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A design for a continuous source, polychromatic beam powder neutron diffractometer based on an energy-dispersive detection methodology is proposed. Such an instrument would offer significantly higher incident neutron flux, with an attendant increase in data collection rate, reducing the time per scan from hours to seconds. Completely passive components are conceived for neutron bandwidth shaping, and detection is achieved using a massively parallel analyzer/detector system, making installation and operation of the instrument comparatively simple. The proposed instrument is intended to be used for rapid structural characterizations of small samples or for kinetic studies of materials undergoing structural or magnetic phase changes.

The polymorphic phase transition of 1,2,4,5-tetrachlorobenzene (TCB) has been investigated using neutron powder diffraction and single-crystal X-ray diffraction. The diffraction experiments show a reversible phase change that occurs as a function of temperature with no apparent loss of sample quality on transition between the two phases. Neutron powder diffraction gives detailed information on the molecular structural changes and lattice parameters from 2 K to room temperature. The structure of the low-temperature form has been elucidated for the first time using single-crystal X-ray diffraction. Comparison of the α and β structures show that they are both based on the same sheet motif, with the differences between the two being very subtle, except in terms of crystal symmetry. Detailed analysis of the structures revealed the changes required for inter-conversion. A computational polymorph search showed that these two sheet structures are more thermodynamically stable than alternative herringbone-type structures.

Metal lite: High-resolution neutron powder diffraction data reveals that the body-centered cubic crystal structure of lithium(0)tetraamine transforms to a simple cubic structure below 22 K. The detailed structure determinations will allow new insights into the coupled structural and electronic properties of the lightest metal.

Once unit-cell dimensions have been determined from a powder diffraction data set and therefore the crystal system is known (e.g.orthorhombic), the method presented by Markvardsen, David, Johnson & Shankland [Acta Cryst.(2001), A57, 47–54] can be used to generate a table ranking the extinction symbols of the given crystal system according to probability. Markvardsenet al.tested a computer program (ExtSym) implementing the method against Pawley refinement outputs generated using theTF12LSprogram [David, Ibberson & Matthewman (1992). Report RAL-92-032. Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, UK]. Here, it is shown thatExtSymcan be used successfully with many well known powder diffraction analysis packages, namelyDASH[David, Shankland, van de Streek, Pidcock, Motherwell & Cole (2006).J. Appl. Cryst.39, 910–915],FullProf[Rodriguez-Carvajal (1993).Physica B,192, 55–69],GSAS[Larson & Von Dreele (1994). Report LAUR 86-748. Los Alamos National Laboratory, New Mexico, USA],PRODD[Wright (2004).Z. Kristallogr.219, 1–11] andTOPAS[Coelho (2003). Bruker AXS GmbH, Karlsruhe, Germany]. In addition, a precise description of the optimal input forExtSymis given to enable other software packages to interface withExtSymand to allow the improvement/modification of existing interfacing scripts.ExtSymtakes as input the powder data in the form of integrated intensities and error estimates for these intensities. The output returned byExtSymis demonstrated to be strongly dependent on the accuracy of these error estimates and the reason for this is explained.ExtSymis tested against a wide range of data sets, confirming the algorithm to be very successful at ranking the published extinction symbol as the most likely.

The phase diagram of cyclopentane has been studied by powder neutron diffraction, providing diffraction patterns for phases I, II, and III, over a range of temperatures and pressures. The putative phase IV was not observed. The structure of the ordered phase III has been solved by single-crystal diffraction. Computational modeling reveals that there are many equienergetic ordered structures for cyclopentane within a small energy range. Molecular dynamics simulations reproduce the structures and diffraction patterns for phases I and III and also show an intermediate disordered phase, which is used to interpret phase II.

