  Recent measurements of an unusual high thermal conductivity of around 1000 W
m-1 K-1 at room temperature in cubic boron arsenide (BAs) confirm predictions
from theory and suggest potential applications of this semiconductor compound
for thermal management applications. Knowledge of the thermal expansion
coefficient and Gr\"uneisen parameter of a material contributes both to the
fundamental understanding of its lattice anharmonicity and to assessing its
utility as a thermal-management material. However, previous theoretical
calculations of the thermal expansion coefficient and Gr\"uneisen parameter of
BAs yield inconsistent results. Here we report the linear thermal expansion
coefficient of BAs obtained from the X-ray diffraction measurements from 300 K
to 773 K. The measurement results are in good agreement with our ab initio
calculations that account for atomic interactions up to fifth nearest
neighbours. With the measured thermal expansion coefficient and specific heat,
a Gr\"uneisen parameter of BAs of 0.84 +/- 0.09 is obtained at 300 K, in
excellent agreement with the value of 0.82 calculated from first principles and
much lower than prior theoretical results. Our results confirm that BAs
exhibits a better thermal expansion coefficient match with commonly used
semiconductors than other high-thermal conductivity materials such as diamond
and cubic boron nitride.
