The spin-density wave (SDW) phase of quenched (TMTSF)2ClO4 was investi
gated by both measurements of narrow band noise and H-1 NMR relaxation
rate. In the rapidly quenched phase, the sharp peak of narrow band vo
ltage noise associated with the sliding motion of SDW is abruptly broa
dened in the narrow temperature range around 1.7 K much lower than the
SDW transition temperature (5.7 K) and becomes undetectable at high t
emperature. This indicates that the spatial coherence of sliding motio
n is decreasing above this temperature. The divergent behavior in NMR
relaxation rate is also observed at 1.7 K in the rapidly quenched phas
e. These results suggests that another phase transition occurs in the
SDW phase. The temperature of this phase boundary decreases proportion
ally with the SDW transition temperature controlled by the anion quenc
hing rate. The mechanism of this transition is closely related with th
at of the SDW transition.