| 1. |
- Carlson, Stefan, et al.
(författare)
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High-pressure transformations of NbO2F
- 2000
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Ingår i: Acta Crystallographica. Section B: Structural Science. - 0108-7681. ; 56:2, s. 189-196
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Tidskriftsartikel (refereegranskat)abstract
- The ReO3-type structure NbO2F, niobium dioxyfluoride, has been studied at high pressures using diamond anvil cells and synchrotron X-ray radiation. High-pressure powder diffraction measurements have been performed up to 40.1 GPa. A phase transition from the cubic (Pm3m) ambient pressure structure to a rhombohedral (R3c) structure at 0.47 GPa has been observed. Rietveld refinements at 1.38, 1.96, 3.20, 6.23, 9.00 and 10.5 GPa showed that the transition involves an a-a-a- tilting of the cation-anion coordination octahedra and a change of the anion-anion arrangement to approach hexagonal close packing. Compression and distortion of the Nb(O/F)6 octahedra is also revealed by the Rietveld refinements. At 17-18 GPa, the diffraction pattern disappears and the structure becomes X-ray amorphous.
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| 2. |
- Hirsch, T.K., et al.
(författare)
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An investigation of H-atom positions in sulfuric acid crystal structures
- 2004
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Ingår i: Acta Crystallographica Section B. - 0108-7681 .- 1600-5740. ; 60:2, s. 179-183
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen conformations in crystalline H2SO4· 8H2O and H2SO4·6.5H2O have been studied using a system developed by Hirsch [(2003), Z. Anorg. Allg. Chem. 629, 666-672]. New H-atom coordinates, as estimated from DFT calculations, are given for these structures. © 2004 International Union of Crystallography Printed in Great Britain - all rights reserved.
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| 3. |
- Höwing, Jonas, et al.
(författare)
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Li3+δVC6O13: a short-range-ordered lithium insertion mechanism
- 2004
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Ingår i: Acta Crystallographica. - 0108-7681. ; B60, s. 382-387
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Tidskriftsartikel (refereegranskat)abstract
- The structures of Li3 V6O13 and Li3+V6O13, 0.3, have been determined by single-crystal X-ray diffraction. Both compounds have the space group C2/m, with very similar cell parameters. In Li3V6O13, the Li atoms are found in the Wyckoff positions 4(i) and 2(b) with multiplicities of four and two, respectively. Since Li3V6O13 exhibits no superstructure reflections, it is concluded that Li3V6O13 contains one disordered lithium ion in an otherwise ordered centrosymmetric structure. On inserting more lithium into the structure, the Li3+V6O13 phase is formed with the homogeneity range 0 < < 1. It is concluded that the site for the extra inserted lithium ion is closely coupled to the position of the disordered lithium ion in Li3V6O13. A mechanism for this behaviour and for the further formation of the Li6V6O13 end-phase in the LixV6O13 system is proposed.
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| 4. |
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| 5. |
- Krogh Andersen, A.M., et al.
(författare)
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Ab Initio Structure Determination and Rietveld Refinement of a High-Temperature Phase of Zirconium Hydrogen Phosphate and a New Polymorph of Zirconium Pyrophosphate from In-Situ Temperature-Resolved Powder Diffraction Data.
