R. J. Nelmes

University of Edinburgh

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M. I. McMahon

University of Edinburgh

119

J. S. Loveday

University of Edinburgh

103

Stefan Klotz

Institut de minéralogie, de physique des matériaux et de cosmochimie

G. Hamel

Institut de minéralogie, de physique des matériaux et de cosmochimie

M. Guthrie

Oak Ridge National Laboratory

Craig L. Bull

University of Edinburgh

David R. Allan

Diamond Light Source

S. A. Belmonte

Ministero della Salute

J. M. Besson

Stress Environnementaux et Biosurveillance des Milieux Aquatiques

Nicolas G. Wright

Colorado School of Mines

Cooperative Institutions

University of Edinburgh

Centre National de la Recherche Scientifique

Rutherford Appleton Laboratory

Sorbonne Université

Institut de minéralogie, de physique des matériaux et de cosmochimie

University College London

European Synchrotron Radiation Facility

Scottish Universities Physics Alliance

Science and Technology Facilities Council

Didcot Community Hospital

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Structural studies of deuterated CsH₂AsO₄in its paraelectric and ferroelectric phases

Journal of Physics C Solid State Physics (1981)

William Hay R. J. Nelmes

The crystal structure of deuterated CsH2AsO4 (DCsDA) has been determined by neutron diffraction at room temperature and at Tc+5K in the tetragonal (paraelectric) phase using single-crystal samples. The structure has also been determined at 77K in the orthorhombic (ferroelectric) phase using an unpoled, unconstrained-and, so, poly-domained-sample. The results for both phases are compared with those obtained from similar studies of KD2PO4. Whilst the structures (and the way they change through the transition) are very alike in most respects, there are some interesting differences. In particular, the 'disordered' D(H) sites in the tetragonal phase are significantly further apart in DCsDA (by approximately 0.05 AA), yet the O-D(H)-O bondlengths are the same within error and the transition temperatures differ by only approximately 20K. This is remarkably like the relationship that obtains between RbH2PO4 and KH2PO4.

Tetragonal crystal system

Orthorhombic crystal system

Crystal (programming language)

10.1088/0022-3719/14/7/009

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Citations (22)

High-Pressure High-Temperature Studies of Structural Ordering in GaSb

physica status solidi (b) (2001)

C. B. Vanpeteghem R. J. Nelmes D.R. Allan M. I. McMahon Andrei Sapelkin

Both angle-dispersive and EXAFS experiments have been carried out to investigate the structural ordering of the high-pressure (hp) and high-pressure high-temperature (hp/ht) phases of GaSb. The diffraction patterns of all the different phases of GaSb have shown an absence of long-range order, while the EXAFS studies have demonstrated the lack of complete short-range order. GaSb is the only semiconductor compound among the III–V and II–VI systems to present such behaviour in all of its different hp and hp/ht phases.

Atmospheric temperature range

Short range order

10.1002/1521-3951(200101)223:2<405::aid-pssb405>3.3.co;2-x

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Neutron-diffraction studies of the heavy-atom structure in h-ordering materials

Ferroelectrics (1991)

M. I. McMahon R. J. Nelmes Ross O. Piltz W. F. Kuhs Nicolas G. Wright

Abstract We have made a high-resolution neutron-diffraction study of KD2PO4 (DKDP) at Tc+5K. The results indicate that above Tc, in the paraelectric phase, the thermal motion of the P atom is highly anharmonic along the z axis, and can be better modelled using a disordered three-site distribution. Analysis of the P-atom thermal motion in a neutron-diffraction study of KH2PO4 (KDP) at 16.5kbars and 10K, close to the point where Tc ↪ ok, indicates that the separation of the disordered P-atom sites remains relatively unchanged on cooling or under pressure. This may suggest that Tc is not directly related to the P-atom separation.

10.1080/00150199108209464

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Citations (3)

Powder diffraction studies of structures at high pressure

AIP conference proceedings (1994)

R. J. Nelmes M. I. McMahon

Significant advances are now being achieved in the quality of crystal‐structure information at high pressure, particularly with powder diffraction techniques, on both neutron and synchrotron sources. A short review is presented of recent progress in angle‐dispersive powder diffraction on synchrotron sources, using an image‐plate area detector. The image‐plate techniques pioneered at the Photon Factory have now been further developed at SRS Daresbury. Recent work at SRS is used to illustrate the gains in data quality achievable with optimized angle‐dispersive techniques.

Powder Diffraction

Factory (object-oriented programming)

10.1063/1.46134

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Neutron powder diffraction at pressures beyond 25 GPa

Applied Physics Letters (1995)

Stefan Klotz J. M. Besson G. Hamel R. J. Nelmes J. S. Loveday

Full structural studies of condensed media under high pressure by neutron powder diffraction have been limited in practice to 2–3 GPa for several decades. This range is in general too small to allow a precise determination of the pressure dependence of atomic coordinates. As a consequence, almost no direct measurements exist, for example, of the pressure dependence of the bond lengths in H2 and the planetary ices. In this letter, a technique is presented which makes it possible to pressurize samples of 35 mm3 volume up to 30 GPa and to collect neutron diffraction patterns in a few hours by time-of-flight techniques. This method provides data which can be treated by Rietveld profile refinement methods, as demonstrated on a sample of D2O ice VII at 26 GPa. This represents a tenfold increase of the pressure range over which refinable neutron diffraction data can be obtained and should have a number of applications in such fields as fundamental physicochemistry, and geo- and planetary sciences.

