The electron density distribution and 3d-orbital electron occupancies for the Fe atom in synthetic triphylite, LiFePO4, have been analysed using single-crystal X-ray diffraction data measured at T= 298 K with Mo Ka (3. =0.71069 ,~) radiation to a resolution corresponding to (sinOmax/3.)= 1.078 ,~-~. Measurements of 3265 reflections gave 944 unique data [Rint(I) = 0.036] with I > 2o,(1). For an atomic multipole density model fitted by least-squares methods R(F) = 0.0174 for all unique reflections. The Fe atom 3d-orbital occupancies have been derived from the multipole population coefficients using point-groupspecific relations. The asphericity of the electron deformation density around the Fe atom is discussed using crystal-field theory and magnetic properties of triphylite. Crystal data: lithium iron(II) phosphate, IntroductionThe crystal structure of LiFePO4, which has been studied by Yakubovich, Simonov & Belov (1977) and described in more detail by Yakubovich, Belokoneva, Tsirelson & Urusov (1990), has an olivine-type structure with a distorted hexagonal * Author to whom correspondence should be addressed. Present address: Crystallography Centre, University of Western Australia, Nedlands, Western Australia 6009, Australia.0108-7681/93/020147-07506.00 anion close packing, where the cations occupy three different positions: a tetrahedral (P) site and two octahedral sites. An ORTEP (Johnson, 1965) plot showing the cation coordination is given in Fig. 1. One octahedral site lies at the inversion centre and the other is in the mirror plane. As usual in olivinetype structures the former octahedral site is occupied by cations with smaller charge (Li) and the latter by cations with larger charge (Fe). The main feature of the LiFePO4 crystal structure consists of olivine-type ribbons extending along the b crystal axis. The Li octahedra protrude from the olivine ribbon and are connected along their edges and with the larger Fe © 1993 International Union of Crystallography 148 LiFePO4 octahedra. PO4 tetrahedra have three of the six edges in common with the cation octahedra. These edges have the shortest lengths and differ significantly from the other O---O distances.The magnetic structure of LiFePO4 has been determined by Santoro & Newnham (1967) from neutron diffraction data. Below the N~el temperature TN = 50 K, the spin vectors associated with these Fe-atom positions are antiparallel and align in an antiferromagnetic array collinear with the b axis. The magnetic space group is Pnma'. AS has been shown (Santoro & Newnham, 1967), the only Fe O---Fe superexchange interactions give rise to antiferromagnetic puckered planes orthogonal to a. There are no direct or superexchange linkages between these planes, and long-range interactions, such as Fe---O---P--O--Fe triple exchange, have been suggested.The present paper describes a study of the electron deformation density in synthetic crystalline triphylite, LiFePO4. A multipole refinement of the X-ray diffraction data has been carried out using different approxim...

Full concentration series of tetragonal melilite‐like Ca2−xNdxGa2+xSi1−xO7 single crystals with desordered structure are grown using Czochralski technique and their precies X‐ray structural analysis is carried out. Intensity, absorption, and luminescence characteristics including orientational ones of Nd2+ ions in Ca2Ga2SiO7 are investigated. Using conventional lamp‐pumping technique pulse stimulated emission at the wavelengths of two channels (4F3/2 → 4I11/m, 13/2) is excied and investigated both, at low and room temperatures. Raman spectra of Ca2Ga2SiO7 are also measured.

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Precise X-ray and high-energy transmission electron diffraction methods were used for the study of electron density and electrostatic potential in MgO crystals. The structure amplitudes were determined and their accuracy estimated using ab initio Hartree-Fock structure amplitudes as criteria. The electrostatic potential distributions, reconstructed using Fourier series from both X-ray and electron diffraction data, are in satisfactory mutual agreement and are similar to the theory. They, however, suffer from restricted experimental resolution and, therefore, the reconstruction of the electrostatic potential via an analytical structural model is preferable. The x model of electron density was adjusted to X-ray experimental structure amplitudes and those calculated by the HartreeFock method. The electrostatic potential, deformation electron density and the Laplacian of the electron density were calculated with this model. The critical points in both experimental and theoretical model electron densities were found and compared with those for procrystals from spherical atoms and ions. A disagreement 11 concerning the type of critical point at (Z,Z,0) in the area of low, near-uniform electron density is observed. It is noted that topological analysis of the electron density in crystals can be related with a close-packing concept.