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This structure has proved rather controversial. (Zachariasen 1926, 1929) originally proposed that the crystal structure of La2O3 and other lanthanide series oxides belong to trigonal space group $P321$ #150. This was immediately criticized by (Pauling, 1929), who suggested the trigonal space group ($P\bar{3}m1$ #164). This was confirmed by (Koehler, 1953). Much later, (Aldebert, 1979) stated that the structure was hexagonal, space group $P6_3/mmc$ #194. This would represent a doubling of the $P\bar{3}m1$ unit cell in the $z$-direction. However, they give structural parameters, used by us and by (Villars, 1991, 2016), which have a density consistent with the $P\bar{3}m1$ structure, and are in reasonable agreement with lattice parameters given in (Pearson, 1958) and (Koehler, 1953). We agree with (Villars, 1991, 2016) that these are close to the correct values for the structure. This structure is very similar to Al$_{3}$Ni$_{2}$ ($D5_{13}$, A3B2_hP5_164_ad_d). We follow (Pearson, 1958) and assign the intermetallics as $D5_{13}$, keeping $D5_{2}$ for oxides and related compounds. The ternary Ce2O2S structure, aka CaAl2Si2, is related to this structure, the sulfur atom replacing the oxygen on the (1a) site.
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W. Zachariasen, Die Kristallstruktur der A–Modifikation von den Sesquioxyden der seltenen Erdmetalle. (La2O3, Ce2O3, Pr2O3, Nd2O3), Z. Phys. Chem. 123U, 134–150 (1926), doi:10.1515/zpch-1926-12309.
W. Zachariasen, Kürzere Originalmitteilungen und Notizen, Zeitschrift für Kristallographie – Crystalline Materials 70, 187–189 (1929), doi:10.1524/zkri.1929.70.1.187.
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W. C. Koehler and E. O. Wollan, Neutron–diffraction study of the structure of the A–form of the rare earth sesquioxides, Acta Cryst. 6, 741–742 (1953), doi:10.1107/S0365110X53002076.
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