A greater understanding of the structure–property relationships of hybrid perovskites for solar cells is crucial for enhancing their performance. The low-temperature phases of formamidinium lead iodide (FAPbI3) have been investigated using rapid neutron powder diffraction. On cooling, the metastable α-polymorph descends in symmetry from the cubic unit cell phase present at room temperature through two successive phase transitions. Between 285 and 140 K a tetragonal phase, adopting the space group P4/mbm, is confirmed and the orientation of the disordered FA cation over this temperature range determined. The cation dynamics have also been investigated, over the same temperature range, at the atomic scale by using ab initio molecular dynamics simulations, which indicate contrasting FA motion in the cubic and tetragonal structures. Below 140 K the neutron powder diffraction data display weak Bragg scattering intensities not immediately indexable to a related unit cell. Data collected at 100 K from N-deuterated FAPbI3 did not reveal any indication of the fundamental Bragg reflections at high d-spacing expected for an expanded supercell model as previously reported. Other hypotheses of a mixture of phases or a simple tetragonal cell below 140 K are also rejected on the basis of the observed data, and our observations are consistent with a locally disordered low-temperature γ-phase.

D20 is a medium to high resolution two-axis diffractometer capable of producing a neutron flux of 108 s−1 cm−2 at the sample position. The 1536 detection cells of its curved linear position sensitive detector (PSD) cover a continuous 2θ range of 153.6° over a total solid angle of 0.27 sr. This combination of a high incident neutron flux and a large detector solid angle provides D20 with the fastest counting rate, at a given resolution, of any reactor-based neutron diffractometer. Different monochromators and take-off angles, plus optional Soller collimators and secondary slits, permit a wide choice in the Q-space range, wavelength, resolution and flux. A high-resolution configuration offers Δd/d ∼ 2 × 10−3. Fast modern counting electronics allow in situ time-resolved experiments at the timescale of a few tens of milliseconds. In addition, a variety of sample environments, including an optional radial oscillating collimator for suppressing parasitic scattering, contribute to a rich scientific programme.

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The ambitious instrument suite for the future European Spallation Source whose civil construction started recently in Lund, Sweden, demands a set of diverse and challenging requirements for the neutron detectors. For instance, the unprecedented high flux expected on the samples to be investigated in neutron diffraction or reflectometry experiments requires detectors that can handle high counting rates, while the investigation of sub-millimeter protein crystals will only be possible with large-area detectors that can achieve a position resolution as low as 200 µm. This has motivated an extensive research and development campaign to advance the state-of-the-art detector and to find new technologies that can reach maturity by the time the ESS will operate at full potential. This paper presents the key detector requirements for three of the Time-of-Flight (TOF) diffraction instrument concepts selected by the Scientific Advisory Committee to advance into the phase of preliminary engineering design. We discuss the detector technologies commonly employed at the existing similar instruments and their major challenges for ESS. The detector technologies selected by the instrument teams to collect the diffraction patterns are also presented. Analytical calculations, Monte-Carlo simulations, and real experimental data are used to develop a generic method to estimate the event rate in the diffraction detectors. We apply this method to make predictions for the future diffraction instruments, and thus provide additional information that can help the instrument teams with the optimisation of the detector designs.

The mixed cation lead iodide perovskite photovoltaics show improved stability following site substitution of cesium ions (Cs +) onto the formamidinium cation sites (FA +) of (CH(NH2)2PbI3 (FAPbI3), and increased resistance to formation of the undesirable ∂-phase. The structural phase behavior of Cs0.1FA0.9PbI3 has been investigated by neutron powder diffraction (NPD), complemented by single crystal and power X-ray diffraction, and photoluminescence spectroscopy. The Cs substitution limit has been determined to be less than 15% and the cubic α-phase Cs0.1FA0.9PbI3 is shown to be synthesizable in bulk and stable at 300 K. On cooling cubic Cs0.1FA0.9PbI3 a slow, second order cubic to tetragonal transition is observed close to 290 K, with variable temperature NPD indicating the presence of the tetragonal βphase, adopting the space group P4/mbm, between 290 K and 180 K. An orthorhombic phase or twinned tetragonal phase is formed below 180 K and the temperature for the further transition to a disordered state is lowered to 125 K compared to that seen in phase pure α-FAPbI3 (140 K). These results demonstrate the importance of understanding the effect of cation site substitution on structure-property relationships in perovskite materials. FA site of the FAPbI3 structure can significantly improve device lifetimes, enhancing the thermal and moisture stability of thin films when compared to pure FAPbI3 15. Yi et al. attributed the enhanced stability provided by the Cs cation to the improved crystallization of the α-phases, as cation mixing in the α-phase is more energetically favorable than that in the δ-phase for the CsPbI3 and FAPbI3 structure types 16. However, there is confusion concerning the structures of the mixed Cs-FA lead iodide perovskites, with contradictory information on whether the composition Cs0.1FA0.9PbI3 adopts a tetragonal or cubic structure at room temperature 17-20 , and the phase behavior of the mixed Cs-FA cation perovskite remains poorly understood. It is crucial that the fundamental structural properties of these mixed Cs-FA cation materials are fully understood in order to appreciate PV device operation across different environments.

