text: Although traditionally thought to form at compositions XYZ and XYZ, studies published after 2015 have discovered and reliably predicted Heusler compounds with atypical compositions such as XYZ and XYZ. Besides these ternary compositions, quaternary Heusler compositions called the double Half-Heusler XYYZ  (e.g. TiFeNiSb) and triple Half-Heusler XXYZ  (for e.g. MgVNiSb) have also been discovered. These "off-stoichiometric" (that is, differing from the well-known XYZ and XYZ compositions) Heuslers are mostly semiconductors in the low temperature T = 0 K limit. The stable compositions and corresponding electrical properties for these compounds can be quite sensitive to temperature and their order-disorder transition temperatures often occur below room-temperatures. Large amounts of defects at the atomic scale in off-stoichiometric Heuslers helps them achieve very low thermal conductivities and make them favorable for thermoelectric applications. The XYZ semiconducting composition is stabilized by the transition metal X playing a dual role (electron donor as well as acceptor) in the structure.