  Pressure effect on superconducting properties of two YB6 samples (Tc = 5.9
and 7.5 K) were investigated by measurements of electrical resistivity,
magnetic susceptibility, and X-ray diffraction in the pressure range up to 320
kbar. Magnetoresistivity measurements down to 60 mK and up to 47 kbar have
shown a negative pressure effect on Tc as well as on the third critical field
Hc3 with the slopes dlnTc/dp = -0.59%/kbar and dlnHc3/dp = -1.1%/kbar,
respectively. The magnetic susceptibility measurements evidenced that the slope
of dlnTc/dp gradually decreases with pressure reaching 3 times smaller value at
112 kbar. The lattice parameter measurements revealed the volume reduction of
14% at 320 kbar. The pressure-volume dependence is described by the Rose-Vinet
equation of state. The obtained relative volume dependence dlnTc/dlnV analyzed
by the McMillan formula for Tc indicates that the reduction of the
superconducting transition temperature is mainly due to hardening of the
Einstein-like phonon mode responsible for the superconducting coupling. This is
confirmed by the analysis of the resistivity measurements in the normal state
up to T = 300 K performed at pressures up to 28 kbar.
