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
La3.266Mn1.1S6: 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.