 12$ K. discovered that triclinic lattice distortion takes place $T_0$, which is further modified in subsequent antiferromagnetic phase. The structural domains can be selected by applying magnetic field, indicating some electronic ordering exists behind and affects anisotropy hidden $T_0$. transition at although order parameter still unknown, associated with maximum $c$-axis parameter. In fields along $[1, 0, 0]$, temperature, named as $T_{\text{s1}}$, deviates from $T_0$ decreases increasing whereas increases. This shows without between $T_{\text{s1}}$ results for $H \parallel [1, 1, 0]$ are also reported.