E.N. Maslen

Electron density and optical anisotropy in rhombohedral carbonates. III: Synchrotron X-ray studies of CaCO3, MgCO3 and MnCO3

Diffraction-deformation electron-density (Δp) images for small, naturally faced single crystals of synthetic calcite (CaCO 3 ), magnesite (MgCO 3 ) and mineral rhodochrosite (MnCO 3 ) were measured with focused λ = 0.7 and 0.9 A synchrotron (SR) X-radiation. Mo Kα (λ = 0.71073 A) structure factors were also measured for MnCO 3 . Lattice mode frequencies predicted from eigenvalues of T and L tensors for CO 3 rigid-group motion in these structures are close to spectroscopic values. High approximate Δp symmetry around the cations increases towards 6/mmm in the sequence CaCO 3 , MgCO 3 to MnCO 3 . The Δp topography near the CO 3 groups shows the influence of the cations, and correlates strongly with the refractive indices, as required for a cause and effect relationship between electron density and optical anisotropy. Aspherical electron density around the Mn atom can be attributed to the effect of a non-ideal octahedral crystal field on the 3d electron distribution. The relationship of the Δp topography near the Mn atom with that near the CO 3 group in MnCO 3 is consistent with magnetic interactions. Space group R3c, hexagonal, Z = 6, T= 295K : CaCO 3 , M r = 100.09, a = 4.988(2), c = 17.068(2) A, V = 367.8(3) A 3 , D x = 2.711 Mg m -3 , μ 0.7 = 1.93 mm -1 , F(000) = 300, R = 0.015, wR = 0.012, S = 3.0 for 437 unique reflections ; MgCO 3 , M r = 84.31, a = 4.632(1), c = 15.007 (2) A, V = 278.8 (2) A 3 , D x = 3.013 Mg m -3 , μ 0.9 = 0.99 mm -1 , F(000) = 252, R = 0.015, wR = 0.021, S = 4.34 for 270 unique reflections ; MnCO 3 , M r = 114.95, a = 4.772(3), c = 15.637 (3) A, V = 308.4 (4) A 3 , D x = 3.713 Mg m -3 , p 0.7 = 5.62 m -1 , F(000) = 330, R = 0.015, wR = 0.039, S = 3.38 for 386 unique reflections of the SR data set and a = 4.773 (1), c = 15.642(1) A, V = 308.6 (1) A 3 , D x = 3.711 Mg m -3 , μ(Mo Kα) = 5.86 mm -1 , R = 0.017, wR = 0.024, S = 2.79 for 368 unique Mo Kα reflections.

X-ray Study of the Electron Density in Calcite, CaCO3

The electron density in synthetic calcite, CaCO 3 , has been determined using diffraction data for a naturally faced single crystal measured with X-ray Mo Kα (λ=0.71073 A) radiation. Extinction corrections that minimize differences between equivalent reflection intensities are closely approximated by the values which optimize the extinction parameter as part of the least-squares structure refinement. Deformation electron densities evaluated with the two techniques are closely similar. There are 0.26 e A -3 high-density maxima in the C-O bonds and 0.28 e A -3 maxima at the 0-atom lone pairs

Synchrotron X-ray study of the electron density in α-Al2O3

Structure factors for synthetic haematite, α-Fe 2 O 3 , have been measured for two small crystals using focused λ=0.7 A synchrotron radiation. The structure factors from the two data sets are consistent. Approximate symmetry in the concordant densities, related more closely to the Fe-Fe geometry than to the nearest-neighbour Fe-O interactions, is similar to that in the corundum α-Al 2 O 3 structure. Deformation density maxima are located at the midpoint of the Fe-Fe vector along the c axis, on a common face for O-octahedra, perpendicular to c. Maxima also occur at the midpoint of the Fe-Fe vector bisecting the edges of the O-octahedra. These results are in accordance with theoretical predictions for metal-metal bonding

A synchrotron X-ray study of the electron density in C-type rare earth oxides

Structure factors for small synthetic crystals of the C-type rare earth (RE) sesquioxides Y2O3, Dy2O3 and Ho2O3 were measured with focused λ = 0.7000 (2) A, synchrotron X-radiation, and for Ho2O3 were re-measured with an MoKα (λ = 0.71073 A) source. Approximate symmetry in the deformation electron density (Δρ) around a RE atom with pseudo-octahedral O coordination matches the cation geometry. Interactions between heavy metal atoms have a pronounced effect on the Δρ map. The electron-density symmetry around a second RE atom is also perturbed significantly by cation–anion interactions. The compounds magnetic properties reflect this complexity. Space group Ia{\bar 3}, cubic, Z = 16, T = 293 K: Y2O3, Mr = 225.82, a = 10.5981 (7) A, V = 1190.4 (2) A3, Dx = 5.040 Mg m−3, μ0.7 = 37.01 mm−1, F(000) = 1632, R = 0.067, wR = 0.067, S = 9.0 (2) for 1098 unique reflections; Dy2O3, Mr = 373.00, a = 10.6706 (7) A, V = 1215.0 (2) A3, Dx = 8.156 Mg m−3, μ0.7 = 44.84 mm−1, F(000) = 2496, R = 0.056, wR = 0.051, S = 7.5 (2) for 1113 unique reflections; Ho2O3, Mr = 377.86, a = 10.606 (2) A, V = 1193.0 (7) A3, Dx = 8.415 Mg m−3, μ0.7 = 48.51 mm−1 F(000) = 2528, R = 0.072, wR = 0.045, S = 9.2 (2) for 1098 unique reflections of the synchrotron data set.

