CRYSTAL STRUCTURE OF THE COMPOUNDS Er2.34Се(Pr)0.66Ge1.28S7

Abstract

Quaternary compounds Er2.34Се(Pr)0.66Ge1.28S7 are formed in the quasi-ternary systems Er2S3–R2S3–GeS2 (R–Ce, Pr) at 770 K. The structure of these compounds was determined from data sets obtained from homogeneous samples weighing 0.8 g recorded at a DRON 4-13 X-ray diffractometer, CuKα radiation, in the range of 10°≤2θ≤100°, scan step 0.05°, 20 s exposure at each point. The computation of the crystal structure of the quaternary compounds Er2.34Се(Pr)0.66Ge1.28S7 was peformed using WinCSD software package. It was determined by powder method that these compounds crystallize in the Dy3Ge1.25S7 structure type (space group P63). The new compounds are formed by substituting Dy atoms in the Dy3Ge1.25S7 structure with atoms of the statistical mixture (2.34 Er + 0.66 Се) or (2.34 Er + 0.66 Pr): r(Dy+3) = 0.0910 nm, averaged radius r(Rr+3 + Ce+3) = 0.0916 nm, r(Er+3 + Pr+3) = 0.0912 nm (was calculated on the basis of the REM content of the statistical mixture). The replacement of Ce atoms with Pr atoms leads to a decrease in the unit cell parameters which is explained mainly by a corresponding decrease in the atomic radii of the above elements. The unit cell contains 22.5 atoms. The coordinates of the atoms and the isotropic displacement parameters have satisfactory values. The 2a site contain Ge2 atoms with 0.28 occupation ratio.The atoms of the statistical mixture (Er + R) form in the structure of the quaternary compounds trigonal antiprisms [(Er + R)7S] that are spherically embedded around the [Ge26S] octahedral; the next sphere around these antiprisms is composed of the [Ge14S] tetrahedra. One of the features of such crystal structure is the formation of two-dimensional networks of sulfur atoms parallel to ab plane. The Er2.34Се(Pr)0.66Ge1.28S7 compounds have mixed covalent ionic nature. The closest packing of sulfur atoms in the Er2.34Се(Pr)0.66Ge1.28S7 structure contains one octahedral void and two tetrahedral voids per unit cell; these are occupied by Ge2 and Ge1 atoms, respectively. A row of prismatic voids that are occupied by the mixture (Er + R) is located between the rows of octahedral and tetrahedral voids. This arrangement of atoms in the structure forms the particular properties of these materials that will be a promising object for the study of physical properties.

Keywords

• рідкісноземельні метали
• кристалічна структура
• рентгенівський метод порошку