Jutta Chrosch

Is "metamictization" of zircon a phase transition?

Abstract Diffuse X-ray scattering from single crystals of metamict zircon reveals residual crystallinity even at high fluences (up to 7.2 × 1018 α-decay events/g). The experimental evidence does not suggest that radiation-induced amorphization is a “phase transition.” The observations are in good agreement with a nonconvergent, heterogeneous model of amorphization in which damage production is a random process of cascade formation and overlap at increasing fluence. Instead of an amorphization transition, the existence of a percolation transition is postulated. At the level of radiation damage near the percolation point, the heterogeneous strain broadening of Xray diffraction profiles is reduced whereas the particle-size broadening increases. Simultaneously, the macroscopic swelling of the zircon becomes larger than the maximum expansion of the unit-cell parameters. A suitable empirical parameter that characterizes this transition is the flux, Ds, at which the macroscopic expansion is identical to the maximum expansion of the crystallographic unit cell. In zircon, Ds = 3.5·1018 α-decay events/g.

Temperature dependence of the domain wall width in LaAlO3

Twin wall related diffuse scattering intensities from a single crystal of LaAlO3 were determined using high-resolution x-ray diffraction methods. Rocking curves were measured for sample temperatures between 295 and 900 K. The wall thickness W was determined by comparing the observed diffraction profiles with structure factor calculations for a tanh(x/W) wall profile. It is shown that W≈20 A at low temperatures and increases slowly with temperature reaching values in excess of 200 A near Tc≈850 K with W∝|T−Tc|−1.

Anti-phase boundaries and phase transitions in titanite; an X-ray diffraction study

Abstract X-ray diffraction rocking curves of titanite, CaTiSiO5, were measured using a novel high-resolution diffractometer. Three Bragg reflections were recorded as a function of temperature and their profiles analyzed in terms of a Gaussian Bragg peak and a diffuse scattering component with an overall Lorentzian shape. The temperature dependence of the Gaussian intensities of the rocking peaks of superstructure reflections, hkl with k + l odd, scale with the long-range order parameter as I ∞ Q2 ∞ |T - Tc|2β, where β = 0.14(1) is the effective order parameter exponent. The diffuse scattering intensity changes little with temperature at T < Tc - 20 K. Strong diffuse scattering is found at T > Tc with centers of their Lorentzian diffraction profiles shifted by Δω ≈ 0.5° with respect to the position of the equivalent Bragg peak. A second phase transition at 825 K is confirmed and the possibility of a third transition at ~ 1150 K is discussed.

Thin domain walls in YBa2Cu3O7−δ and their rocking curves an x-ray diffraction study

Abstract Twin walls were investigated of single crystals of the high- T c superconductor YBa 2 Cu 3 O 7−δ (YBCO) using a high-resolution X-ray diffractometer. Rocking curves were recorded for the (029)/(209), (02.10)/(20.10) and (02.11)/(20.11) pairs of Bragg peaks. The reflections were analysed using a wall profile e=e O tanh( r w ) , where w is the effective wall thickness and r the distance from the wall centre. The wall thickness was determined to be 7 A ±2 A.

Diffuse X-ray scattering in WO3

Abstract The diffuse scattering in WO3 has been investigated using high-resolution X-ray methods. The structure of the (040)/(400) diffraction profiles in a densely twinned orthorhombic crystal has been mapped. The scattering is characterised by strong, well defined Bragg peaks with a diffuse streak between these of intensity three orders of magnitude lower, arising from the domain walls. Comparison with a simple analytical model for the scattering from such a microstructure suggests an effective domain wall width of around 4 unit cells.

