The electron-density distribution in crystals of CuAIO 2 was investigated on the basis of X-ray intensity data * To whom correspondence should be addressed.0108-7681/83/050564-06501.50 collected by single-crystal diffractometry. The crystal has a delafossite CuFeO 2 structure, containing the Cu ~ ion with linear twofold coordination. Several peaks observed on the difference Fourier maps after the spherical-atom refinement were well explained by assuming aspherical scattering factors of electrons in © 1983 International Union of Crystallography IntroductionThe d ~° ions such as Cu ÷, Ag r, Au ÷ or Hg 2+ have filled d orbitals and, therefore, spherical electron-density distributions are expected for these ions. However, Orgel (1966) proposed the d-s-hybridized orbital model for d l° ions with linear twofold coordinations (e.g. Cu r in cuprite, Cu20). As the energy difference between 3d and 4s orbitals is small, the 3d? orbital whose lobes extend toward the ligands can hybridize with the empty 4s orbital to form hybridized orbitals 1/X/~(d~2-s) and 1/V/2(dz 2 + s).One of the orbitals, 1/V/2(d~2 -s), has no lobe in the direction of the ligands so that the linear coordination can be stabilized by ~e aspherical electron configuration with empty 1/V/2(dz~ + s) and filled 1/v~(d,~ -s) orbitals.Delafossite CuFeO2-type compounds with formula A+B3+O2 have a very anisotropic structure (Fig. 1). It is constructed by the alternate stacking of edge-shared {BO~}oo octahedral layers and A+-ion layers perpendicular to the c axis. Each A r ion is coordinated linearly by two 0 2-ions. The A+ ion is restricted to a d '° or a d 9 ion, while many trivalent cations can enter into the octahedral site. Rogers, Shannon, Prewitt & Gillson (1971) Martin, Rees & Mitschler (1982) studied the chargedensity distribution in Mn2(CO)~ 0 from the X-ray diffraction data and refined the electron populations in the 4s orbital of Mn by the least-squares method for the first time. They obtained the electron configuration 3dS"254s TM, but the value of the 4s population is incorrect, because it exceeds the maximum population. No trial has been reported which deals with the charge-density distributions of transition-metal compounds by taking into account hybridization schemes.The present study shows that the deformation of the electron-density distribution in crystals of CuAIO2 is well explained by assuming d-s hybridization with linear combination of the 3dz~ and 4s orbitals for the valence electrons of the Cu + ion. ExperimentalPolycrystalline CuAIO 2 was prepared by solid-state reaction of Cu20 and A1203 at 1373 K in N 2 atmosphere. The cell dimensions were determined by least-squares procedure from high-angle powder X-ray diffraction data measured at 293 K using Cu K~t radiation; these dimensions are given in the Abstract together with other crystal data, which agree well with previous values (Ishiguro, Kitazawa, Mizutani & Kato, 1981; Ishiguro, Ishizawa, Mizutani & Kato, 1982). D m was measured by a pycnometer.Single crystals of CuAIO 2 w...

DREAM (Blessing, 1986) and the recently developed multipole refinement and analyses package XD (KOiitzantszk.-y et aL, 1994, we will present charge density analyses results on Creatine Monohydrate using both the Seed-Skewness method and the elliptical peak-mask method.An interactive version of this abstract can be obtain from the