Gabrielle Donnay
Carnegie Institution of Washington Washington, D. C. and J. W. Gryder The Johns Hopkins University Baltimore, Maryland
I I1
Textbook Errors, 57
The Oxygen C 0 0 d h t i 0 n ~of Lithium
The prevalent notion1 that lithium has only one coordination number with oxygen calls for revision: both tetrahedral and octahedral coordinations do occur. One of the most universal substitutions throughout the mineral kingdom is Lif Al+a,usually in disorder, substituting for Mg+2 and Fe+Zin their octahedra, or trigonal autiprisms, of oxygens, hydroxyls or fluorines. The most important natural sources of lithium are the aluminum silicates LiAISitOe and K(Li, Al)&Si, A1)4010 (OH, F)f (spodumene and lepidolite), and the iron manganese phosphate Li(Fe, Mn)POa (t,i-iphylite). In these minerals, crystal structure determinat,ions ( 1 , 2 , 3 ) show Li+ to have six nearest 0, OH, F neighbors. LiN03and LiI03 are exan~plesof simple chemicals with sixfold oxygen coordination about lithium. The coordination polyhedra are octahedra, but not necessarily regular ones. Reliable Li-0 distancp range from 2.0 to 2.4 A, with an average value of 2.16 A (4). Most chemistry textbooks state or imply that lithium is always tetrahedrally surrounded by four oxygen atoms. Compounds such as LizO, LiOH, LiOH.H20, LiClO,, LiC104.3H20 and Li&03 show this coordination. Among important minerals with four oxygens about lithium are eucryptite, a-LiilSiOn (5),and petalite, LiAISi,OIO (6). Reliable Li-0-distances for 4coordination Zange from 1.86 to 2.05 A, with an average value of 1.98 A (4.. The lithium rad~uscomnronly found in the literature
+
is 0.60 A. I t is obtained from the observed Li-0 diztance of 2.00 in LizOusing an oxygen radius of 1.40 A. To compute the radius ratio (Li/O), the univalent radius of oxygen must be used: 0.60/1.76 = 0.34. The expected tetrahedral range of radius ratios extends from 0.225 to 0.414 so that 0.34, falling near the center of the range, leads to the prediction of 4-fold coordination for lithium. But crystal radii, in general, are defined for NaC1-type "standard crystals," so that an octahedral Li-0 distance should be considered, rather than the shorter tetrahedral one. Using the best experimental value, Li-0 = 2.16 A (4), for six-coordinated lithium and keeping the oxygen radius 1.40 A, we obtain a lithium radius of 0.76 8. The radius ratio (Li/O) = 0.43 then leads us to predict both 4 and 6 nearest oxygen neighbors, similar to the case for Al+3,where (A1/0) = 0.41. Thus the "prediction" can be brought in line with observations. Crystal-structure detern~inationscan now be made with a precision that makes the usefulness of numerical values for ionic radii doubtful indeed. The above observed ranges of Li-0 distances for a given coordination emphasize this point. Rather than stress a new radius for lithium ion, we wish to point out that Li+ surrounds itself with oxygens both tetrahedrally and octahedrally and does not show any predictable preference. We are indebted to Professor Rlchard bl. Noyes for calling our attention to the possible usefulness of this. Literature Cited (1) WARREN, B. E., AND BISCOE, J., Zeitsehr. Kristallogr., 80,391
Suggestions of material suitable for this column and guest columos suitable for publication directly should he sent with as many details as possible, and particularly with references to modern textbooks, to Karol J. Mysels, Department of Chemistry, University of Southern California, Lo8 Angeles 7, California. Since the oumose of this column is to orevent the snread and
standard books,
,,02,, ,As"A,.
(2) HENDRICKS, S. B., AND JEFFERSON, M. E., Am. Mineral., 24, 729 (19391. B:, AND STRUNZ, H.,Zeitsch~.Kristallogr., 81, 415 (3) GO~BNER, 118.121. --, \--
(4) "Int. Tables for X-ray Crystnllogr~phy," Kynoch Press, Birmingham, England, 1962, Val. 111, p. 258. ( 5 ) WINKLER, H. G. F., Acta C v s t . , 6,99 (1953). (6)ZEMANN-HEDLICK, A,, AND ZEMANN: J., Aeta Cryst., 8, 781 (1955).
Volume 42, Number 4, April 1965
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