  Ternary nitride materials hold promise for many optical, electronic, and
refractory applications yet their preparation via solid-state synthesis remains
challenging. Often, high pressures or reactive gasses are used to manipulate
the effective chemical potential of nitrogen, yet these strategies require
specialized equipment. Here we report on a simple two-step synthesis using
ion-exchange reactions that yield rocksalt-derived MgZrN$_2$ and Mg$_2$NbN$_3$,
as well as layered MgMoN$_2$. All three compounds show nearly
temperature-independent and weak paramagnetic responses to an applied magnetic
field at cryogenic temperatures indicating phase pure products. The key to
synthesizing these ternary materials is an initial low-temperature step
(300-450 $^{\circ}$C) to promote Mg-M-N bond formation. Then the products are
annealed (800-900 $^{\circ}$C) to increase crystalline domains of the ternary
product. Calorimetry experiments reveal that initial reaction temperatures are
determined by phase transitions of reaction precursors, whereas heating
directly to high temperatures results in decomposition. These two-step
reactions provide a rational guide to material discovery of other bulk ternary
nitrides.