The complex phase behavior of cyclohexene, C6H10, has been characterized between 2 K and its melting point at 170 K using complementary high-resolution neutron powder diffraction and single-crystal X-ray diffraction techniques, and the crystal structures of the three known ambient-pressure phases have been determined. Phase II is the sole orientationally ordered structure of cyclohexene, crystallizing in the triclinic space group P1. Phase I and metastable phase III may be characterized by the mode of orientational disorder corresponding to free uniaxial rotation and ring inversion of the molecules, respectively. Phase I is cubic with space group Pa3, and the molecule is located on a 3-fold axis with Z′ = 1/3. Phase III is monoclinic with Z′ = 2 and space group P21/c, but with pseudocubic character inherited from the phase I structure. In this phase, only one of the molecules in the asymmetric unit is disordered and exhibits ring inversion which freezes at the glass transition temperature around 80 K.

In situ synchrotron X-ray powder diffraction is used to study the kinetics of cation ordering and ordered domain growth in the microwave dielectric electroceramic Ba3ZnTa2O9 (BZT) with a time resolution of 15 s and processing temperatures of up to 1500 °C. This permits the measurement of the activation energy for cation ordering and the identification of different mechanisms for ordered domain growth, associated with the formation of the Ba8ZnTa6O24 impurity phase due to zinc loss.

High resolution neutron powder diffraction data recorded at 4.5 K confirm the orthorhombic structure of methyl iodide reported previously by Kawaguchi et al. (1973) from an X-ray single-crystal study at 193 K. The present study, using neutron powder diffraction, provides more precise structural parameters and reliable atomic co-ordinates for the hydrogen atoms enabling the methyl group libration to be analysed as a function of temperature up to 193 K.

A 21R-type mixed conducting oxygen-deficient hexagonal perovskite Ba7Y2Mn3Ti2O20 was synthesized and its structure characterized by variable temperature neutron powder diffraction. The structure is characterized by the presence of both BaO2 and BaO3 layers. In the oxygen-stoichiometric material, the vacancies are ordered in c‘-BaO2 layers. Extra oxygen is accommodated by intergrowth of the vacancy-ordered c‘-BaO2 layers with cubic c-BaO3-x layers (x ≅ 1), which have disordered 2/3 oxygen occupancy. This material displays mixed electronic and oxide ionic conduction at high temperature, and the role of the two distinct and partially occupied anion sites within the BaO2 layers in permitting the oxygen mobility is discussed.

Background The response of molecular crystal structures to changes in externally applied conditions such as temperature and pressure are the result of a complex balance between strong intramolecular bonding, medium strength intermolecular interactions such as hydrogen bonds, and weaker intermolecular van der Waals contacts. At high pressure the additional thermodynamic requirement to fill space efficiently becomes increasingly important. Results The crystal structure of piperidine-d11 has been determined at 2 K and at room temperature at pressures between 0.22 and 1.09 GPa. Unit cell dimensions have been determined between 2 and 255 K, and at pressures up to 2.77 GPa at room temperature. All measurements were made using neutron powder diffraction. The crystal structure features chains of molecules formed by NH…N H-bonds with van der Waals interactions between the chains. Although the H-bonds are the strongest intermolecular contacts, the majority of the sublimation enthalpy may be ascribed to weaker but more numerous van der Waals interactions. Conclusions Analysis of the thermal expansion data in the light of phonon frequencies determined in periodic DFT calculations indicates that the expansion at very low temperature is governed by external lattice modes, but above 100 K the influence of intramolecular ring-flexing modes also becomes significant. The principal directions of thermal expansion are determined by the sensitivity of different van der Waals interactions to changes in distance. The principal values of the strain developed on application of pressure are similarly oriented to those determined in the variable-temperature study, but more isotropic because of the need to minimise volume by filling interstitial voids at elevated pressure.

