Distortion mode anomalies in bulk PrNiO$_3$

We investigate the emergence of the charge order in PrNiO$_3$ below the metal-insulator transition using high resolution neutron powder diffraction data analyzed in terms of symmetry-adapted distortion modes. Our analysis allows to identify the contribution of the different modes to the global distortion in a broad temperature range (10-300K), and reveals an abrupt increase of all mode amplitudes at $T_{MIT}$ = $T_{N}$ =130K. It also uncovers a previously unnoticed anomalous temperature dependence of all mode amplitudes in the insulating phase which persists down to $T = 0.6 \times \, T_{MIT}$ $\sim$ 80K. This unexpected behaviour suggests that the symmetry of the insulating phase could be actually lower, in line with the theoretically predicted appearance of additional ferroelectric distortions below $T_N$.