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\noindent Dear Editors of Nature Materials,\\ 
\\ 
One of the central
challenges in materials science is the rational design of functional
and multifunctional materials.  Improved and novel properties can
often be discovered by searching within a large structure family
exhibiting a great variety of properties.  In addition to providing a
large number of candidate parent materials, the existence of a common
structure family provides enormous opportunities for designing
functional solid solutions and epitaxial superlattices.  Historically,
the prototypical families of this kind have been the zincblende
semiconductors and the perovskite oxides.  However, other families,
including the Heusler and half-Heusler compounds, have started to
receive increasing attention, with several recent publications in
Nature Materials.\\ 
\\ 
In our manuscript, entitled ``Half-Heusler
semiconductors as piezoelectrics,'' we point out that the half-Heusler
family has been completely overlooked as a very promising source of
piezoelectric materials. While we could find no report in the
literature of any previous discussion or measurement of a
piezoelectric coefficient in this class of compounds, our first
principles results show that many known semiconducting half-Heusler
compounds have a large piezoelectric response. Furthermore, a
high-throughput first-principles search of a large space of
hypothetical half-Heusler materials reveals compounds with even better
performance.\\ 
\\ 
Our first-principles analysis thus provides guidance
for experimental efforts at directed synthesis to realize new
materials with enhanced piezoelectric properties suitable for
practical applications, further extending the multifunctionality of
the half- Heusler family. In addition, the implementation of our
high-throughput first-principles approach advances the development of
rational materials design. Our paper will thus be of great interest to
both experimental and theoretical readers of Nature Materials.\\ 
\\
Thank you for your consideration,\\ 
\\ 
Prof. Karin M. Rabe\\
Department of Physics and Astronomy\\ Rutgers, the State University of
New Jersey\\ Piscataway, NJ 08854\\ rabe@physics.rutgers.edu

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