High-resolution neutron powder diffraction has been used in order to characterize the order–disorder transition in monoclinic cyclo-octasulphur. Rapid data collection and the novel use of geometrically constrained refinements has enabled a direct and precise determination of the order parameter, based on molecular site occupancies, to be made. The transition is critical and continuous; with a transition temperature, Tc = 198.4 (3) K, and a critical exponent, β = 0.28 (3), which is indicative of three-dimensional ordering. Difficulties encountered as a consequence of the low thermal conductivity of the sample are discussed.

The crystal structure of phase III of perdeuterocyclohexanone, C6D10O, has been determined at 5 K using high-resolution neutron powder diffraction. Below its melting point of 245 K cyclohexanone forms a plastic crystal in the space group Fm\bar 3m. On cooling below 225 K the crystal transforms to the monoclinic phase III structure in the space group P21/n. The orthorhombic phase II structure exists under high pressure, but the triple point for all three phases is close to atmospheric pressure. Details of the phase II structure are also reported at 4.8 kbar (273 K) and ambient pressure. The phase behaviour of the compound and isotope effects are discussed.

In situ methods are ideally suited to study the development of property-critical structural features during material processing. As an example of their potential in the area of complex oxide electroceramics, here, we apply X-ray synchrotron and neutron powder diffraction to investigate the ordering processes responsible for optimizing the microwave dielectric properties of the commercial electroceramic barium zinc tantalate (BZT) doped with strontium gallate. The collection of synchrotron diffraction data with high resolution and high intensity during processing has allowed the development of cation site order and growth of ordered domains to be followed during the multi-stage thermal treatment processing used by industry. Complementary neutron diffraction data reveal changes in the cation–oxygen bonding associated with development of the cation-ordered trigonal perovskite structure. The effects of material loss with processing at temperatures of up to 1525°C are discussed following experiments using open and closed systems. The complex nature of the ordering is confirmed by the coexistence of two BZT phases and the formation of a further zinc-deficient phase. The implications of these observations and relation to the industrial processing procedures are discussed.

The phase II crystal structure of perdeuterated cyclohexene oxide has been solved from neutron powder diffraction data at 5 K utilising a simulated annealing method. The structure is triclinic, space group P 1 ¯ , with a unit cell volume of 268 A3. The refined molecular conformation is shown to be in close agreement with recent ab initio calculations. The p–T phase diagram determined using DTA and details of the phase I structure are also reported.

The room-temperature crystal structure of the heavy fermion antiferromagnet Ce2RhIn8, dicerium rhodium octaindide, has been studied by a combination of high-resolution synchrotron X-ray reciprocal-space mapping of single crystals and high-resolution time-of-flight neutron powder diffraction. The structure is disordered, exhibiting a complex interplay of non-periodic, partially correlated planar defects, coexistence and segregation of polytypic phases (induced by periodic planar `defects'), mosaicity (i.e. domain misalignment) and non-uniform strain. These effects evolve as a function of temperature in a complicated way, but they remain down to low temperatures. The room-temperature diffraction data are best represented by a complex mixture of two polytypic phases, which are affected by non-periodic, partially correlated planar defects, differ slightly in their tetragonal structures, and exhibit different mosaicities and strain values. Therefore, Ce2RhIn8 approaches the paracrystalline state, rather than the classic crystalline state and thus several of the concepts of conventional single-crystal crystallography are inapplicable. The structural results are discussed in the context of the role of disorder in the heavy-fermion state and in the interplay between superconductivity and magnetism.

Figure 2 and its caption were incorrect, along with a line of the text. Full details can be found in the PDF; these corrections do not change the conclusions of the paper.

Neutron powder diffraction has been used to study the two title compounds, Sr2HoRuO6 and Sr2TbRuO6, in order to determine the crystal and magnetic structures of the materials. Both materials have a distorted double perovskite structure and at temperatures below 30-40 K long-range magnetic order is observed. The. magnetic. structure for each compound consists of two interpenetrating Type I arrangements, one for the ruthenium sublattice and one for the rare earth sublattice, which are antiferromagnetically coupled to each other and order at the same temperature. From variable temperature measurements the combination of magnetic interactions and their relative strengths was deduced. The antiferromagnetic Ru-O-O-Ru interaction is approximately constant in the compounds, but it is a factor of 2.5 and 0.7 times stronger than the antiferromagnetic Ru-O-Ho and Ru-O-Tb interactions respectively.

