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Structure of Potassium Calcium Hexanitrocuprate( 11) at 295°K. Evidence for a Static Jahn-Teller Distortion Shozo Takagi,Ia P. Galen Lenhert,*Ih and Melvin D. Joesten*'" Contribution f i o m the Departments of Chemistry und Physics, Vanderbilt Unirjersity , Nashville, Tennessee 37235. Received February 4, 1974 Abstract: Crystals of potassium calcium hexanitrocuprate(II), K2CaCu(NOs)6, usually have two or more orthorhombic lattices, space groyp Fmmm, with identical unit cells containing four formula units (u = 10.743 (3), b = 10.325 ( 2 ) , c = 10.318 (2) A). Data collected from a crystal with two lattices were used to solve and refine each of the two crystal structures which showed identical hexanitrocuprate(I1)ions with different orientations. A more precise result was obtained from H crystal showing negligible diffraction from subsidiary lattices. These data (fourcircle diffractometer, 28 scan, Mo K a radiation 0 < 20 < 90", 1310 independent reflections) were refined by fullmatrix least squares to R, = 0.032. The C L I Ngroup ~ has tetragonal symmetry with Cu-N distances of 2.313 (l), 2.052 (l), and 2.050 (1) A. This structure is compared with preliminary results for orthorhombic K2PbCu(N01)6 at 193'K (cell dimensions of a = 10.72 (l), b = 10.74 (l), c = 10.51 (1) A) which suggest the presence of a compressed tetragonal distortion. These results from K2CaCu(N02)6at 295°K and K2PbCu(N02)6 at 193°K provide direct experimental evidence for the static-dynamic Jahn-Teller distortion.
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he high symmetry of Cu(I1) in tris(octamethylpyr0phosphoramide)copper(II) perchlorate, potassium lead hexanitrocuprate(II), and tris(ethy1enediamine)copper(I1) sulfate4 has been attributed to a dynamic Jahn-Teller effect. Low temperature X-ray5 and epr results6 for the first compound are consistent with the dynamic-static Jahn-Teller theory proposed by Abragam and P r y ~ e . 7 , This ~ theory assumes the presence of three mutually perpendicular tetragonal distortions of equal energy. Rapid oscillation among the three distortions gives an average structure with high symmetry, but at low temperatures static distortions appear. The transition temperature depends on the compound. We are studying several of these compounds by X-ray crystallography at temperatures where the static distortions are present in order to provide experimental data on the magnitude and direction of distortions which are attributable to the Jahn-Teller effect. Systems which appear to be undergoing a dynamic-static Jahn-Teller effect are ideal since other factors which may cause distortions are either not present or negligible. A recent report'" on the low temperature X-ray study of the static distortion around Co(I1) in the strong field environment of K2BaCo(N0& indicates the feasibility of this approach. Complete structure determinations for the series K2MCu(N02),, where M is Pb, Ba, or Ca are particularly appropriate for several reasons. X-Ray powder data" have shown that the symmetry of the unit cell of (1) (a) Department of Chemistry. (b) Department of Physics. (2) M. D. Joesten, M. S . Hussain, and P. G. Lenhert, I m r g . Cheni., 9, 151 (1970). (3) D. L. Cullen and E. C. Lingafelter, Inorg. Chem., 10, 1264 (1971). (4) D. L. Cullen and E. C. Lingafelter, Inorg. Chem., 9, 1858 (1970). (5) S. Takagi, P. G. Lenhert, and M. D. Joesten, unpublished results. ( 6 ) R. C. Koch, M. D. Joesten, and J. H. Venable, Jr., J . Chem. Phys., 59,6312 (1973). (7) A. Abragam and M. H. L. Pryce, Proc. Phys. Soc., Loiidon, Sect. A , 63, 409 (1950). ( 8 ) A. Abragam and M. H. L. Pryce, Proc. Roj,. Soc., Ser. A , 206, 164 (1951). (9) P. T. Miller, P. G. Lenhert, and M. D. Joesten, Inorg. Chem., 12,218 (1973). (10) J. A. Bertrand, D. A. Carpenter, and A. R. Kalyanaraman, Inorg. Chim. Acta, 5, 113 (1971).
K2PbCu(N0& is lowered either by replacing Pb(I1) with Ca(I1) or with Ba(I1) or by lowering the temperature. Hathaway, Dudley, and Nicholls12 have predicted an elongated tetragonal environment for Cu(I1) in the barium salt on the basis of single-crystal polarized electronic spectra and unit cell parameters. However, Harrowfield and Pilbrow have interpreted singlecrystal epr studies of K2PbCu(N0& below 280°K in terms of tetragonally compressed environment for Cu(11). Since the corresponding K?MNi(NOs)6series is cubic for M = Ca, Ba, or Pb,".l4 the differences observed for the KeMCu(NO& series must be due to Jahn-Teller effects. Complete structure information on K,Ca(N02)s and K2BaCu(N02)6at room temperature and K2PbCu(NO2& at low temperature would verify the type of distortion and provide information on the magnitude of the distortion. The present paper is a re,,ort of the potassium calcium hexanitrocuprate(I1) structure at 295 OK. Experimental Section Greenish black crystals of K2CaCu(NOl)6were obtained by the method of Elliott. Hathaway. and Sladell with the substitution of copper(I1) nitrate for copper(l1) acetate. Photographs of several different crjstals at room temperature showed two or more lattices with one usually more prominent than the others. When two lattices are about equally prominent. h e observe that they represent identical unit cells but with one lattice rotated from the other. The first lattice can be brought into coincidence with the second by a 90- rotation about ( I ( b into c) followed by a 92.2- rotation about the new direction of b (c into ( I ) . Thus LI c, h o. and c b. approximately. Crystals which contain more than one lattice are not transparent. Precession photographs showing systematic absences for hhl with /I + X. X t 1. or / I + I odd indicated one of three orthorhombic space groups: Fmmni. Fmnl2. and F222.Ia The space group Fmii7m \+as assumed and the successful refinement of the structure confirms that choice.
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(11) H. Elliott, B. J. Hathaway, and R. C. Slade, I w r g . Cbem., 5, 669
(1966). (12) B. J. Hathab\ay, R . J. Dudley, and P. Nicholls, J . Cheni. SOC.A , 1845 (1969). (13) B. V. Harro\\field and J. R. Pilbrow, Proc. P ~ J , sSoc., . Lotidoiz (Solid Srure P b j , ~ . )6,, 755 (1973). (14) D. Reinsn, C . Friebcl, a n d I
