  A series of garnets of formula Er3+xGa5-xO12 is described, for which we
report the crystal structures for both polycrystalline and single-crystal
samples. The x limit in the garnet phase is between 0.5 and 0.6 under our
conditions, with the Er fully occupying the normal garnet site plus
half-occupying the octahedral site at x = 0.5 in place of the Ga normally
present. Long-range antiferromagnetic order with spin ice-like frustration is
suggested by the transition temperature (TN=0.8K) being much lower than the
Curie-Weiss theta. The magnetic ordering temperature does not depend on the Er
excess, but there is increasing residual entropy as the Er excess is increased,
highlighting the potential for unusual magnetic behavior in this system.