The results on deprotonation and cation partitioning as a function of temperature in the tetrahedral and octahedral sites of a phengite 3 T , (K 0.94 Na 0.02 ) ∑0.96 (Al 1.43 Mg 0.33 Fe 0.25 ) ∑2.01 (Si 3.47 Al 0.53 ) ∑4 O 10 (OH) 2 , from Cima Pal (western Alps) are presented and discussed. They have been obtained by a synergic approach based on neutron powder diffraction from 293 K to 873 K, and on Mossbauer spectroscopy at ambient conditions, of the sample before and after heating. Order is observed at room temperature in both tetrahedral (Si fully occupies T 1) and octahedral (Al almost fills M 2) sites. Upon heating, deprotonation [mainly according to the reaction 2[(OH) - + Fe 2+ ] → 2O 2- + 2Fe 3+ + H 2 ↑], Fe oxidation and inter-site cation re-ordering reactions take place. Al moves by ~ 0.1 atoms from M 2 to M 3, and is replaced by Mg and Fe; Fe 2+ and Fe 3+ occupy different M -sites, after cooling, whereas Fe 2+ was partitioned on two sites, one site shared with Fe 3+ , before heating.

The perovskite Ba3ZnTa2O9 (BZT) is the parent of a family of high dielectric constant, low-loss oxides currently applied in mobile telecommunications base stations. The preparation of BZT microwave dielectric resonators with optimal dielectric properties requires careful attention to thermal treatment to control the formation of domains in which the Ta and Zn cations order on the octahedral sites of the perovskite structure. In this paper we use powder X-ray and neutron diffraction together with X-ray fluorescence and electron microscopy to demonstrate the strong correlation between the size of the cation-ordered domains, the extent of the ordering within them, and their composition, particularly the ZnO content. Phase separation into two trigonal BZT phases, which differ in the extent of cation order and the size of the ordered domains, is a common feature of all the synthetic procedures. A zinc-deficient impurity phase of nominal stoichiometry Ba8ZnTa6O24, involving face- as well as corner-sharing octah...

High-resolution powder neutron diffraction data (HRPD, ISIS Facility, U.K.) were collected on (H3O)Zr2(PO4)3 between 4 and 300 K. In the range 175-180 K, a ferroelastic first-order phase transition from the rhombohedral R3̄c NASICON structure to a monoclinic distorted one was discovered. Rietveld structure refinements were performed at 300 K (a = 8.74848(3), c = 23.7598(1) Å) and 4.5 K (space group C2/c or Cc, a = 15.0663(1), b = 8.7878(1), c = 9.3611(1) Å, β = 122.498(1)°). At RT, the hydronium ion H3O+ is pyramidal disordered over two inversion-related configurations. At LT, H3O+ becomes nearly planar, with two H atoms ordered and hydrogen-bonded to neighboring oxygens, and the third H atom disordered over two close positions with 2/3 and 1/3 statistical occupancies. Relations with the structural features of hydronium in other structures and the nature of order-disorder are discussed. An atomistic mechanism for proton conductivity is proposed.

