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  • 1
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray analysis of RbTiOAsO4 (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 785-792 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for rubidium oxotitanium arsenate, RbTiOAsO4, were measured at 293 K with focused synchrotron X-radiation [λ = 0.7500 (9) Å] using a fast avalanche photodiode counter. The accurate synchrotron single-crystal data are of sufficient quality and resolution to detect the splitting positions of the Rb cations at room temperature. Strong accumulation of the Δρ difference charge density near the Rb atoms at a distance of ∼0.5 Å in the −c direction can be attributed to the partial occupancy of additional sites related by pseudosymmetry. This type of static and/or dynamic disorder is temperature-dependent and seems to be universal for the KTiOPO4 family of compounds. The best modelling of the experimental data was obtained with the Rb atom in split positions described within the harmonic approximation and the multipole functions for the other atoms. The Δρ density features in the Ti—O and As—O covalent bonds can be related to the linear and non-linear susceptibility using bond-polarization theory. The charge-density maps reflect the anisotropy of non-linear susceptibility, which is larger for directions where locally antisymmetric components of Δρ are strong.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768100006285
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  • 2
    Electronic Resource
    Electronic Resource
    Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 980-987 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for strontium/chromium- (Sr/Cr) and barium- (Ba) doped potassium titanyl phosphate (KTiOPO4, KTP) were measured with focused synchrotron X-radiation [0.75000 (9) Å] using a fast avalanche photodiode counter. Space group Pna21, Z = 8, a = 12.786 (2), b = 6.3927 (8), c = 10.5585 (9) Å, T = 293 (1) K, R = 0.028 (SrCrKTP); a = 12.851 (6), b = 6.418 (3), c = 10.620 (5) Å, T = 120 (1) K, R = 0.031 (BaKTP). The refinement of the dopant positions showed that Ba2+ is positioned in the larger of the two K cavities of KTP, while the smaller Sr2+ ion is located in both. Split positions are found for the strontium dopant in both cavities and they are located in the positive c direction from the potassium cation. The chromium dopant has two different oxidation states, namely +III and +VI; in both states the dopant is located inside the TiO6 octahedra. The two structures show slightly less distorted TiO6 octahedra than pure KTP.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768100012866
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  • 3
    Electronic Resource
    Electronic Resource
    Structure of and electron density in RbTiOAsO4 at 9.6 K (1999)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 712-720 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for rubidium titanyl arsenate, RbTiOAsO4, were measured with Mo Kα radiation (λ = 0.71069 Å) at 9.6 and 295 K. The data show that there is no phase transition between room temperature and 9.6 K. The space group is Pna21. Unit-cell parameters are a = 13.218 (1), b = 6.6500 (9) and c = 10.761 (1) Å at 9.6 K, and a = 13.261 (2), b = 6.6791 (8) and c = 10.769 (1) Å at 295 K. As the temperature was lowered from 295 to 9.6 K the Rb atoms moved along the c axis in the direction of the polarization vector, while no significant change was noted for the Ti–O–As network. Strong accumulation and polarization of the difference electron density (Δρ) in exceptionally short covalent Ti—O bonds alternates with the depleted density in long Ti—O bonds. The Δρ near the Ti atoms is polarized and aligned in the negative c direction in accordance with the ferroelectric properties of this material. However, the electron density near the Rb atoms is depleted in this direction and the excess Δρ is moved further away from the nuclei along the c vector.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S010876819900395X
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  • 4
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray analysis of the electron density in CoF2 and ZnF2 (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 128-135 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Accurate structure factors for small crystals of the rutile-type structures CoF2, cobalt difluoride, and ZnF2, zinc difluoride, have been measured with focused λ = 0.8400 (2) Å synchrotron X-radiation at room temperature. Phenomenological structural trends across the full series of rutile-type transition metal difluorides are analysed, showing the importance of the metal atom in the degree of distortion of the metal–F6 octahedra in these structures. Multipole models reveal strong asphericities in the electron density surrounding the transition metals, which are consistent with expectations from crystal field theory and the structural trends in these compounds. Transition metal 3d-orbital populations were computed from the multipole refinement parameters, showing significant repopulation of orbitals compared with the free atom, particularly for CoF2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768100017353
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  • 5
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray study of Er3Al5O12 and Yb3Al5O12 garnets (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 136-141 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for Er3Al5O12 and Yb3Al5O12 garnets were measured using focused synchrotron X-radiation, with λ = 0.7500 (2) and 0.7000 (2) Å, respectively. The difference electron density maps for Er3Al5O12 and Yb3Al5O12 were similar, as expected. This was attributed to the 4f electrons being shielded, which reduces their effectiveness in chemical bonding and the relative position of the rare-earth atoms in the periodic table. The symmetry of the difference electron density around the rare-earth atoms was found to reflect that of the cation geometry, emphasizing the importance of second nearest-neighbor interactions. This is consistent with the view that oxide-type structures may be regarded as a packed array of cations with anions in the interstices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768100019923
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  • 6
