  The crystalline structures of superionic high temperature copper selenides
Cu$_{2-x}$Se ($0 \le x \le 0.25$) produced by Mechanical Alloying were
investigated using X-ray diffraction (XRD) technique. The measured XRD patterns
showed the presence of the peaks corresponding to the crystalline superionic
high temperature $\alpha$-Cu$_2$Se phase in the as-milled sample, and its
structural data were determined by means of a Rietveld refinement procedure.
After a heat treatment in argon at 200$^\circ$C for 90 h, this phase transforms
to the superionic high temperature $\alpha$-Cu$_{1.8}$Se phase, whose
structural data where also determined through the Rietveld refinement. In this
phase, a very low occupation of the trigonal 32(f) sites ($\sim 3$%) by Cu ions
is found. In order to explain the evolution of the phases in the samples, two
possible mechanisms are suggested: the high mobility of Cu ions in superionic
phases and the intense diffusive processes in the interfacial component of
samples produced by Mechanical Alloying.
