Структурні дослідження халькогенідів Ce,5R1,5PbSi2S8 та Pr1,5R0,5PbSi2S8 (R – Tb, Y, Er)

Abstract

The crystal structure of the chalcogenide phases Ce0,5R'1,5PbSi2S8 and Pr1,5R'0,5PbSi2S8 (R' – Tb, Y, Er) have been determined by means X-ray powder diffraction and Raman spectroscopy. These phases are separate compositions of solid solutions of Ce2-xR'xPbSi2S8 and Pr2-xR'xPbSi2S8 (x = 1,5 and x = 0,5). It was found that these phases crystallizes in structural type La2PbSi2S8 (space group R3c , Pearson symbol hR78), which is derived from the crystalline structure of Eu3As2S8 (2Eu3+ ® 2R3+, Eu2+ ® Pb2+). Those researches made it possible to determine the parameters of a unit cell of six chalcogenide phases: a = 8.8913(4) Å, c = 26.275(2) Å (for Ce0,5Tb1,5PbSi2S8); a = 8.8848(6) Å, c = 26.169(2) Å (for Ce0,5Y1,5PbSi2S8); a = 8.8604(8) Å, c = 26.045(3) Å (for Ce0,5Er1,5PbSi2S8); a = 8.9337(7) Å, c = 26.485(2) Å (for Pr1,5Tb0,5PbSi2S8); a = 8.9390(1) Å, c = 26.504 (4) Å (for Pr1,5Y0,5PbSi2S8) and a = 8.9363(8) Å, c = 26.484(3) Å (for Pr1,5Er0,5PbSi2S8). In the structure of the chalcogenide phases Ce0,5R'1,5PbSi2S8 and Pr1,5R'0,5PbSi2S8, the rare earth element and Lead atoms are statistically distributed in crystallographic position 18e and coordinate the eight Sulfur atoms around themselves, Si atoms occupy correct point system 12c and form tetrahedras. Found that the quantitative ratio between the components R : R' = 16.7 : 50.00 and R : R' = 50.00 : 16.7 (R' – Tb, Y, Er) in correct point system 18e influences the magnitude of change in the parameters of the unit cell in series Ce0,5Tb1,5PbSi2S8 – Ce0,5Y1,5PbSi2S8 – Ce0,5Er1,5PbSi2S8 (I) and Pr1,5Tb0,5PbSi2S8 – Pr1,5Y0,5PbSi2S8 – Pr1,5Er0,5PbSi2S8. In row (I), parameter a decreases from 8.8913 to 8.8604 (Å), and с with – from 26.275 to 26.045 (Å), in row (II) the change in the unit cell parameters is practically absent. Raman researches of synthesized chalcogenides have shown that the replacement of a rare earth element in the constituent composition of solid solutions does not significantly affect their vibration properties, but in some cases can significantly affect the structural perfection of crystals. Simultaneous addition rare earth elements with ionic radii that are different, into the structure of the investigated chalcogenide phases, leads to the formation structural defects in the crystals. It appears as certain features in the low-frequency region of the Raman spectrum and in a significant increase in the halfwidth in the Raman spectra. Keywords: crystal structure; Raman scattering; rare earth metals; chalcogenides.