  The superconducting and normal state characteristics of yttrium hexaboride
(YB$_6$) have been investigated for the single crystals with a transition
temperatures $T_c$ ranging between 6 K and 7.6 K. The extracted set of
microscopic parameters [the coherence length $\xi$(0) $\sim$ 320$\div$340
${\AA}$, the penetration depth $\lambda$(0) $\sim$ 1100$\div$1600 ${\AA}$ and
the mean free path of charge carriers $l$ = 31$\div$58 ${\AA}$, the
Ginzburg-Landau-Maki parameters $\kappa$$_{1,2}$(0) $\sim$ 3.3$\div$4.8 and the
superconducting gap $\Delta$(0) $\sim$ 10.3$\div$14.8 K] confirms the type II
superconductivity in "dirty limit" ($\xi$$\gg$ $l$) with a medium to strong
electron-phonon interaction (the electron-phonon interaction constant
$\lambda_{e-ph}$ = 0.93$\div$0.96) and $s$-type pairing of charge carriers in
this compound [2$\Delta$(0)$/k_BT_c$ $\approx$ 4]. The comparative analysis of
charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic
(heat capacity, magnetization) properties in the normal state in YB$_6$ allowed
to detect a transition into the cage-glass state at $T^*$ $\sim$ 50 K with a
static disorder in the arrangement of the Y$^{3+}$ ions. We argue that the
significant $T_c$ variations in the YB$_6$ single crystals are determined by
two main factors: (i) the superconductivity enhancement is related with the
increase of the number of isolated vacancies, both at yttrium and boron sites,
which leads to the development of an instability in the hexaboride lattice;
(ii) the $T_c$ depression is additionally stimulated by the spin polarization
of conduction electrons emerged and enhanced by the magnetic field in the
vicinity of defect complexes in the YB$_6$ matrix.
