Polynitrides
Polynitrides are solid chemical compounds with a large amount of nitrogen, beyond what would be expected from valencies. Some with N2 ions are termed pernitrides. Azides are not considered polynitrides, although pentazolates are.
They are formed at high pressures and mostly at high temperature. But some are metastable when decompressed to standard conditions. They are high energy density materials, capable of exploding with high power. Some are very hard, almost as strong as diamond.