The Crystal Structure of Lanthanum Silicon Triarsenide (LaSiAs3)

Abstract

National Chemical Laboratory for Industry Yatabe, Tsukuba-gun, Ibaraki 305 Japan _Lanthanum silicon triarsenide (F-type), LaSiAs3, crystallizes in the orthorhombic system, space group Pbca, with a=6 084( 1 ), b=5 986( 1 ), c=26 235( 3 ), Åz=8. The single crystals were prepared by cooling the melt slowly from 1000°C under the arsenic pressure of 2.5 atm. The crystal structure was determined from 1272 independent X-ray diffraction intensities by the Patterson method, and refined to R=0.063 by the full-matrix least squares method. It has a pseudo face-centered lattice and is closely related to the layer structure of CeSiP₃ (P-type), [Pn2₁a, a=5.861, b=5.712, c=25.295 Å] previously reported. The layer sequence along c-axis can be described as
[As₂]-[La][Si₂As₄][La]As₂[La]Si₂As₄-La-As₂La-La distance is 4.27 A in the [La] layer. La is coordinated to five As's of the [Si₂As₄]layer in 3.09-3.18 Å and to four As's of the [As₂] layer in 3.20-3.23Å. The As-As distances in the [As₂] layer are 2.99 and 3.04 Å. The [Si₂As₄] layer has the same structure as the corresponding layer of CeSiP₃. In this layer, alternate Si and As atoms form a twodimensional infinite network of six-membered rings with boat conformation, Si-As distances being 2.34-2.37 A. The [As₂] layer seems to have no long range ordering of the zigzag chain which is observed in the CeSiP₃ structure.