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray Imaging of the Electron Density in RFeO3 (R = Y, Ho) Using an APD Detector (1998)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 5 (1998), S. 1309-1316 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Structure factors for small hydrothermally grown yttrium and holmium orthoferrites, RFeO3 (R = Y, Ho), were measured with focused synchrotron radiation at wavelengths of 0.70 and 0.84 Å using both scintillation and high-speed avalanche photodiode (APD) detectors. Resulting APD Δρ images showed striking correlations between aspherical electron densities around Fe and rare-earth metals. Approximate high symmetry in the Δρ images indicates that cations deform the electron density far more strongly than the O atoms. The Ho—Fe magnetic interactions appear to affect the electron density distribution of the Fe atoms and the magnetic phase transitions. Space group Pnma, orthorhombic, YFeO3 (APD): Mr = 192.76, a = 5.5931 (3), b = 7.6102 (4), c = 5.2806 (3) Å, V = 224.77 (2) Å3, Z = 4, Dx = 5.695 Mg m−3, μ0.84 = 15.56 mm−1, F(000) = 356, T = 293 K, R = 0.045, wR = 0.073, S = 4.83 (9) for 1282 unique reflections; HoFeO3 (APD): Mr = 268.78, a = 5.5922 (3), b = 7.6157 (5), c = 5.2798 (3) Å, V = 224.86 (2) Å3, Z = 4, Dx = 7.939 Mg m−3, μ0.84 = 61.98 mm−1, F(000) = 356, T = 293 K, R = 0.036, wR = 0.037, S = 3.07 (6) for 1284 unique reflections.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049598001447
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  • 7
    Electronic Resource
    Electronic Resource
    Crystals of a mutant form of ribosomal protein L22 rendering bacterial ribosomes resistant to erythromycin (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 1150-1152 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A mutant form of Thermus thermophilus ribosomal protein L22 responsible for erythromycin resistance has been overexpressed in Escherichia coli, purified to homogeneity and crystallized using the hanging-drop vapour-diffusion technique. While several different crystallization conditions were found, only one set of conditions yielded crystals suitable for X-ray diffraction analysis. These crystals grow as thick plates, with unit-cell parameters a = 31.8, b = 86.59, c = 38.96 Å, β = 104.47°. The crystals belong to the space group P21 and diffract to 1.8 Å resolution. On the basis of density calculations, two monomers are predicted per asymmetric unit (VM = 2.06 Å3 Da−1), with a solvent content of 40%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S090744490100868X
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  • 8
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray study of the electron density in RFeO3 (R = Nd, Dy) (1999)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 1-7 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for small hydrothermally grown orthorhombic neodymium and dysprosium iron oxides RFeO3 (R = Nd, Dy) were measured with focused λ = 0.84 Å synchrotron radiation using a fast avalanche photodiode (APD) counter. Higher symmetry in the deformation electron density (Δρ) maps reflects the coordination of the magnetic atoms in the structure and implies that Δρ is sensitive to the cation–cation magnetic interactions. Within a given compound the local Δρ topographies around the R and Fe atoms are similar, but differ markedly between compounds. The difference in Δρ near the Nd and Dy atoms is associated with the different type of spin reorientation transitions in these compounds. The R–Fe magnetic interactions determine the magnetic phase transitions and consequently affect the electron distribution. Synchrotron radiation diffraction imaging of the electron density provides an additional tool for probing the magnetic properties of materials.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768198005400
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  • 9
    Electronic Resource
    Electronic Resource
    Synchrotron X-ray analysis of the electron density in HoFe2 (1999)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 321-326 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Structure factors for a small holmium diiron HoFe2 Laves crystal were measured with focused λ = 0.75 Å synchrotron X-radiation using a fast avalanche photodiode (APD) counter. The deformation electron density (Δρ) maps are remarkable for significant excess electron density midway between the Ho atoms, which is not dissimilar to the peaks attributed to classic `covalent bonding' in C and Si crystals. These residual electrons accumulate at the centres of the kagomé net hexagons and form, with the Fe atoms, a triangular lattice which is characterized by more stable magnetic order than the kagomé net. Similar peaks occur along the Ho–Fe and Fe–Fe contacts. These results confirm the hypothesis that 5d electrons of the rare-earth atoms are important in the spin-coupling mechanism for HoFe2-type compounds. The 5d electrons are far less localized than the 4f electrons, and considerable 5d–5d and 5d–3d orbital overlap occurs between neighbouring atoms. Aspherical electron density near the Ho nuclei can be associated with the Ho 4f subshell electrons. Strong depletions of the Δρ near the atoms along the Ho–Ho, Ho–Fe and Fe–Fe vectors are indications of exchange repulsion. The effect of anharmonicity on the Δρ is insignificant.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768198016759
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  • 10
    Electronic Resource
    Electronic Resource
    Electron density in the sodium vanadium oxide bronze β-NaxV2O5 at 9 K (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 244-250 
    Details
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure and electron density in sodium vanadium oxide bronze, β-NaxV2O5 [x = 0.282 (3)], have been studied by accurate Mo Kα X-ray diffraction measurements at 9.6 (3) K. No noticeable difference in the crystal structures at room temperature and 9.6 K has been observed. No superstructure reflections, previously found by Kanai, Kagoshima & Nagasawa [(1982), J. Phys. Soc. Jpn, 51, 697–698], have been detected at low temperature. Analysis of the deformation electron density has revealed the presence of the quasi-two-dimensional sheets of the —V—O—V—O— bonds in the structure. The electron density in the different chemical bonds within each of the three crystallographically independent VO6 polyhedra noticeably varies, although there is no clear evidence that the three crystallographically independent V atoms have different valence states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108768101002038
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