- 2000
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Ingår i: Acta Crystallographica Section B. - : Wiley. - 0108-7681. ; 56:4, s. 618-625
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Tidskriftsartikel (refereegranskat)abstract
- The collected in situ temperature-resolved synchrotron powder data revealed that the transformation of the recently reported three-dimensional τ-Zr(HPO4)2 to cubic ZrP2O7 goes through two intermediate phases. The first intermediate phase, ρ-Zr(HPO4)2, is formed in a reversible phase transition at 598 K, which involves both rearrangement and disordering of the hydrogen phosphate groups of τ-Zr(HPO4)2. At 688 K condensation of the hydrogen phosphate groups leads to the formation of the second intermediate, a new polymorph of zirconium pyrophosphate (β-ZrP2O7). Heating above 973 K results in the gradual transformation of β-ZrP2O7 to cubic zirconium pyrophosphate (α-ZrP2O7). The crystal structures of the two intermediate phases were solved from the in situ powder diffraction data using direct methods and refined using the Rietveld method. Both phases are orthorhombic, space group Pnnm and Z = 2. The lattice parameters for the two phases are: ρ-Zr(HPO4)2: a = 8.1935 (2), b = 7.7090 (2), c = 5.4080 (1) Å; β-ZrP2O7: a = 8.3127 (5), b = 6.6389 (4), c = 5.3407 (3) Å. The formation mechanism for the new zirconium pyrophosphate polymorph, β-ZrP2O7, is discussed in relation to structurally restricted soft chemistry
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| 6. |
- Krogh Andersen, A.M., et al.
(författare)
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High-Pressure Structures of α- and δ-ZrMo2O8
- 2001
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Ingår i: Acta Crystallographica Section B Structural science. - : Wiley. - 0108-7681. ; 57:1, s. 20-26
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Tidskriftsartikel (refereegranskat)abstract
- In situ high-pressure synchrotron X-ray powder diffraction studies of trigonal α-ZrMo2O8, zirconium molybdate, have been performed from ambient conditions to 1.9 GPa, over the α–δ phase transition at 1.06–1.11 GPa. The monoclinic structure of δ-ZrMo2O8, stable between 1.1 and 2.5 GPa at 298 K, has been solved by direct methods and refined using the Rietveld method. Significant distortions of the ZrO6 and MoO4 polyhedral elements are observed for δ-ZrMo2O8, as compared to the ambient conditions of the α-phase, while the packing of anions becomes more symmetric at high pressure.
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| 7. |
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| 8. |
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| 9. |
- Norberg, Stefan, 1972, et al.
(författare)
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Cation movement and phase transitions in KTP isostructures; X-ray study of sodium-doped KTP at 10.5 K
- 2003
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Ingår i: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 59:3, s. 353-360
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Tidskriftsartikel (refereegranskat)abstract
- An accurate structure model of sodium-doped potassium titanyl phosphate, (Na0.114K0.886) K(TiO)(2)(PO4)(2), has been determined at 10.5 K by single-crystal X-ray diffraction. In addition to the low-temperature data, X-ray intensities have been collected at room temperature. When the temperature was decreased from room temperature to 10.5 K, both potassium cations moved 0.033 (2) Angstrom along the c-axis, i.e. in the polar direction within the rigid Ti-O-P network. This alkaline metal ion displacement can be related to the Abrahams-Jamieson-Kurtz T-C criteria for oxygen framework ferroelectrics. Potassium titanyl phosphate (KTP) is a well known material for second harmonic generation (SHG), and the influence of sodium dopant on the TiO6 octahedral geometry and SHG is discussed. The material studied crystallizes in the space group Pna2(1) with Z = 4, a = 12.7919 (5), b = 6.3798 (4), c = 10.5880 (7) Angstrom, V = 864.08 (9) Angstrom(3), T = 10.5 (3) K and R = 0.023.
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| 10. |
- Norberg, Stefan, 1972, et al.
(författare)
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Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation
- 2001
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Ingår i: Acta Crystallographica Section B: Structural Science. - 0108-7681 .- 1600-5740. ; 56:6, s. 980-987
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Tidskriftsartikel (refereegranskat)abstract
- 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) Angstrom] using a fast avalanche photodiode counter. Space group Pna2(1), Z = 8,a = 12.786 (2), b = 6.3927 (8), c = 10.5585 (9) Angstrom, T = 293 (1) K, R = 0.028 (SrCrKTP); a = 12.851 (6), b = 6.418 (3), c = 10.620 (5) Angstrom, 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.
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