Eightfold Superstructure in K2Gd2Sb2Se9 and K2La2Sb2S9 Caused by Three-Dimensional Ordering of the 5s2 Lone Pair of Sb3+ Ions

Kyoung Shin Choi, Jason A. Hanko, Mercouri G. Kanatzidis

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50 Citations (Scopus)

Abstract

The new isostructural compounds, K2Gd2Sb2Se9 and K2La2Sb2S9, were discovered by the molten polychalcogenide salt method. They crystallize in the orthorhombic space group Pbam with a=11.4880(3) Å, b=17.6612(1) Å, c=4.2201(1) Å, and Z=2 for K2Gd2Sb2Se9 and a=11.2080(5) Å, b=16.8781(8) Å, c=4.2419(2) Å, and Z=2 for K2La2Sb2S9. The compounds have a three-dimensional [M2Sb2Q9]2- framework (M=Gd, La; Q=Se, S) with K+-ion-filled channels running along the c axis. The coordination geometry around the rare earth atom is best described as a bicapped trigonal prism. The Gd3+/La3+ centered trigonal prisms share triangular faces with neighboring prisms forming one-dimensional columns along the c axis. The columns are connected to each other to form sheets by sharing Se/S atoms on the capping sites of the trigonal prisms. Sb3+ ions are stabilized in distorted octahedral sites. The SbQ6(Q=Se, S) octahedra share edges with neighboring octahedra making an infinite chain along the c axis and bridging Gd/La layers together to make the whole framework three-dimensional. In both compounds, Sb atoms appear to be positionally disordered over two crystallographically different sites with half occupancy. This disorder was removed upon elucidation of a 2a×2b×2c superstructure, which more accurately describes the positional ordering of Sb atoms in the structure. The superstructure of K2Gd2Sb2Se9 was refined in the monoclinic space group C2/m with a=22.8783(4) Å, b=8.4062(2) Å, c=20.970(1) Å, β=123.022(1)°, and Z=8. These compounds are semiconductors with band gap values of 1.33 eV for K2Gd2Sb2Se9 and 2.20 eV for K2La2Sb2S9. Magnetic susceptibility measurements indicate no apparent magnetic coupling between the Gd3+ centers showing Curie-Weiss behavior with μeff=7.96 B.M. K2Gd2Sb2Se9 melts congruently at 598°C while K2La2Sb2S9 decomposes gradually above 400°C. The Raman spectra show the diselenide stretching vibration in K2Gd2Sb2Se9 at 266 cm-1 and the disulfide stretching vibration in K2La2Sb2S9 at 473 cm-1.

Original languageEnglish
Pages (from-to)309-319
Number of pages11
JournalJournal of Solid State Chemistry
Volume147
Issue number1
DOIs
Publication statusPublished - Oct 1999

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Condensed Matter Physics
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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