The crystal structures of the mixed valence compound Bi(IIII)3Bi(V)O7 (1) and isotypic Bi(III)3Sb(V)O7 (2) were determined ab initio from powder diffraction data. At room temperature, both structures crystallize in space group P 1 (Z=2) with a=6.7253(2), b=6.9950(2), c=7.7961(2) A, α=72.566(2), β=88.842(2), γ=76.925(2)°, and V=340.39(2) A3 for (1) and a=6.6044(3), b=7.0146(3), c=7.6048(3) A, α=73.388(2), β=89.225(2), γ=76.190(2)°, and V=327.28(2) A3 for (2). Combined Rietveld refinements based on both synchrotron and neutron powder patterns were performed (T=295 K, R-Bragg=5.4% for (1), R-Bragg=6.4% for (2)). The crystal structures contain five crystallographically different anion polyhedra in two different coordination types and can be related to the CaF2-type structure with the cations forming a distorted face centered cubic lattice and the oxygen atoms filling positions close to seven of the eight tetrahedral cavities.

We present a detailed study of the structure of the polymer phase of AC 60 (A=K, Rb) using data obtained from high-resolution neutron powder diffraction. We are able to obtain stable Rietveld refinements of both structures with no constraints on any carbon atom. KC 60 has lattice parameters a =9.952 A, b =9.091 A, c =14.372 A in space group P nmn and RbC 60 has a =10.125 A, b =9.086 A, c =14.207 A and β =90.316° in space group I12/ m 1. We notice significant distortion throughout the ball from its original icosahedral symmetry. Coordinates of the atoms will allow more accurate theoretical calculation of the physical properties of these fullerides and might shed light on the differences among the cations and the dimensionality of their electronic properties.

High-resolution powder neutron diffraction has been used to study the crystal structure of the Mg11B2 superconductor (Tc = 39.2 K) prepared at high pressure in the temperature range 2 to 293 K. The experimental data provide clear evidence of temperature-independent phase inhomogeneity, which was modelled by coexisting Mg1+δB2 phases. Precise structural information for temperatures near Tc also reveals extremely small volume and c-axis discontinuities, consistent with the existence of a specific heat anomaly at Tc.

The hitherto unknown crystal structure of ozone was determined from conventional X-ray and high resolution neutron powder diffraction data. Ozone crystallises in the orthorhombic space group Pbca with 8 formula units per unit cell. The lattice parameters at 5 K are: a =778.88(2) pm, b =669.73(2) pm, c =684.13(2) pm. The geometry determined for an ozone molecule in the solid is found in good agreement with the well-known gas phase structure. In particular, there is no significant deviation from ideal C 2v symmetry. The efficient molecular packing is generated by three mutually perpendicular glide planes. Within this packing the orientation of the molecules is such that their polar axes lie roughly parallel or anti-parallel to the c -axis and secondly, neglecting their elevation along z , the atoms of one molecule are almost perfectly aligned with terminal but not central atoms of neighbouring units. This gives rise to parquet type patterns perpendicular to the c -axis. As a consequence solid ozone comprises a number of structural fragments with short intermolecular contacts between the terminal atoms of the O 3 triangles. Of these an ozone dimer with C i point symmetry is the simplest. The unit cell dimensions of solid ozone have been determined as a function of temperature. The negative coefficient of thermal expansion obtained for the b lattice parameter is explained by structural peculiarities of parquet type patterns.

Neutron powder diffraction data of deuterated clathrate hydrates of carbon dioxide and xenon were measured in order to investigate the effects of the motion of guest molecules on the structure of the surrounding hydrogen-bonded network. Rietveld analysis revealed that the magnitude of the thermal parameters of the host atoms of the CO2 clathrate hydrate depends strongly on the crystallographic site, while those of the Xe clathrate hydrate do not depend on the site. The host lattice structure of the Xe clathrate hydrate may be a close representation of the hypothetical empty clathrate hydrate, because of the simple spherical shape of the Xe molecule. The site dependence of the thermal parameters of the host atoms in the CO2 clathrate hydrate are attributed to the motion of the CO2 molecules in the cages. The CO2 molecule rotates anisotropically about the symmetry axis inside the tetrakaidecahedral cage. We suppose that the surrounding deuterium atoms are temporarily attracted and released by the oxygen ato...

