[en] The research conducted in project part F of the Verbundvorhaben ''Grundlegende Untersuchungen zur Immobilisierung langlebiger Radionuklide mittels Einbau in endlagerrelevante Keramiken'' is described. Results of this research in work packages 2 (structural characterization), AP3 (radiation damage) and AP4 (thermodynamic data) are presented.

[en] The synchrotron-radiation research at DESY is reviewed. The following topics are dealt with: Research highlights, research platforms and outstations, light sources, new technologies and developments. (HSI)

[en] Three high-quality single-crystal X-ray diffraction data sets have been measured under very different conditions on a structurally simple, but magnetically complex, coordination polymer, (Mn(HCOO)₂(H₂O)₂)∞ (1). The first data set is a conventional 100(2) K Mo_Kα data set, the second is a very high resolution 100(2) K data set measured on a second-generation synchrotron source, while the third data set was measured with a tiny crystal on a high brilliance third-generation synchrotron source at 16(2) K. Furthermore, the magnetic susceptibility (χ) and the heat capacity (C_p) have been measured from 2 to 300 K on pressed powder. The charge density of 1 was determined from multipole modeling of the experimental structure factors, and overall there is good agreement between the densities obtained separately from the three data sets. When considering the fine density features, the two 100 K data sets agree well with each other, but show small differences to the 16 K data set. Comparison with ab initio theory suggests that the 16 K APS data set provides the most accurate density. Topological analysis of the metal-ligand bonding, experimental 3d orbital populations on the Mn atoms, and Bader atomic charges indicate quite ionic, high-spin metal atoms. This picture is supported by the effective moment estimated from the magnetization measurements (5.840(2)μ_B), but it is at variance with earlier spin density measurements from polarized neutron diffraction. The magnetic ordering originates from superexchange involving covalent interactions with the ligands, and non-ionic effects are observed in the static deformation density maps as well as in plots of the valence shell charge concentrations. Overall, the present study provides a benchmark charge density that can be used in comparison with future metal formate dihydrate charge densities.

[en] High-pressure behaviours of isomorphous HoMn₂O₅ and BiMn₂O₅ (Pbam, Z = 4) were studied, using Raman spectroscopy and single-crystal x-ray diffraction, with the samples in diamond anvil cells pressurized to 10.09 GPa and 8.54 GPa, respectively, at room temperature. No phase transition was detected in either material. The two manganates are most compressible along the a axis with the lattice contractions along the b and c directions nearly the same. Their P-V data fitted with the second-order Birch-Murnaghan equation of state (B' = 4.0) give the bulk moduli B₀ = 138(2) GPa for BiMn₂O₅ and B₀ = 173(3) GPa for HoMn₂O₅. The difference in bulk moduli results from BiMn₂O₅ being more compressible than HoMn₂O₅ along the a axis. Our study shows that the Mn-O frameworks in isomorphous RMn₂O₅ compounds (R = Bi and rare earth) are not rigid at high pressures, but rather adapt to the behaviour of the coordination spheres around the R atoms. Bond valence calculations indicate that the pressure-induced charge transfers between the two Mn sites have different characters for the two compounds.

[en] High pressure behaviour of disordered pyrochlore CsMgInF₆ (Pnma, Z=4) has been studied with powder and single-crystal X-ray diffraction to 8.0 and 6.94 GPa, respectively, in diamond anvil cells at room temperature. The material is structurally stable to at least 8.0 GPa with no ordering of the In³⁺ and Mg²⁺ cations. The P-V data are fitted by a Birch-Murnaghan equation of state with the zero-pressure bulk modulus B₀=33.4(3) GPa and the unit-cell volume at ambient pressure V₀=603.2(4) A³ for the first pressure derivative of the bulk modulus B'=4.00. The major contribution to the bulk compressibility arises from the changes in the coordination sphere around the Cs atoms. The effect of hydrostatic pressure on the crystal structure of CsMgInF₆ is comparable to the effect of chemical pressure induced by the incorporation of ions of different sizes into the A and B sites in defect AB²⁺B'³⁺F₆ pyrochlores. - Graphical abstract: Crystal structure of CsMgInF₆ (Pnma, Z=4) at 6.04 GPa. Yellow, red, and blue symbols represent the Cs, (Mg/In), and F atoms, respectively. Octahedra around the (In1/Mg1) and (In2/Mg2) atoms are drawn gray and cyan, respectively.

[en] The high-pressure behavior of KIO₃ was studied up to 30 GPa using single crystal and powder x-ray diffraction, Raman spectroscopy, second harmonic generation (SHG) experiments and density functional theory (DFT)-based calculations. Triclinic KIO₃ shows two pressure-induced structural phase transitions at 7 GPa and at 14 GPa. Single crystal x-ray diffraction at 8.7(1) GPa was employed to solve the structure of the first high-pressure phase (space group R3, a = 5.89(1) Å, α = 62.4(1)°). The bulk modulus, B, of this phase was obtained by fitting a second order Birch-Murnaghan equation of state (eos) to synchrotron x-ray powder diffraction data resulting in B_exp,second = 67(3) GPa. The DFT model gave B_DFT,second = 70.9 GPa, and, for a third order Birch-Murnaghan eos, B_DFT,third = 67.9 GPa with a pressure derivative of B_DFT,third^'=5.9. Both high-pressure transformations were detectable by Raman spectroscopy and the observation of second harmonic signals. The presence of strong SHG signals shows that all high-pressure phases are acentric. By using different pressure media, we showed that the transition pressures are very strongly influenced by shear stresses. Earlier work on low- and high-temperature transitions was complemented by low-temperature heat capacity measurements. We found no evidence for the presence of an orientational glass, in contrast to earlier dielectric studies, but consistent with earlier low-temperature diffraction studies. (paper)