This article reports a comprehensive investigation of the average and local structure of La 5.6 WO 12À , which has excellent mixed proton, electron and oxide ion conduction suitable for device applications. Synchrotron X-ray and neutron powder diffraction show that a cubic fluorite supercell describes the average structure, with highly disordered lanthanum and oxide positions. On average, the tungsten sites are sixfold coordinated and a trace [3.7 (1.3)%] of anti-site disorder is detected. In addition to sharp Bragg reflections, strong diffuse neutron scattering is observed, which hints at short-range order. Plausible local configurations are considered and it is shown that the defect chemistry implies a simple 'chemical exchange' interaction that favours ordered WO 6 octahedra. The local model is confirmed by synchrotron X-ray pair distribution function analysis and EXAFS experiments performed at the La K and W L 3 edges. It is shown that ordered domains of $3.5 nm are found, implying that mixed conduction in La 5.6 WO 12À is associated with a defective glassy-like anion sublattice. The origins of this ground state are proposed to lie in the nonbipartite nature of the face-centred cubic lattice and the pairwise interactions which link the orientation of neighbouring octahedral WO 6 sites. This 'function through frustration' could provide a means of designing new mixed conductors. research papers J. Appl. Cryst. (2016). 49, 997-1008 Tobias Scherb et al. Nanoscale order in La 5.6 WO 12À 999 Figure 1Observed, calculated and difference profiles for the Rietveld fit to the synchrotron X-ray diffraction profile of La 5.6 WO 12À , highlighting the region of the 10 6 2 reflection. The peak asymmetry is well fitted by the minimal broadening model discussed in the text.

The key design parameters for the ESS provide new opportunities in neutron diffraction. The long pulse at 5 MW is most intense and about two orders of magnitude larger than the neutron flux of today's leading pulsed sources. The peak brightness clearly exceeds those of the existing short pulse spallation sources. Tailoring the pulse with fast choppers results in a very flexible time-resolution and yields a unique versatility for measuring either with highest resolution or highest intensity for tiny samples or real-time studies of chemical reactions. This versatility is a characteristic feature of the proposed DREAM powder diffractometer [1], which can ultimately offer a d-resolution of 0.00028 Å. Other interesting features with respect to efficiency are the simultaneous use of the thermal and cold ESS moderators, by use of a solid Si bender which will be reflecting the cold neutrons into the incident beam, while transmitting the thermal neutrons. The detectors are based on a new technology using B-10 coated cathodes in inclined geometry [2] covering a large solid angle with position sensitivity appropriate for powder and single crystal diffraction. The instrument has entered into construction in early 2017. The project scope of the instrument has been set with a budget to deliver a world leading neutron powder diffractometer already with the start of user operation at the ESS in 2023. The design has been driven by the broad science case received from the European user community. These cases emphasize the needs for neutron diffraction for small or complex samples, in-situ studies of batteries, metal-organic framework structures, and phase-studies with weak signals related to magnetism and superconductivity. Possible upgrade options cover high-pressure studies with diamond anvil cells and polarized neutrons distinguishing magnetic diffraction or removing the typical large background of hydrogenous materials. With an additional detector for small scattering angles, DREAM will probe multiple length scales within a Q-range from 0.01 to 25 1/Å, which is sufficient for PDF-studies as well as for small angle scattering of nanoparticles. Simulating and benchmarking the instrument to world leading instruments demonstrates a far superior performance. [1] Schweika, W. et al. (2016) J. Physics Conf. Ser. 746 012013. [2] Modzel, G. et al. (2014) NIM A 743 90-95.

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