  Two modifications of CeCuSn were prepared from the elements: the
high-temperature (beta) modification crystallizes directly from the quenched
sample, while the low-temperature (alpha) modification forms after annealing at
700 deg C for one month. Both modifications were investigated by X-ray powder
and single crystal diffraction. We find for alpha-CeCuSn a structure of ZrBeSi
type, space group P63/mmc, a = 458.2(1), c = 793.7(2) pm, wR2 = 0.0727, 148 F2
values, 8 variable parameters. In the case of beta-CeCuSn we find the NdPtSb
type structure, space group P63mc, a = 458.4(1), c = 785.8(2) pm, wR2 = 0.0764,
233 F2 values, 11 variable parameters. The copper and tin atoms build up layers
of ordered [Cu3Sn3] hexagons. The layers are planar in beta-CeCuSn, however,
with highly anisotropic displacements of the copper and tin atoms. In
alpha-CeCuSn a puckering effect is observed resulting in a decrease of the c
lattice parameter. Both modifications of CeCuSn exhibit antiferromagnetic
ordering, however, there is a considerable difference in their magnetic
behaviour. We show the anomalies in the physical properties of the alpha- and
beta- modifications of CeCuSn by Mossbauer spectroscopy,magnetic and specific
heat measurements and explain their structure-property relations.