[en] The crystal structure and stability of NaTh₂F₉ have been studied using thermal analysis, powder X-ray diffraction at atmospheric conditions, and single-crystal X-ray diffraction at high pressure. Sodium dithorium fluoride is stable at least up to 5.0 GPa at room temperature and to 954 K at ambient pressure. In contrast to earlier investigations, which have reported the structure to be cubic (I4-bar3m, Z=4), we observe a tetragonal distortion of the lattice. The actual crystal structure (I4-bar2m, Z=4) is twinned and composed of corner-sharing distorted ThF₉ tricapped trigonal prisms and distorted NaF₆ octahedra. The twinning element is a three-fold axis from cubic symmetry. The ThF₉ polyhedra are rigid and it is the volume changes of the octahedra around the Na atoms that have the major contribution to the bulk compressibility. The zero-pressure bulk modulus B₀ and the unit-cell volume at ambient pressure V₀ are equal to 99(6) GPa and 663.1(1.0) A³, respectively, with the fixed first pressure derivative of the bulk modulus B'=4.00. An inspection of the known crystalline phases in the system NaF-ThF₄ reveals that their bulk moduli increase with the increasing ThF₄ content. - Graphical abstract: Crystal structure of NaTh₂F₉ (I4-bar2m, Z=4) at ambient conditions. The polyhedra around the Th atoms are drawn

[en] The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi₂Ga₄O₉, was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grueneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The densityfunctional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi³⁺ is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure. (orig.)

[en] Powder X-ray diffraction experiments of sillenites Bi₁₂M O₂₀ (M=Si, Ge, Ti) were performed with synchrotron radiation at the ESRF, Grenoble, at pressures up to 39 GPa (M=Si), 50 GPa (M=Ge), and 37 GPa (M=Ti), respectively. These three sillenites were investigated for the first time over such a large pressure range and they show no phase transition up to the highest pressures achieved. Birch–Murnaghan equations of state of third order were fitted to the pressure dependence of the lattice parameters. The resulting bulk moduli B₀ and their pressure derivatives B′ are B₀=63(1)GPa with B′=6.6(3) (M=Si), B₀=63.0(5)GPa with B′=5.90(7) (M=Ge), and B₀=48(1)GPa with B′=9.4(5) (M=Ti). In the case of Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ the equation of state fits were restricted to pressures below 15 GPa, because the experimental data were affected by non-hydrostatic stress at higher pressures. The equation of state of neon was redetermined (V₀=11.7(6)cm³/mol, B₀=5(1)GPa, B′=5.5(3)GPa) and its use as an internal pressure standard for diffraction experiments is discussed. A second cubic phase could be identified in our Bi₁₂TiO₂₀ sample. This was attributed to eulytite-type Bi₄Ti₃O₁₂, which has B₀=50.9(8)GPa and B′=6.9(3). - Graphical abstract: X-ray diffraction at high pressure → Equation of state. - Highlights: • Powder X-ray diffraction experiments of sillenites were performed at high pressures. • Equations of state of sillenites Bi₁₂MO₂₀ (M=Si, Ge, Ti) were determined. • The sillenite structure was shown to be stable at high hydrostatic pressure. • Sillenites are sensitive to non-hydrostatic pressure conditions in neon-loaded DACs. • An eulytite-type Bi₄Ti₃O₁₂ phase was found to exist up to 10 GPa

[en] We report results of a powder x-ray diffraction (XRD) study of vanadium sesquioxide, V₂O₃, under pressurization in a neon pressure-transmitting medium up to 57 GPa. We have established a bulk modulus value for corundum-type V₂O₃ of B₀ = 150 GPa at B′ = 4. This bulk modulus value is the lowest among those known for the corundum-type-structured oxides, e.g. Al₂O₃, α-Fe₂O₃, Cr₂O₃, Ti₂O₃, and α-Ga₂O₃. We have proposed that this might be related to the difference in the electronic band structures: at room temperature V₂O₃ is metallic, but the above corundum-structured sesquioxides are semiconducting or insulating. Around ∼21–27 and ∼50 GPa we registered changes in the XRD patterns that might be addressed to phase transitions. These transitions were sluggish upon room-temperature compression, and hence we additionally facilitated them by the laser heating of one sample. We have refined the XRD patterns of only the first high-pressure phase in an orthorhombic lattice of a Rh₂O₃(II)-type. Our findings significantly extend the knowledge of the P–T phase diagram of V₂O₃ and advance the understanding of its properties. We speculate that the elastic properties of V₂O₃ can be closely linked to its electronic band structure and, consequently, we propose that slightly doped V₂O₃ (e.g. with Cr) could be a potential candidate for systems in which the bulk modulus value may be remarkably switched by moderate pressure or temperature. (paper)