ИСТИНА

Mineral francisite Cu3Bi(SeO3)2O2Cl is characterized by the rare combination of chemical elements met in this mineral only. It was shown that Bi can be substituted by Y or lanthanide1, 2, thus resulting in the formation of a family of isostructural compounds. This family of compounds is a good example of a situation where correlations between composition, crystal structure and physical properties can be observed and evaluated experimentally. The complex investigation can be done on Ln substituted francisites owing to the fact that the full row of rare earth compounds is available for experimental study. It should be noted, however, that the presence of lanthanides in the francisite’s structure makes its magnetic response significantly more complicated, in part due to the complex interplay of rare earth and transition metal anisotropies. The complications can be avoided in cases of nonmagnetic trivalent Ln metals, i.e. La, Eu and Lu. In this study synthesis, crystal structure and magnetic properties of Cu3M(SeO3)2O2Cl (M = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) are presented. Interrelation between chemical composition, crystal structure and magnetic behavior of francisite-like compounds is discussed. Polycrystalline samples were obtained by solid-state reaction in evacuated quartz tubes. CuO, CuCl2, SeO2, M2O3 (M = Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu), TbOCl and PrOCl were used as starting compounds. All preparatory procedures with SeO2 were carried out in a dry box purged with argon. Preliminary characterization by powder X-ray diffraction confirmed phase purity according to the previously suggested structural model. Structure refinement was performed by Rietveld method on powder samples using JANA 2006 software. As the initial model, the structural data for Cu3Bi(SeO3)2O2Cl were used. The magnetization and susceptibility data were taken over the temperature range 2–400 K in applied field strengths up to 7 T.All compounds obtained crystallize in orthorhombic Pmmn space group. A nearly linear dependence of the unit cell parameters of lanthanide selenite oxohalides with a francisite-type structure on the ionic radius of Ln3+ (CN = 8) is observed. It was shown that all compounds of francisite family reach longrange ordered antiferromagnetic state at low temperatures and exhibit field-induced metamagnetic transition prior to full saturation in moderate magnetic field.This study was supported by RFBR grant №16-03-00463a. 1. P.S. Berdonosov, V.A. Dolgikh, Russ. J. Inorg. Chem., 2008, 53, 1353- 1358. 2. R. Berrigan, B.M. Gatehouse, Acta Cryst., 1996, C52, 496-497.