A model for verifying and developing the fundamental ideas underlying the e
rgodic hypothesis is proposed. The model describes the dynamics of the spin
subsystem formed by impurity charges with spin I and a small g factor in a
crystal immersed in a strong constant external magnetic field under condit
ions where the spin system of the nuclei in the crystal is isolated from th
e other degrees of freedom. The additive integral of motion is the projecti
on of the total spin of the subsystem onto the external field. Attention is
focused mainly on the case of I = 1/2. It is shown that the ergodic hypoth
esis holds if the correlation radius is finite in the initial state and tha
t the ergodic hypothesis is violated if the initial state is sharply locali
zed or has global correlation. The nonergodicity of the Li-8-Li-6 spin subs
ystem, which is a convenient object for experimental investigations of spin
dynamics, is revealed. An estimate is obtained for the time for transition
from a sharply localized disturbance of the canonical distribution to a qu
asiequilibrium state. (C) 1999 American Institute of Physics. [S1063-7761(9
9)02010-7].