  In an attempt to clarify conflicting published data, we report new
measurements of specific heat, resistivity, magnetic susceptibility, and
thermal expansivity up to room temperature for the 6 K superconductor ZrB12,
using well-characterized single crystals with a residual resistivity ratio >9.
The specific heat gives the bulk result 2*delta(0)/(k_B*Tc)=3.7 for the
superconducting gap ratio, and excludes multiple gaps and d-wave symmetry for
the Cooper pairs. The Sommerfeld constant gamma_n = 0.34 mJ K-2 gat-1 and the
magnetic susceptibility xsi = -2.1 x 10^-5 indicate a low density of states at
the Fermi level. The Debye temperature theta_D is in the range 1000-1200 K near
zero and room temperature, but decreases by a factor of ~2 at ~35 K. The
specific heat and resistivity curves are inverted to yield approximations of
the phonon density of states F(omega) and the spectral electron-phonon
scattering function alpha_tr^2 F(omega), respectively. Both unveil a 15 meV
mode, attributed to Zr vibrations in oversized B cages, which gives rise to
electron-phonon coupling. The thermal expansivity further shows that this mode
is anharmonic, while the vanishingly small discontinuity at Tc establishes that
the cell volume is nearly optimal with respect to Tc.