Thermal annealing of radiation damaged titanite

Abstract Radiogenic impurities of 400 to 800 ppm U and Th in titanite, CaTiSiO5, lead to moderate radiation damage (≈1.5 × 1018 α-decay events/g) and therefore to partial amorphization (≈30%). Powder X-ray diffraction on such damaged titanite from the Cardiff locality in Canada shows that two modifications of the crystalline material coexist. Both modifications are structurally β phase but differ systematically in their lattice parameters and also in their chemical composition. One modification exhibits strong particle size broadening in X-ray diffraction patterns, whereas it is almost unstrained with respect to fully annealed titanite. The other modification shows large strain broadening and increased specific volume (about 3%) due to a high concentration of defects. The unstrained modification consists of small nucleation centers in the damaged material, and it grows when the sample is annealed. At annealing temperatures above 823 K, this modification dominates rapidly and replaces the strained titanite. The results of Rietveld refinement of the annealed samples and of the time evolution of isothermal annealing studies are discussed. The analysis of volume strain and of structural strain resulting from the peak profiles suggests a temperature-dependent activation energy for the recrystallization process, with EA ≈ 380 kJ/mol at T > 873 K and EA ≈ 500 kJ/mol at temperatures 773 K < T < 873 K.

Superconducting YBCO single crystals with macroscopically tetragonal phase

We have carried out a series of investigations in an attempt to demonstrate the existence of tweed tetragonal phase in oxygen-rich rich pure YBCO7−δ (δ<0.6). A technique for producing such a phase was developed which involved heating the fully oxygenated orthorhombic crystal to just above its orthorhombic-tetragonal phase transition temperature for a short period in a high-oxygen-partial-pressure environment, followed by rapid quenching to room temperature. X-ray rocking curves and electron microscopy studies provided the evidence of the tweed tetragonal phase, and AC susceptibility measurements confirmed these crystals to be superconducting with a sharp transition.

Local fluctuations in feldspar frameworks

Abstract At any set of thermodynamic conditions a mineral will have some well defined equilibrium crystal structure. However, this structure can be locally disturbed by crystal defects, such as domain walls or solute atoms. This distorted structure will only affect a finite volume within the crystal, but the need to retain continuity within the crystal means that this volume must be non-zero. This means, for example, that the boundary between two domains will include a transition zone from one domain’s crystal structure to that of the other domain. Thick twin domain walls can be studied quantitatively, by measuring the intensity of diffuse diffraction between pairs of twin-related Bragg peaks. In alkali feldspar (Or30) at room temperature, these walls are approximately 25 Å thick. Similarly, a single solute atom in a mineral will only affect a small region within a crystal. As a result, chemical mixing will only occur in a substitutional solid solution once there is significant overlap between the strain fields around individual solute atoms. This causes the ‘plateau effect’, where the properties of a phase transition are independent of composition. In alkali feldspar, this plateau extends from albite to 2% Or, which corresponds to a strain field radius of 10 Å. These phenomena can be modelled using Ginzburg-Landau theory, which predicts that the range of these strain fields will increase as the temperature is raised to Tc. This has been confirmed by measuring the thickness of twin walls as a function of temperature.

High-resolution X-ray diffraction analysis of magnetically aligned high-Tc superconducting ceramics

Abstract A large range of magnetically treated superconducting ceramics has been investigated using new high-resolution X-ray diffractometers in combination with different integration techniques to obtain quantitative and qualitative results on the degree of alignment. The diffractometers combine and enhance the flexibility of a 4-circle diffractometer with the possibility to use it as a bond camera and as grazing incidence surface diffractometer. The X-ray beam is strictly monochromatic (CuKα 1 ) and focused with a long focal length on the sample. The sample goniometer can be rotated and shifted around three perpendicular axes and two translation directions, respectively. The instruments are equipped with 1D and 2D detectors and their distance to the sample can be varied between 10 and 30 cm. In our contribution we describe the experimental methods connected with this special equipment, their application to the analysis of the degree of alignment in high T c superconducting ceramics and compare the results to those obtained by standard diffraction methods.

High-resolution X-ray diffraction from microstructures

Abstract Novel high-resolution X-ray diffractometers have been used to investigate a variety of microstructures such as twins, tweed, and domain boundaries in ferroelastic materials. It is shown that copious information can be extracted using our experimental methods. The instruments are equipped with 1D and 2D detectors, where the distance to the sample can be varied to enhance the intrinsic resolution. Both the specimen and the detectors possess several degrees of freedom which include sample tilts and rotations as well as translations. In our contribution we introduce briefly the experimental and theoretical methods connected with this special equipment and describe its application to the characterization of a wide variety of specimens such as single crystals, ceramic materials and thin films.