  The merger of two neutron stars or of a neutron star and a black hole
typically results in the formation of a post-merger accretion disk. Outflows
from disks may dominate the overall ejecta from mergers and be a major source
of r-process nuclei in our universe. We explore the parameter space of such
disks, their outflows, and r-process yields by performing three-dimensional
general-relativistic magnetohydrodynamic (GRMHD) simulations with weak
interactions and approximate neutrino transport. We discuss the mapping between
initial binary parameters and the parameter space of resulting disks, chiefly
characterized by their initial accretion rate. We demonstrate the existence of
an ignition threshold for weak interactions at around $\sim 10^{-3}
M_\odot\mathrm{s}^{-1}$ for typical parameters by means of analytic
calculations and numerical simulations. We find a degenerate, self-regulated,
neutrino-cooled regime above the threshold and an advection dominated regime
below the threshold. Excess heating in the absence of neutrino cooling below
the threshold leads to $\gtrsim 60\%$ of the initial disk mass being ejected in
outflows, with typical velocities $\sim (0.1-0.2)c$, compared to $\lesssim
40\%$ at $\sim (0.1-0.15)c$ above the threshold. While disks below the
threshold show suppressed production of light r-process elements, disks above
the threshold can produce the entire range of r-process elements in good
agreement with the observed solar system abundances. Disks below the ignition
threshold may produce an overabundance of actinides seen in actinide-boost
stars. As gravitational-wave detectors start to sample the neutron star merger
parameter space, different disk realizations may be observable via their
associated kilonova emission.