A synchrotron X-ray study of the electron density in YFeO3

Structure factors for synthetic yttrium orthoferrite, YFeO 3 , measured with MoKα (λ = 0.71073 A) X-radiation and using a smaller crystal with focused λ = 0.75 A synchrotron radiation are broadly consistent. Approximate symmetry in concordant difference densities indicates that cations deform the electron density more strongly than O atoms, although the deformation-density (Δp) map symmetry is lower than that expected from Fe cation interactions alone. The maps local symmetry is influenced strongly by the Y cations, but not by neighbouring O anions. Space group Pnma, orthorhombic, M r = 192.76, a = 5.5877 (3), b = 7.5951(4), c = 5.2743 (2) A, V = 223.84 (2) A 3 , Z = 4, D x = 5.719 Mg m -3 , μ 0.75 = 29.82 mm -1 , F(000) = 356, T = 293 K, R = 0.026, wR = 0.028, S = 5.4 (1) for the 1039 unique reflections of the synchrotron data set.

A synchrotron X-ray study of the electron density in SmFeO3

Structure factors for small, hydrothermally grown samarium orthoferrite, SmFeO3, were measured with focused λ = 0.7 A synchrotron radiation. Approximate high symmetry in the Δρ images indicates that cations deform the electron density far more strongly than the O atoms. The most obvious effect is on distribution of the Fe atoms. The influence of the nearest low-symmetry (Cs) O coordination on the electron density of the Sm cation is weak by comparison with that of the Sm–Fe interactions, as is illustrated by the high symmetry of the Δρ map near the Sm atom. The Sm–Fe interactions appear to affect the magnetic ordering and spin configuration of the Fe atoms. Space group Pnma, orthorhombic, Mr = 254.20, a = 5.6001 (3), b = 7.7060 (7), c = 5.3995 (6) A, V = 233.01 (4) A3, Z = 4, Dx = 7.246 Mg m−3, μ0.7 = 28.5 mm−1, F(000) = 448, T = 293 K, R = 0.017, wR = 0.021, S = 3.83 (8) for 1329 unique reflections.

Accurate synchrotron radiation Δρ maps for K2SiF6 and K2PdCl6

Difference electron densities for the structurally isomorphic title compounds have been measured using 0.9 and 0.7 A synchrotron radiation as well as Mo Kα (λ=0.71069 A) radiation. The merits of using synchrotron radiation for such accurate electron density studies are confirmed. The noise level in the 0.9 A K 2 SiF 6 Δρ maps is low, and justifies confidence in physical properties predicted from the one-electron density. Similar Δρ features are present in both compounds. The redistribution of electron density indicated by the Δρ maps, which mainly reflects the effect of exchange interactions, does not support simple predictions from ionic and orbital models for the bonding in these structures

Synchrotron X-ray study of Er3Al5O12 and Yb3Al5O12 garnets.

Structure factors for Er(3)Al(5)O(12) and Yb(3)Al(5)O(12) garnets were measured using focused synchrotron X-radiation, with lambda = 0.7500 (2) and 0.7000 (2) A, respectively. The difference electron density maps for Er(3)Al(5)O(12) and Yb(3)Al(5)O(12) 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.

Synchrotron X-ray electron density in the layered LaOCl structure

The deformation density Δρ in lanthanum oxychloride, LaOCl, has been determined for a small, naturally faced single-crystal using 0.7 A synchrotron X-radiation. Accumulation of electron density in the interlayer region stabilizes the structure that contains layers of positively charged Cl atoms. Space group P4/nmm, tetragonal, Mr = 190.36, a = 4.1198 (7), c = 6.883 (2) A, V = 116.8 (5) A3, Z = 2, Dx = 5.411 Mg m−3, μ0.7 = 17.28 mm−1, F(000) = 164, T = 293 K, R = 0.011, wR = 0.011, S = 1.98 (7) for 416 unique reflections.

X-ray study of the electron density in magnesite MgCO3

The electron density in a naturally faced mineral crystal of MgCO 3 was re-measured with Mo Kα (λ= 0.71073 A) diffraction data, using extinction corrections that minimize differences between equivalent reflection intensities. The deformation electron density has 0.66 e A -3 , density maxima in the C-O bonds and 0.35 e A -3 maxima at the O-atom lone pairs. Space group R3c, hexagonal, M r =84.31, a= 4.635 (1), c=15.023 (2) A, V=279.5 (1) A 3 , Z=6, D x =3.005 Mg m -3 , μ (Mo Kα)=0.48 mm -1 , F(000)=252, T=293 K, R=0.022, wR=0.025, S= 5.87 for 332 unique reflections