The structure of Al2(CD3)6 has been determined at 4.5 K by powder neutron diffraction. The molecule crystallizes in space group C2/c. Salient bond distances and angles are as follows: r(Al−Cb) = 2.145(7)/2.146(8) A, r(Al−Ct) = 1.945(6)/1.926(5) A, r(Al···Al) = 2.700(10) A, r(C−D) = 1.061(4)−1.118(5) A; ∠Cb−Al−Cb = 102.0(3)°, ∠Ct−Al−Ct = 125.8(3)°, ∠Cb−Al−Ct = 108.8(4)°, ∠D−C−D = 99.0(4)−111.6(5)°. The bridging CD3 groups adopt a staggered conformation with respect to each other in a molecule with C2h symmetry, and there is no evidence in the structure for the involvement of the C−D bonds in Al−CD3−Al bridge bonding. Variable-temperature studies show the lattice constant b to exhibit unusual behavior, initially decreasing with increasing temperature. This phenomenon is interpreted in terms of increasing thermal motion of the molecule with increasing temperature.

Neutron diffraction experiments on the room- and low-temperature phases of K3H(SeO4)2 (Ttrs = 21 K) and K3D(SeO4)2 (Ttrs = 105 K) have been performed using the time-of-flight high-resolution powder diffractometer HRPD at the ISIS pulsed neutron source. Data were collected at 2 and 30 K for K3H(SeO4)2 and 2 and 130 K for K3D(SeO4)2 over the d-spacing range 0.7-2.4 A. The room-temperature phases of the crystals have monoclinic structure belonging to the space group A2/a. Rietveld refinement combined with the published dielectric and calorimetric studies reveals that H/D atoms are disordered over two symmetry-equivalent positions. The hydrogen bonds are non-linear with an O-H/DO angle of 175(1)°. The hydrogen bond length is 2.495(1) A in K3H(SeO4)2 and 2.533(2) A in K3D(SeO4)2. The separation between the half occupancy hydrogen positions is 0.32(1) A for K3H(SeO4)2 and 0.550(4) A for K3D(SeO4)2. The relation between these structural parameters and the isotope effect in the transition temperature is discussed. Diffraction patterns of the low-temperature phases indicated that the structure change accompanying the transition is very small.

The ZrW2O8 family of materials has been shown to display the unusual property of an isotropic bulk contraction in volume as a function of temperature. We report here on negative thermal expansion properties and oxygen migration at very low temperatures in ZrW2-xMoxO8 phases. ZrWMoO8 shows significant negative thermal expansion (αl = −8.7 × 10-6 K-1) and oxygen migration at temperatures as low as 200 K with an activation energy of 0.23 eV. We are unaware of oxygen migration at such low temperatures in other electronic insulators. The low-temperature migration is associated with an order−disorder transition between the α and β forms of the material with ΔVdisorder = −0.24%. A negative volume change for an order−disorder transition is unusual.

Neutron diffraction profiles at high resolution (HRPD, ISIS Facility, U.K.) were collected on powder samples of LiZr2(PO4)3 at 20°C (β′phase, monoclinic P21/n, Z=4; a=8.81277(4), b=8.94520(5), c=12.37540(6) A, β=90.801(1)°) and at 350°C (β phase, orthorhombic Pbna, Z=4; a=8.84303(5), b=8.94120(6 ), c=12.41301(8) A). All Li sites were located by difference Fourier maps in tetrahedral coordination, and both structures were Rietveld-refined to wRp=0.0353 (β′) and 0.0429 (β). The β′ structure is a distortion of β, with a [100] pseudo-twofold axis x, 1/2−y,−z relating all atoms but lithium, which is fully ordered (〈Li–O〉=2.02 A). In the β phase, Li is disordered over four sites, of which two (Li1 and Li2) are symmetry-independent with occupancies 0.34(1) and 0.16(1), respectively, and 〈Li–O〉=2.17 and 2.16 A. The disorder fully explains the higher Li+ mobility in the β with respect to the β′ phase observed from ionic conductivity data. Mechanisms of ion transport are proposed, and the relationship to the NASICON-type α′/α phase of LiZr2(PO4)3 is analyzed in detail.

