  The distribution of chemically similar transition-metal ions is a fundamental
issue in the study of herbertsmithite-type kagome antiferromagnets. Using
synchrotron radiation, we have performed resonant powder x-ray diffractions on
newly synthesized CoCu3(OH)6Cl2, which provide an exact distribution of
transition-metal ions in the frustrated antiferromagnet. Both magnetic
susceptibility and specific heat measurements are quantitatively consistent
with the occupation fractions determined by resonant x-ray diffraction. The
distribution of transition-metal ions and residual magnetic entropy suggest a
novel low temperature (T < 4 K) magnetism, where the interlayer triangular
spins undergo a spin-glass freezing while the kagome spins still keep highly
frustrated.
