Soliton lattice to metal: A first order phase transition

$\text{In-situ}$ measurements of the electron spin resonance of Na⁺_y (CH)^?y]_x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.