  Chemically active Brownian particles with surface catalytic reactions may
repel each other due to diffusiophoretic interactions in the reaction and
product concentration fields. The system behavior can be described by a
`chemical' coupling parameter $\Gamma_c$ that compares the strength of
diffusiophoretic repulsion to Brownian motion, and by a mapping to the
classical electrostatic One Component Plasma (OCP) system. When confined to a
constant-volume domain, Body-Centered Cubic crystals spontaneously form from
random initial configurations when the repulsion is strong enough to overcome
Brownian motion. Face-Centered Cubic crystals may also be stable. The `melting
point' of the `liquid-to-crystal transition' occurs at $\Gamma_c\approx140$ for
both BCC and FCC lattices.
