We have carried out an x-ray study of solid parahydrogen-neon mixtures
within the entire concentration range and at temperatures from 5 K to
the respective solidus. The initial samples were prepared by depositi
on from the gas onto a cool substrate. At low neon fractions x(Ne) we
have found a number of anomalous phenomena: 1) the excess volume per i
mpurity is a large positive value al a striking variance with Vegard's
rule; 2) at low temperatures, two systems of hcp reflexes are observe
d with lattice parameters close to those for the pure components; 3) t
he increase in temperature up to the melting point results in that in
alloys with x(Ne) > 0.25% both systems of reflexes simultaneously chan
ge the symmetry to a fcc one, the fcc structure persisting on subseque
nt temperature cycling. The above listed anomalies can be qualitativel
y explained on the premise that the neon atom in the hydrogen lattice
together with the nearest coordinate shell forms a metastable complex.