P. D. Hatton

The structure of the paraelectric and incommensurate phases of TlGaSe2

The structure of TlGaSe2 has been studied by triple-crystal X-ray scattering techniques. At room temperature in the paraelectric phase the structure is believed to be C2/c, but the authors found that some of the (h, h, l) Bragg reflections, those with h odd, are broadened along the c* direction. This is shown to arise because of faults in the stacking of the atomic sheets perpendicular to c*. A simple model is developed for the scattering which suggests that on average a stacking fault occurs in the single crystal about once every four planes. Between 117 and 110 K the crystal has an incommensurate structure with a wavevector ( delta , delta , 1/4) where delta approximately=0.02 reciprocal lattice units. At 110 K, delta jumps discontinuously to zero to produce a commensurate phase. These results confirm the earlier suggestions of an incommensurate phase in TlGaSe2.

Characterization of nanometer‐scale epitaxial structures by grazing‐incidence x‐ray diffraction and specular reflectivity

X‐ray rocking curve analysis is a powerful and nondestructive technique for the characterization of heteroepitaxial structures. Conventionally, measurements are performed in symmetrical scattering geometry using a double‐crystal x‐ray diffractometer but the technique can be extended to the study of very thin layers (<200 A) by the use of glancing‐incidence scattering geometry and a triple‐crystal diffractometer. These structures can also be studied by the technique of total external x‐ray reflectivity. This is sensitive to the electron‐density profile of the heterostructure as a function of depth. By combining the above techniques we have found it possible to obtain structural information on layers as thin as 20 A. Such measurements permit accurate measurement of individual layer thicknesses and interface roughnesses on the angstrom level. The lattice parameter strain can be obtained by modeling of the intensity distribution of the crystal truncation rod.

Direct observation of orbital ordering in La0.5Sr1.5MnO4 using soft X-ray diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

An imaging plate system for high‐pressure powder diffraction: The data processing side

The use of an imaging plate as a two‐dimensional (2‐D) detector removes many of the difficulties that arise in performing angle‐dispersive powder diffraction at high pressures in a diamond‐anvil cell. Due to the 2‐D nature of the imaging plate, a substantial part of each Debye Scherrer ring is intercepted and recorded. The averaging of the intensities around a ring so as to create a conventional one‐dimensional (1‐D) powder pattern results in a significant improvement in counting statistics and powder averaging, both severe problems in high‐pressure diffraction due to the very small sample volumes involved. For an accurately known plate geometry the 2‐D to 1‐D conversion is straightforward; however, considerable complications arise when inaccuracies in plate to sample distance, plate orientations, poor powder averaging/preferred orientation, and the presence of diamond Bragg spots are considered. The current status of the software used to analyze the imaging plate data is presented along with test data to illustrate its use.The use of an imaging plate as a two‐dimensional (2‐D) detector removes many of the difficulties that arise in performing angle‐dispersive powder diffraction at high pressures in a diamond‐anvil cell. Due to the 2‐D nature of the imaging plate, a substantial part of each Debye Scherrer ring is intercepted and recorded. The averaging of the intensities around a ring so as to create a conventional one‐dimensional (1‐D) powder pattern results in a significant improvement in counting statistics and powder averaging, both severe problems in high‐pressure diffraction due to the very small sample volumes involved. For an accurately known plate geometry the 2‐D to 1‐D conversion is straightforward; however, considerable complications arise when inaccuracies in plate to sample distance, plate orientations, poor powder averaging/preferred orientation, and the presence of diamond Bragg spots are considered. The current status of the software used to analyze the imaging plate data is presented along with test data to...

The structure and dynamics of solid benzene. I: A neutron powder diffraction study of deuterated benzene from 4 K to the melting point

The crystal structure of deuterated benzene has been investigated over the complete temperature range 4–280 K. Our results are in excellent agreement with previous studies, recorded at a limited number of temperatures, and display a smooth variation of the lattice parameters over the complete temperature range. No evidence of the suggested low‐temperature phase transitions, or marked premelting effects have been found. Detailed measurements of the lattice parameters close to the melting point display an anomalous behavior of the b lattice parameter and the orientation of the molecules which is believed to be evidence of a disordering transition at temperatures above the melting point.

Nature of the magnetic order and origin of induced ferroelectricity in TbMnO3

The magnetic structures which endow TbMnO(3) with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L(2) edge, Mn 3d moments; Tb M(4) edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures.

Angle‐dispersive powder‐diffraction techniques for crystal structure refinement at high pressure

An imaging‐plate system designed for the full Rietveld refinement of crystal structures at high pressure is described. Emphasis is given to techniques that have been developed to obtain data free from contaminating diffraction peaks. Initial results from studies of III–V semiconductors and La2CuO4 are given.

Resonant Soft X-Ray Scattering Investigation of Orbital and Magnetic Ordering in La0.5Sr1.5MnO4.

We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn L-2,L-3 edges in La0.5Sr1.5MnO4. The orderings display complex energy features close to t ...

X-ray scattering studies of 2H-NbSe2, a superconductor and charge density wave material, under high external magnetic fields

Using x-ray scattering we have measured the response of the incommensurate charge-density wave (CDW) modulation in the quasi-low-dimensional material 2H-NbSe2 to applied magnetic fields at low temperatures. The application of a magnetic field, either perpendicular or parallel to the layers of a single crystal of 2H-NbSe2, caused no significant change to either the correlation length or the intensity of the CDW satellites for magnetic fields up to 10 T. These results suggest that the enhancement of the resistance observed in low-dimensional CDW materials exposed to the applied magnetic field does not result from an appreciable conversion of carriers from the normal state to the CDW state. In addition 2H-NbSe2 is a superconductor at low temperatures (Tc = 7.2 K), whilst still within the incommensurate CDW state. This material therefore affords an opportunity to study any interaction between the CDW and superconducting condensates. High magnetic fields can suppress the superconducting state yet no change in the incommensurate CDW satellite correlation length or intensity was observed at high applied magnetic fields. These results conflict with spectroscopic measurements, which suggest a coupling between the superconducting gap excitations and the CDW.

Structural studies of semiconductors at very high pressures

Abstract High pressure angle dispersive X-ray powder diffraction studies have been carried out on the IV–VI semiconductors PbX,(X = S, Se, Te) using image plate detection. Exploiting the high resolution of this method, structural refinements of the high pressure intermediate phases of the lead chalcogenides suggests that the cell type adopted by all three is monoclinic, as opposed to the previously accepted orthorhombic structures for each.