  The outer crust of a cold nonaccreting neutron star has been generally
assumed to be stratified into different layers, each of which consists of a
pure body-centered cubic ionic crystal in a charge compensating background of
highly degenerate electrons. The validity of this assumption is examined by
analyzing the stability of multinary ionic compounds in dense stellar matter.
It is thus shown that their stability against phase separation is uniquely
determined by their structure and their composition irrespective of the stellar
conditions. However, equilibrium with respect to weak and strong nuclear
processes imposes very stringent constraints on the composition of multinary
compounds, and thereby on their formation. By examining different cubic and
noncubic lattices, it is found that substitutional compounds having the same
structure as cesium chloride are the most likely to exist in the outer crust of
a nonaccreting neutron star. The presence of ternary compounds is also
investigated. Very accurate analytical expressions are obtained for the
threshold pressure, as well as for the densities of the different phases
irrespective of the degree of relativity of the electron gas. Finally,
numerical calculations of the ground-state structure and of the equation of
state of the outer crust of a cold nonaccreting neutron star are carried out
using recent experimental and microscopic nuclear mass tables.
