  Oscillation of particles in a dust crystal formed in a low-pressure
radio-frequency gas discharge under microgravity conditions is studied.
Analysis of experimental data obtained in our previous study shows that the
oscillations are highly isotropic and nearly homogeneous in the bulk of a dust
crystal; oscillations of the neighboring particles are significantly
correlated. We demonstrate that the standard deviation of the particle
radius-vector along with the local particle number density fully define the
coupling parameter of the particle subsystem. The latter proves to be of the
order of 100, which is two orders of magnitude lower than the coupling
parameter estimated for the Brownian diffusion of particles with the gas
temperature. This means significant kinetic overheating of particles under
stationary conditions. A theoretical interpretation of the large amplitude of
oscillation implies the increase of particle charge fluctuations in the dust
crystal. The theoretical estimates are based on the ionization equation of
state for the complex plasma and the equation for the plasma perturbation
evolution. They are shown to match the results of experimental data processing.
Estimated order of magnitude of the coupling parameter accounts for the
existence of the solid-liquid phase transition observed for similar systems in
experiments.
