  The electronic and local structural properties of CuO under pressure have
been investigated by means of X-ray absorption spectroscopy (XAS) at Cu K edge
and ab-initio calculations, up to 17 GPa. The crystal structure of CuO consists
of Cu motifs within CuO$_4$ square planar units and two elongated apical Cu-O
bonds. The CuO$_4$ square planar units are stable in the studied pressure
range, with Cu-O distances that are approximately constant up to 5 GPa, and
then decrease slightly up to 17 GPa. In contrast, the elongated Cu-O apical
distances decrease continuously with pressure in the studied range. An
anomalous increase of the mean square relative displacement (EXAFS Debye
Waller, \sigma$^2$) of the elongated Cu-O path is observed from 5 GPa up to 13
GPa, when a drastic reduction takes place in \sigma$^2$. This is interpreted in
terms of local dynamic disorder along the apical Cu-O path. At higher pressures
(P>13 GPa), the local structure of Cu$^{2+}$ changes from a 4-fold square
planar to a 4+2 Jahn-Teller distorted octahedral ion. We interpret these
results in terms of the tendency of the Cu$^{2+}$ ion to form favorable
interactions with the apical O atoms. Also, the decrease in Cu-O apical
distance caused by compression softens the normal mode associated with the
out-of-plane Cu movement. CuO is predicted to have an anomalous rise in
permittivity with pressure as well as modest piezoelectricity in the 5-13 GPa
pressure range. In addition, the near edge features in our XAS experiment show
a discontinuity and a change of tendency at 5 GPa. For P < 5 GPa the evolution
of the edge shoulder is ascribed to purely electronic effects which also affect
the charge transfer integral. This is linked to a charge migration from the Cu
to O, but also to an increase of the energy band gap, which show a change of
tendency occurring also at 5 GPa.
