R. O. Piltz

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...

Order-disorder behaviour in the transition of PbTiO3

Abstract Single-crystal neutron-diffraction studies of PbTiO3, in its tetragonal (ferroelectric) phase at room temperature, at Tc - 60 K and at Tc - 20 K, and in its cubic phase at Tc+ 2 K, Tc + 50 K and Tc + 100 K, give evidence that the phase transition, at T, = 763 K, is order-disorder in character. In particular, the Pb atoms can be described as being disordered over six sites above Tc, each displaced -0.2 1 along the ⟨001⟩ directions, and then ordering onto one of these sites below Tc.

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.

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.

High-pressure neutron-diffraction studies of KH2PO4-type phase transitions as Tc tends to ok

Abstract A neutron-diffraction study has been carried out on KH2PO4 at 16.5kbars and 10K, close to the point at which the transition temperature, Tc, falls to OK at 17.1kbars. The results suggest that under these conditions, the H atoms are still disordered over two sites with a separation, δ, of 0.22(2)A. Comparison with results from previous structural studies of other other H-bonded systems suggests that Tc is linearly related to δ in each case and that each system has the same critical value of δ as Tc ↪

Neutron-diffraction studies of the relationship between Tc and h-bond dimensions in H-ordering transitions

Abstract Neutron-diffraction studies have been carried out on KH2PO4, and other H-bonded materials exhibiting a similar H-ordering phase transition, just above the transition temperature Tc, at atmospheric pressure and under applied hydrostatic pressures up to 19 kbar. The results show a systematic relationship between Tc, and δ, where δ is the separation between the two sites over which the H atoms are 50:50 disordered above Tc.

Pressure dependence of the structure of La-Sr-Cu-O

Abstract A high-resolution neutron powder diffraction study has been carried out to study the effect of pressure on the structure of the oxide superconductor La 1.85 Sr 0.15 CuO 4 in its room-temperature tetragonal phase. The refined atomic coordinates indicate an anomalous behaviour of the bond linking the cooper atom to the apical oxygen atom (Cu-O2) of the CuO 6 octahedra. The length of this bond increases significantly when the pressure is raised from its atmospheric value to 6 kbar, before decreasing again when the pressure is increased further to 10 kbar. By contrast, the superconducting transition temperature increases linearly with pressure to well above 10 kbar. This result differs from recent work on other oxide superconductors in which increases in T c have been attributed to linearly related decreases in the corresponding apical Cu-O distance.

Possible disorder of the P atom in KD2PO4

Very high-resolution neutron diffraction data have been collected from KD2PO4 (DKPD) at Tc + 5 K. Analysis of these data shows that the P-atom distribution is highly anharmonic, so much so that it can be modelled as disorder over three sites–a central site at the mean position, and two others displaced symmetrically along ±z, where z is the direction that becomes the polar axis of the low-temperature ferroelectric (FE) phase below Tc. The displaced sites correspond to the two FE configurations of the PO4 groups. It is concluded that the population of these sites is 40% in each, leaving only 20% at the central site. This indicates a much greater preponderance of the FE configurations just above Tc than predicted by the standard model of phase transitions of this type.

Neutron-diffraction studies of the geometric isotope effect in h-ordering transitions

Abstract Neutron-diffraction studies have been carried out on PbHPO4 and its deuterated form PbDPO4 just above their respective H(D)-ordering phase transition temperatures, Tc, at atmospheric pressure. For PbDPO4, data have also been collected under pressure. The results are compared with a similar study of KH2PO4 and KD2PO4. In both systems, the large change in Tc on deuteration at atmospheric pressure appears to be attributable simply to the accompanying increase in δ, where δ is the separation between the two sites over which the H(D) atoms are disordered above Tc.

Angle-dispersive powder diffraction at high pressure using an image-plate area detector

Abstract 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 and to a general method of processing the diffraction data. The advantages of using pressure cells that allow full diffraction patterns to be collected are also described. Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991