  Half-Heusler compounds (space group Fm3m) has garnered increasing attention
in recent years in the thermoelectric community. Three decades ago, refractory
RNiSn half-Heusler compounds (R represents refractory metals such as Hf, Zr,
Ti) were found to be narrow-gap semiconductors with large Seebeck coefficients
in 100s of micro-volt per Kelvin. Today, half-Heusler (HH) compounds have
emerged as promising thermoelectric materials in the intermediate temperature
range (400-800oC). HH materials are endowed with good thermal stability and
scalability. Thermoelectric n-p modules based on HH materials demonstrate
conversion efficiency near 10% and power density output near 9 W/cm2. The
objective of this article is to present a historical account of the research
and development of thermoelectric half-Heusler compounds. Particularly, there
have been notable achievements since 2012 thanks to the emergence of new
approaches. As a result, ZT has risen from ~1 to 1.5. The various advances made
since the early 1990s to the present are recounted by categorizing half-Heusler
materials into three generations (Gen): Gen-1 Gen-2, and Gen-3 HH materials.