Rietveld Refinement

Powder Diffraction

10.1063/1.113350

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Citations (91)

Phase transitions in CdTe to 5 GPa

Physical review. B, Condensed matter (1993)

M. I. McMahon R. J. Nelmes Nicolas G. Wright D.R. Allan

Angle-dispersive powder-diffraction techniques using an image-plate area detector have been used to study the high-pressure phases of CdTe to 5 GPa. The recently discovered cinnabar phase between the zinc-blende and NaCl phases is found to be stable on both pressure increase and decrease. No pressure range of single-phase cinnabar has been found on pressure increase, but a single phase is obtained on pressure decrease, between 3.6 and 2.7 GPa. The cinnabar phase has an average bulk modulus of 32(1) GPa and is significantly more compressible than the zinc-blende and NaCl phases. All three phases are site ordered. Each atom in the cinnabar structure has two nearest-neighbor distances of \ensuremath{\sim}2.74 \AA{} (a bonds) and two of \ensuremath{\sim}2.94 \AA{} (b bonds), and the structure is thus close to fourfold coordinated. The next-nearest-neighbor distances are two of \ensuremath{\sim}3.5 \AA{} (c bonds). The a bonds are very similar in length to the nearest-neighbor distance in the zinc-blende phase, and the b bonds increase with pressure towards the nearest-neighbor distance in the NaCl phase. The c-bond distance and the interbond angles also become closer to the geometry of the NaCl structure with increasing pressure, but there are still large discontinuities in bond lengths and angles at the cinnabar-to-NaCl transition.

Cinnabar

Molecular geometry

10.1103/physrevb.48.16246

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Citations (87)

High-pressure structures and phase transformations in elemental metals (vol 35, pg 943, 2006)

Chemical Society Reviews (2006)

M. I. McMahon R. J. Nelmes

Source

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Structural studies of potassium titanyl phosphate (ktp) under high pressure

Ferroelectrics (1991)

David R. Allan J. S. Loveday R. J. Nelmes P. A. Thomas

Abstract The crystal structure of KTiOPO4 (KTP) has been determined as a function of pressure towards its phase transition at 5.5 GPa. The principal changes with pressure are in the cage site environments of the potassium atoms and in the linking bond angles of the TiO6-PO4 framework. The distortions of the TiO6 octahedra, which have been linked to the second harmonic generating properties of KTP, remain almost unchanged. An initial study of the structure in the high-pressure phase (at 6.2 GPa) suggests that the primitive orthorhombic cell is retained with a 5% reduction in the unit-cell volume at the transition.

Potassium titanyl phosphate

Orthorhombic crystal system

10.1080/00150199108209467

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Citations (8)

High-pressure crystal structure of HgTe-IV

Physical review. B, Condensed matter (1996)

M. I. McMahon Nicolas G. Wright David R. Allan R. J. Nelmes

The crystal structure of HgTe-IV has been determined using angle-dispersive diffraction techniques, with synchrotron radiation and an image-plate area detector. The structure is not \ensuremath{\beta} tin, as previously reported, but is found to be site ordered and orthorhombic with space group Cmcm. This is the same structure as has recently been found for CdTe-III/IV and ZnTe-III. However, HgTe exhibits a behavior different from either CdTe or ZnTe, in having a strongly first-order transition from a NaCl phase (HgTe-III) to Cmcm. The volume decrease at the transition is 1.2(1)%, and the Cmcm structure just above the transition is strongly distorted from NaCl. Two slightly different, but distinct, configurations of the structure give very similar fits to the diffraction data. Both configurations have significantly different coordination from CdTe-III/IV and ZnTe-III. \textcopyright{} 1996 The American Physical Society.

Orthorhombic crystal system

Cadmium telluride photovoltaics

10.1103/physrevb.53.2163

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Citations (38)

Equation of state of lithium deuteride from neutron diffraction under high pressure

Physical review. B, Condensed matter (1992)

Jean Besson G. Weill G. Hamel R. J. Nelmes J. S. Loveday

Time-of-flight neutron-diffraction spectra of powder samples of $^{7}\mathrm{LiD}$ have been recorded at 300 K, up to 10 GPa at the ISIS spallation source. The experimental setup consists of a 2.5-MN hydraulic press, 50 kg in mass, acting on a toroidal anvil cell. This allows compression of 100-${\mathrm{mm}}^{3}$ samples to over 10 GPa, with better stress homogeneity than flat-anvil devices. The equation of state of $^{7}\mathrm{LiD}$ has been related to the NaCl pressure scale with a precision of 1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ on the volume variation under pressure. Comparison of existing calculations with the present data shows variable agreement, depending on the calculational hypotheses. Reassessment of the divergent theoretical estimates for the pressure of band crossing in lithium hydrides reconciles them to a common prediction of about 180 GPa for the pressure of semi- metallization. However, the present work cannot reduce the wide separation of the estimates for the B1\ensuremath{\rightarrow}B2 structural-transition pressure.

Spallation Neutron Source

10.1103/physrevb.45.2613

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Citations (71)