  Half-Heusler phases (space group F43m, C1b) have recently captured much
attention as promising thermoelectric materials for heat-to-electric power
conversion in the mid-to-high temperature range. The most studied ones are the
RNiSn-type half-Heusler compounds, where R represents refractory metals Hf, Zr,
and Ti. These compounds have shown a high-power factor and high-power density,
as well as good material stability and scalability. Due to their high thermal
conductivity, however, the dimensionless figure of merit (zT) of these
materials has stagnated near 1 for a long time. Since 2013, the verifiable ZT
of half-Heusler compounds has risen from 1 to near 1.5 for both n- and p-type
compounds in the temperature range of 500 to 900 degrees C. In this brief
review, we summarize recent advances as well as approaches in achieving the
high ZT reported. In particular, we discuss the less-exploited strain-relief
effect and dopant resonant state effect studied by the author and his
collaborators in more detail. Finally, we point out directions for further
development.
  Keywords: half-Heusler compounds; figure of merit; power density; lattice
disorder; dopant resonant states
