Overlooked trigonal symmetry in structures reported with monoclinic centred Bravais lattices; trigonal description of Li₈Pb₃, PtTe, Pt₃Te₄, Pt₂Te₃, LiFe₆Ge₄, LiFe₆Ge₅, CaGa₆Te₁₀ and La_3.266Mn_1.1S₆

Abstract

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: hR 12, R [unk] m , a = 3.963, c = 19.98 Å, Z = 6 Pt 3 Te 4 : hR 21, R [unk] m , a = 3.988, c = 35.39 Å, Z = 3 Pt 2 Te 3 : hR 30, R [unk] m , a = 4.003, c = 50.89 Å, Z = 6 Li 8 Pb 3 : hR 33, R [unk] m , a = 4.757, c = 32.05 Å, Z = 3 LiFe 6 Ge 4 : hR 33, R [unk] m , a = 5.045, c = 19.66 Å, Z = 3 LiFe 6 Ge 5 : hR 72, R [unk] m , a = 5.048, c = 43.64 Å, Z = 6 CaGa 6 Te 10 : hR 105-3, R 32, a = 14.42, c = 17.65 Å, Z = 6 La 3.266 Mn 1.1 S 6 : hR 165-9.3, R 3 m , 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 Cs 4 PbCl 6 is shown to be identical with earlier determined trigonal Cs 4 PbCl 6 . The structures of the Pt tellurides may be interpreted as an intergrowth of Pt 2 Te 2 (PtTe-type) and PtTe 2 (CdI 2 -type) layers, whereas the structure of LiFe 6 Ge 5 contains 14 Å thick MgFe 6 Ge 6 -type slabs (alternating Zr 4 Al 3 - and CeCo 3 B 2 -type layers), stacked with a shift. In LiFe 6 Ge 4 similar, but thinner slabs are found. CaGa 6 Te 10 is a normal valence compound where all Ga atoms centre Te 4 tetrahedra to form an anionic tetrahedron complex where each individual GaTe 4 tetrahedron is connected to the surrounding tetrahedra via 6 Ga-Te-Ga links.