The crystal structures of perdeuterated m -xylene, space group Pbca , a =10.1550(1), b =7.4649(1), c=16.8814(1) A , V=1279.72(2) A 3 , Z =8, ρ calc =1.1021(1) g cm −3 and perdeuterated p -xylene, space group P 2 1 / n , a =5.7337(1), b =4.9485(1), c=11.1385(1) A , β =100.7130(1)°, V=310.52(2) A 3 , Z =2, ρ calc =1.2431(1) g cm −3 at 4.5 K have been refined against high-resolution neutron powder diffraction data. The previously unknown structure of m -xylene was solved from the neutron powder diffraction data utilising a simulated annealing method and verified by a conventional Direct Methods solution using X-ray single crystal data. The inter-molecular contacts determined in the structures of the meta and para compounds are closely related.

High-resolution neutron powder diffraction is particularly suited for structure solution and refinement of low-melting point organic crystals of moderate complexity. Here we present the ab initio structure solution and refinement of stable-phase and metastable-phase perdeuterated acetaldehyde (m.p.: 151 K). In the stable phase, the molecule crystallises in the space group P21/c (no. 14) with a=3.9069(1) A , b=5.4224(1) A , c=12.1868(1) A , β=94.2970(2)° at 5 K, Z=4. In the metastable phase, the molecule crystallises in the space group Pna21 (no. 33) with a=5.1973(1) A , b=6.9791(1) A , c=6.9712(1) A at 5 K, Z=4. The metastable to stable phase transition is characterised by heat capacity measurements.

Inelastic neutron scattering (INS) spectra of ethene and propene adsorbed on carbon at ca. 20 K have been recorded and compared with the spectra of the solid alkenes. Differences between the alkenes in the INS spectra are attributed to the propene molecule experiencing less molecular recoil. Normal coordinate analysis reveals both alkenes to be present as physisorbed, disordered layers on carbon and that the interaction with the surface is weaker than in the corresponding crystal. On the basis of neutron powder diffraction measurements, the splitting in the C–CH3 torsion mode of solid propene is assigned to correlation field effects.

The high-resolution powder diffractometer (HRPD) has been upgraded with the installation of a 100 m supermirror guide. The guide is a unique ballistic design that is elliptic in both horizontal and vertical cross-section and utilizes a graded supermirror coating of between m=1.5 in the center to m=3 at the beam entry and exit points. Flux gains are considerable when compared to the performance of the original m=1 guide. The shallower radius of curvature (36 km) of the new guide also permits transmission of short wavelength neutrons previously cut off. The flux gains have been achieved whilst retaining the high resolution at backscattering, Δd/d∼5×10−4. Results following initial commissioning studies are described highlighting the new capabilities of the instrument and new scientific opportunities resulting from the upgrade are discussed.

The thermal expansion of the orthorhombic \ensuremath{\gamma} phase of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ has been measured using neutron powder diffraction from 4.6 to 410 K, where it transforms to the cubic \ensuremath{\alpha} phase. At low temperature, $\ensuremath{\gamma}{\ensuremath{-}\mathrm{Z}\mathrm{r}\mathrm{W}}_{2}{\mathrm{O}}_{8}$ has a negative thermal expansion, but the thermal expansion becomes less negative with increasing temperature and is slightly positive at room temperature. This behavior can be explained in terms of the contributing phonon modes: At low temperature, the vibrational modes lead to a negative thermal expansion, but additional modes that become active upon increasing temperature add positive contributions. Above room temperature, the a and b axes increase more sharply while the c axis reverses its behavior and decreases with increasing temperature. This unusual behavior can be explained in terms of a thermally activated process, presumed to result from oxygen-atom migration, that makes an additional contribution to the thermal expansion.

