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TIIE Cltl’STAL STRUCTUIW OF MONOCLINIC Sm&‘ BY DONT. CROMER Universily of California, Los Alamos Scientific Laboralory, Los Alamos, New Mexico Rerciued December 8 . 1968
I n this papcr the structure of the type “B” rare earth sesquioxide is reported. This form of SmaOsis monoclinic, probable space group C2/m with a = 14.177, b = 3.63r, c = 8.847, all 10.01 A,, and p 90.06 f 0.03”. There are six forrniilri units per unit cell. Thcre are layers pf composition Sm,Oa at u = 0 and = I/*. Parallel to the zy-plane, layers of oxygen atoms alternate with hivers of snrntmum s t o m . Each samarium atom has seven oxygen neighbors at a n average distance of about 2.46 A. =I
Introduction The rare earth sesquioxides have long been known to exist iri three different crystalline forms. The “A” structure t#ype is hexagonal and is observed in the rare eart)hoxides from La through Nd. The “C” type is cubic and is found in the rare earth oxides from Sm through Lii. A third form, called the “B” type, has been known to exist but its structure has heretofore been unknown. GoldSchmidt, Ulrich and Barth2 described type B as pseudotrigonal, orthorhombic or monoclinic. Crystallographic data for single crystals of Sm203,type B, recently have been published by Douglass and S t a r i t ~ k y . These ~ crystals were prepared by Dr. I. R. Tannenbaum, of this Laboratory, by fusing Smz03in an oxyacetylene flame a t 3000 to 3500”. Douglass and Staritzky found this compound to be monoclinic, space group C2/m, C2 or Cm witJh a = 14.177, b = 3.6%, c = 8.847, all fO.O1 A., and p = 09.96 =k 0.03”. Their measured density is 7.68 g./cma3and the calculated density with six formula units per unit cell is 7.74 g./ ~ 3 1 1 . ~ . After they had finished their work a single crystal fragment of the material was given to the author for a structure determination. Experimental
C2/m. Space group Cm would then require t8hat all atoms be in the twofold positions 2a. It seemed probable that such a simple compound would have more symmetry than this and the actual stxucture dct
