(3 + 1)-dimensional crystal and antiferromagnetic structures in CeRuSn

At 320 K, the crystal structure of CeRuSn is commensurate with the related CeCoAl-type of structure by the doubling of the c lattice parameter. However, with lowering the temperature it becomes incommensurate with x and z position parameters at all three elemental sites being modulated as one moves along the c-axis. The resulting crystal structure can be conveniently described within the superspace formalism in (3 + 1) dimensions. The modulation vector, after initially strong temperature dependence, approaches a value close to qnuc = (0 0 0.35). Below TN = 2.8 (1) K, CeRuSn orders antiferromagnetically with a propagation vector qmag = (0 0 0.175), i.e. with the magnetic unit cell doubled along the c-axis direction with respect to the incommensurate crystal structure. Ce moments appear to be nearly collinear, confined to the a-c plane, forming ferromagnetically coupled pairs. Their magnitudes are modulated between 0.11 and 0.95 μB as one moves along the c-axis.