Crystal and magnetic structure of theR15Si9C compounds(R = Ho, Er, Tb)

Abstract

The synthesis of the new compoundsR15Si9C withR = Sm, Gd–Er,Y and R15Ge9C with R = Ce, Pr and Nd has been recently reported; these compounds crystallize in the hexagonalLa15Ge9Fe structuretype, hP50-P63mc,Z = 2 (orderedsuperstructure of La5Ge3 (Mn5Si3-type, hP 16-P63/mcm,Z = 2)). Here we report the results of a neutron diffraction investigation thatwe have performed to study the crystal and magnetic structures of theR15Si9C compoundswith R = Tb, Ho and Er.All three compounds see the establishment of commensurate magnetic order with a predominantlyferromagnetic interaction. Details of mixed antiferro–ferromagnetic spin arrangements (κ = [000]) (forTb15Si9C andHo15Si9C) or of purelyferromagnetic ordering (Er15Si9C), and of their temperature dependence, are given and linked to the different coordination ofthe four dissimilar rare earth sites. In the Tb and Ho compounds the thermal evolution ofthe magnetic moment values strongly differs between the different R sites. The positionoccupied by the principal carbon has been determined (Wyckoff site 2b) and the existence of asecond position available for the interstitial carbon (Wyckoff site 2a) has been revealed forR = Ho, Tb. Moreover, in the Tb and Ho compounds the magnetic moment value of the rare earth siteR4, surrounding the second interstitial carbon site, is strongly reduced if compared to the valueon the other rare earth sites. The magnetic transition temperatures of all three compounds, i.e.TC = 130, 43 and45 K for Tb15Si9C, Ho15Si9C and Er15Si9C, are remarkably high compared to those of the parentR5Si3 compounds. The magnetic behaviour of the partly filledTb5Si3C0.25 is reported.