High-resolution powder neutron diffraction data have been collected at room temperature on the triclinic alpha' phase of LiZr2(PO4)(3) (HRPB, ISIS Facility, U.K.). Rietveld refinement analysis (space group C (1) over bar; Z = 4; a = 15.0718(2), b = 8.8556(1), c = 9.1233(1) Angstrom, alpha = 89.660(1), beta = 123.912(1), gamma = 90.429(1)degrees) and Fourier difference maps have revealed the presence of monoclinic pseudo-symmetry -x, y, 1/2 - z for all atoms but lithium, which is disordered over two sites Li1 and Li2 with occupancies 0.71(3) and 0.29(3), respectively. The final agreement indexes are R-p = 0.0672, wR(p) = 0.0919, R(F-2) = 0.0603 for 92 refined parameters. Both Li1 and Li2 are in distorted tetrahedral coordinations, with (Li-O) = 2.09 and 2.18 Angstrom, respectively. Pairs of centrosymmetrical Li1 sites are located within half of the available M' holes of the NASICON structure, which are split into two independent sets by triclinic breaking of symmetry. This accounts for the lower conductivity of the triclinic phase. (C) 1999 Elsevier Science B.V. All rights reserved

Strontium and calcium amide are ionic compounds crystallizing in a tetragonally distorted anatase structure type. They include asymmetrically charged amide ions (NH2-/ND2-) which resemble water molecules in their structure and in their charge distribution. By means of temperature-dependent X-ray scattering measurements an anomalous thermal expansion was observed for both compounds (295 K for Sr(ND2)2, 370 K for Ca(ND2)2). Fourier analyses based on high-resolution neutron powder diffraction experiments carried out in a large temperature range show especially for the deuterium atoms a remarkably large, anisotropic spatial distribution of the probability density function. It is caused by strongly anisotropic librations of the amide ions. The amplitudes of their normal librations were analyzed using a rigid-body model to parametrize the atomic mean-square displacements of the atoms belonging to the amide ions. They can be described as composed of strong rocking librations with weak temperature dependence and ...

Potassium amide (KNH2/KND2) is an ionic molecular crystal. The amide ions (NH2−/ND2−)—isoelectronic with the water molecule OH2—display orientational disorder in the cubic and tetragonal high–temperature phases. In the monoclinic low–temperature phase they are orientationally ordered. We investigated the deuterated compound KND2 in all three phases using high resolution neutron powder diffraction and determined the time–averaged probability density function of the deuterons in detail. The structure of the intermediate tetragonal phase was solved, and deviations of the reorientations of the amide ions in the cubic phase from free rotational diffusion were quantified. From the structural properties of the different phases of KND2 and the structural changes at the phase transitions conclusions on the respective dynamics of the amide ions can be drawn.

High-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi206, RbAlSi20", CsAlSi206, and KFeSi206. The temperature-dependent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-0 bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of comer-linked (AI,Si)04 tetrahedra about [001] leads to collapse of the (111) structural channels and concomitant volume reduction.

The crystal structure of perdeuterodimethyl sulfoxide, (CD3)2SO, has been refined at 2 and 100 K, and characterized as a function of temperature up to 275 K against high-resolution neutron powder diffraction data. The structure determined previously by Thomas, Shoemaker & Eriks [Acta Cryst. (1966), 21, 12-20] (T=278 K) is shown to remain down to 2 K. At 2 K, the S-O bond distance is 1.495 (2) A. The fact that the molecule is distorted from ideal Cs point symmetry may be explained by the short D...O contacts of the respective methyl groups.

K1−x(NH4)xH2PO4 mixed crystals which in a certain composition range exhibit proton glass properties were studied using neutron diffraction and quasielastic neutron scattering. Anomalous temperature behaviour of the structure and rotational diffusion of ammonium groups are discussed.