  The magnetic properties of alkali-metal peroxychromate K$_2$NaCrO$_8$ are
governed by the $S = 1/2$ pentavalent chromium cation, Cr$^{5+}$. Specific
heat, magnetocalorimetry, ac magnetic susceptibility, torque magnetometry, and
inelastic neutron scattering data have been acquired over a wide range of
temperature, down to 60 mK, and magnetic field, up to 18 T. The magnetic
interactions are quasi-two-dimensional prior to long-range ordering, where $T_N
= 1.66$ K in $H = 0$. In the $T \to 0$ limit, the magnetic field tuned
antiferromagnetic-ferromagnetic phase transition suggests a critical field $H_c
= 7.270$ T and a critical exponent $\alpha = 0.481 \pm 0.004$. The neutron data
indicate the magnetic interactions may extend over intra-planar
nearest-neighbors and inter-planar next-nearest-neighbor spins.
