  We measured the specific heat, the magnetization, and the magnetoresistance
of a single crystal of ZrB12, which is superconducting below Tc ~ 6 K. The
specific heat in zero field shows a BCS-type superconducting transition. The
normal- to superconducting-state transition changes from first order (with a
latent heat) to second order (without latent heat) with increasing magnetic
field, indicating that the pure compound is a low-kappa, type-II/1
superconductor in the classification of Auer and Ullmaier [J. Auer and H.
Ullmaier, Phys. Rev.B 7, 136 (1973)]. This behavior is confirmed by
magnetization measurements. The H-T phase diagram based on specific-heat and
magnetization data yields Hc2(0) =550 G for the bulk upper critical field,
whereas the critical field defined by vanishing resistance is a surface
critical field Hc3(0) ~ 1000 G.
