Overlooked trigonal symmetry in structures reported with monoclinic centred Bravais lattices; trigonal description of Li8Pb3, PtTe, Pt3Te4, Pt2Te3, LiFe6Ge4, LiFe6Ge5, CaGa6Te10 and La3.266Mn1.1S6

Abstract

About 250 inorganic structure types, reported with a monoclinic centred unit cell, were tested for overlooked trigonal symmetry and the metrics of the different “best” monoclinic cells describing an hR Bravais lattice were studied. Three-fold axes were detected in the compounds listed below, for which the crystallographic data were converted to the corresponding trigonal space groups: PtTe: hR12, R[unk]m, a = 3.963, c = 19.98 Å, Z = 6 Pt₃Te₄: hR21, R[unk]m, a = 3.988, c = 35.39 Å, Z = 3 Pt₂Te₃: hR30, R[unk]m, a = 4.003, c = 50.89 Å, Z = 6 Li₈Pb₃: hR33, R[unk]m, a = 4.757, c = 32.05 Å, Z = 3 LiFe₆Ge₄: hR33, R[unk]m, a = 5.045, c = 19.66 Å, Z = 3 LiFe₆Ge₅: hR72, R[unk]m, a = 5.048, c = 43.64 Å, Z = 6 CaGa₆Te₁₀: hR105-3, R32, a = 14.42, c = 17.65 Å, Z = 6 La_3.266Mn_1.1S₆: hR165-9.3, R3m, a = 14.08, c = 21.80 Å, Z = 15 It is recommended that these data be tested in future refinements. The recently refined “monoclinic” structure of Cs₄PbCl₆ is shown to be identical with earlier determined trigonal Cs₄PbCl₆. The structures of the Pt tellurides may be interpreted as an intergrowth of Pt₂Te₂ (PtTe-type) and PtTe₂ (CdI₂-type) layers, whereas the structure of LiFe₆Ge₅ contains 14 Å thick MgFe₆Ge₆-type slabs (alternating Zr₄Al₃- and CeCo₃B₂-type layers), stacked with a shift. In LiFe₆Ge₄ similar, but thinner slabs are found. CaGa₆Te₁₀ is a normal valence compound where all Ga atoms centre Te₄ tetrahedra to form an anionic tetrahedron complex where each individual GaTe₄ tetrahedron is connected to the surrounding tetrahedra via 6 Ga-Te-Ga links.