  Magnetic susceptibility of the isostructural Ce(Ni{1-x}Cu{x})5 alloys (0< x
<0.9) was studied as a function of the hydrostatic pressure up to 2 kbar at
fixed temperatures 77.3 and 300 K, using a pendulum-type magnetometer. A
pronounced magnitude of the pressure effect is found to be negative in sign and
to depend strongly and non-monotonously on the Cu content, showing a sharp
maximum in vicinity of x = 0.4. The experimental results are discussed in terms
of the Ce valence change under pressure. It has been concluded that the
fractional occupation of the f-states, which corresponds to the half-integer
valence of Ce ion (3.5), is favorable for the valence instability in alloys
studied. For the reference CeNi5 compound the main contributions to magnetic
susceptibility and their volume dependence are calculated ab initio within the
local spin density approximation (LSDA), and appeared to be in close agreement
with experimental data.
