Crystallochemical investigation of rhombic samarium-, holmium- and ytterbium trifluorides

An X-ray study of low-temperature modifications of rhombic compounds SmF3, HoF3 and YbF3 has been carried out. In all the three cases systematic attenuations suggest a mmmPsub(n)-a diffraction symmetry group with the two corresponding Fyodorov groups Dsub(2h)sup(16)=Psub(n)ma and Csub(2v)sup(9)=Psub(n)21a. The trifluorides are shown to be isostructural to cementite Fe3C, with the structure based upon distorted trigonal prisms, i.e. polyhedrons of rare earth cations. Anomalous variation of some individual cation-anion distances accounts for the unusual variation of unit cell parameters. The parameters a and b steadily decrease with the cation radius, while the parameter c, which slightly decreases from SmF3 to HoF3, sharply grows in the HoF3-LuF3 series. The variation of lattice parameters reproduces that of interatomic distances