NIMROD is the Near and InterMediate Range Order Diffractometer of the ISIS second target station. Its design is optimized for structural studies of disordered materials and liquids on a continuous length scale that extends from the atomic, upward of 30 nm, while maintaining subatomic distance resolution. This capability is achieved by matching a low and wider angle array of high efficiency neutron scintillation detectors to the broad band-pass radiation delivered by a hybrid liquid water and liquid hydrogen neutron moderator assembly. The capabilities of the instrument bridge the gap between conventional small angle neutron scattering and wide angle diffraction through the use of a common calibration procedure for the entire length scale. This allows the instrument to obtain information on nanoscale systems and processes that are quantitatively linked to the local atomic and molecular order of the materials under investigation.

Among the over eighteen different forms of water ice, only the common hexagonal phase and a cubic phase are present in nature on Earth. 1,2 The existence of these two polytypes, almost degenerate in energy, represents one of the most important and unresolved topics in the physics of ice. [3][4][5] It is now widely recognised that all the samples of "cubic ice" obtained so far are instead a stacking-disordered form of ice I (i.e. ice Isd), in which both hexagonal and cubic stacking sequences of hydrogen-bonded water molecules are present. [6][7][8] Here we describe a new method to obtain cubic ice Ic in large quantities, and demonstrate its unprecedented structural purity from two independent neutron diffraction experiments performed on two of the leading neutron diffraction instruments in Europe.Stacking disordered forms of cubic ice are generally prepared by low-pressure vapour deposition, 9 or more commonly, by the back-transformation, at room pressure and low temperature, of amorphous 10 or crystalline high-pressure ice polymorphs. [11][12][13] We have prepared for the first time structurally pure ice Ic by the transformation of a powder of ice XVII at room pressure by increasing temperature. Ice XVII is a novel metastable phase of pure ice, obtained from the high-pressure hydrogen filled ice in the C 0 -phase. 14,15 This low density solid water phase has the characteristic of being highly porous, and, unique among the various stable and metastable phases of ice, exhibits a structure comprising only pentagonal rings of water molecules. 15,16 Ice XVII can be maintained at room pressure only up to about 130 K, above which it undergoes a phase transition similar to that mentioned above for the amorphous 10 and high-pressure crystalline 11,12 forms. Whilst the end-product of all of these transitions, above 200 K, is the ordinary hexagonal form of ice (ice Ih), the remarkable difference between ice XVII and the other forms is the nature of the intermediate state, where, instead of stacking-disordered ice, we find a structurally-pure form of cubic ice (true ice Ic).The transition can be easily detected by Raman spectroscopy, which is also a valuable method to study the transition kinetics as a function of either temperature or time. The stretching frequency region (b), measured at 50 K. (c): Frequency position of the OH stretching band (centre of the Lorentzian curve fitting the major band) during the transition ice XVII -ice Ic, while performing a 0.1 K/min temperature ramp (blue line and dots), or as a function of time at constant temperature T = 139.5 K (red line and dots). (d): Width of the OH stretching band (from the Lorentzian fit) measured during the same thermal treatments as in (c).Raman spectra of the two phases, ice XVII and ice Ic, present marked differences, both in the lattice modes (150-350 cm −1 ) and OH stretching region (3000-3500 cm −1 ). In the first region ( Fig. 1(a)) the differences concern both the position of the peaks and the shape of the whole band, while for the OH stretching mode (Fig....

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The structure of the recently identified metastable ice XVII, obtained by release of hydrogen from the C 0 D 2 O-H 2 compound (filled ice), has been accurately measured by neutron powder diffraction. The diffraction pattern is indexed with a hexagonal cell and can be refined with space group P6 1 22 so to obtain accurate values of the oxygen and deuterium positions.The values of the lattice constants at three temperatures between 25 to 100 K are reported, and their behavior is compared with that of ice Ih. Ice XVII is a porous solid that, if exposed to H 2 gas, may adsorb a substantial amount of it. Monitoring this effect at a constant temperature of 50 K, we have observed that the two lattice constants show opposite behavior, a increases and c decreases, with the volume showing a linear increase. At temperatures higher than 130 K the metastability of this form of porous ice is lost and the sample transforms into ice Ih.

The cold neutron imaging and diffraction instrument IMAT at the second target station of the pulsed neutron source ISIS is currently being commissioned and prepared for user operation. IMAT will enable white-beam neutron radiography and tomography. One of the benefits of operating on a pulsed source is to determine the neutron energy via a time of flight measurement, thus enabling energy-selective and energy-dispersive neutron imaging, for maximizing image contrasts between given materials and for mapping structure and microstructure properties. We survey the hardware and software components for data collection and image analysis on IMAT, and provide a step-by-step procedure for operating the instrument for energy-dispersive imaging using a two-phase metal test object as an example.