  Critical neutron scattering measurements have been performed on
CsCo$_{0.83}$Mg$_{0.17}$Br$_3$, a dilute stacked triangular lattice (STL) Ising
antiferromagnet (AF). A two component lineshape associated with the critical
fluctuations appears at a temperature coincident with T$_{N1}$ observed in pure
CsCoBr$_3$. Such scattering is indicative of fluctuations in prototypical
random field Ising model (RFIM) systems. The random field domain state arises
in this case due to geometrical frustration within the STL Ising AF, which
gives rise to a 3 sublattice Neel state, in which one sublattice is disordered.
Magnetic vacancies nucleate AF domains in which the vacancies reside on the
disordered sublattice thereby generating a RFIM state in the absence of an
applied magnetic field.
