  The crystal structure of the superconductor MgCxNi3 is reported as a function
of carbon concentration determined by powder neutron diffraction. The
single-phase perovskite structure was found in only a narrow range of carbon
content, 0.88 < x < 1.0. The superconducting transition temperature was found
to decrease systematically with decreasing carbon concentration. The
introduction of carbon vacancies has a significant effect on the positions of
the Ni atoms. No evidence for long range magnetic ordering was seen by neutron
diffraction for carbon stoichiometries within the perovskite phase stability
range.
