The crystal structure of lanthanum silicon triarsenide (LaSiAs3)

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) A, z = 8. The singlecrystals were prepared by cooling the melt slowly from 10000C under the arsenic pressure of 2.5atm. 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 A] previously reported. The layer sequence along c-axis can be described as [As2] [La] [Si2As4] [La] [As2] [La] [Si2As4] [La] [As2] La-La distance is 4.27 A in the [La] layer. La is coordinated to five As's of the [Si2As4] layer in 3.09-3.18A and to four As's of the [As2] layer in 3.20-3.23A. The As-As distances in the [As2] layer are 2.99 and 3.04A. The [Si2As4] layer has the same structure as the corresponding layer of CeSip3. In this layer, alternate Si and As atoms form a two-dimensional infinite network of six-membered rings with boat conformation, Si-As distances being 2.34-2.37A. The [As2] layer seems to have no long range ordering of the zigzag chain which is observed in the CeSiP3 structure. (author)