  The semimetallic Group V elements display a wealth of correlated electron
phenomena due to a small indirect band overlap that leads to relatively small,
but equal, numbers of holes and electrons at the Fermi energy with high
mobility. Their electronic bonding characteristics produce a unique crystal
structure, the rhombohedral A7 structure, which accommodates lone pairs on each
site. Here we show that the A7 structure can display chemical ordering of Sb
and As, which were previously thought to mix randomly. Our structural
characterization of the compound SbAs is performed by single-crystal and
high-resolution synchrotron x-ray diffraction, and neutron and x-ray pair
distribution function analysis. All least-squares refinements indicate ordering
of Sb and As, resulting in a GeTe-type structure without inversion symmetry.
High-temperature diffraction studies reveal an ordering transition around 550
K. Transport and infrared reflectivity measurements, along with
first-principles calculations, confirm that SbAs is a semimetal, albeit with a
direct band separation larger than that of Sb or As. Because even subtle
substitutions in the semimetals, notably Bi_{1-x}Sb_x, can open semiconducting
energy gaps, a further investigation of the interplay between chemical ordering
and electronic structure on the A7 lattice is warranted.
