Synthesis and crystal structure of RMgSi2 compounds (R=La, Ce, Pr, Nd), a particular example of linear intergrowth

A new series of rare earth compounds with stoichiometry RMgSi2 (R=La, Ce, Pr, Nd) is reported. The single crystal X-ray diffraction showed that CeMgSi2, which melts congruently at 1200 °C, crystallizes in a new tetragonal structure type (I41/amd, tI32, a=4.2652(4) Å, c=36.830(4) Å, Z=8; wR2=0.042 (19 parameters, 393F02), R1=0.018 (297F0>4σF0). The crystal structure of CeMgSi2 can be formally built up by alternating along the z direction four CeMg2Si2-type CeMg2Si2 slabs with four AlB2-type CeSi2 slabs, one after the other. The structural model obtained from a CeMgSi2 single crystal has been confirmed for the La, Pr and Nd homologous compounds by means of Rietveld refinement. The trend of the unit-cell parameters, plotted versus the R3+ ionic radius, shows a linear behaviour, which strongly suggests a trivalent state for the Ce atoms. An analysis of the features of this new structure is reported, in comparison with the other known CeMg2Si2/AlB2-type linear intergrowth compounds.