Space groups trigonal/hexagonal

Note Trigonal space group R3c (hexagonal setting) number of formula units of Lap gSrp 4Cop gFep 2O3 in a unit cell Z = 6. Unit-cell parameters a = h = 5.46974(18) A, c = 13.3693 (5) A, a = p = 90°, 7 = 120° unit-cell volume 346.40(2) A g, occupancy x, y, z, fractional coordinates f/, isotropic atomic displacement parameters equivalent isotropic atomic displacement parameter. 

Another funny problem is how to view the relationship between trigonal, hexagonal, and rhombohedral space groups. For an interesting discussion see Ashcroft and Mermin [5 p. 125 footnote]. 

Table 4 Numbers and stacking schemes of close-packed [AF3] layers in trigonal and hexagonal fluoride perovskites , elpasolites , and derivatives and typical compounds (cubic perovskite included, structure types in italics, space groups in parentheses)...

The high-temperature /3 form is also hexagonal, belonging to the space group POs/mmc. The cell constants are a = 397 and c = 3175 pm 114). The thorium is 6-coordinate with layers of trigonal prismatic and antiprismatic iodine coordination. The Th-I distance is 320 pm, very similar to the values for Thl4 and This. The Th-Th distance is 397 pm. 

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