This work provides an up-to-date account of the use of electron-volt neutron spectroscopy in materials research. This is a growing area of neutron science, capitalising upon the unique insights provided by epithermal neutrons on the behaviour and properties of an increasing number of complex materials. As such, the present work builds upon the aims and scope of a previous contribution to this journal back in 2005, whose primary focus was on a detailed description of the theoretical foundations of the technique and their application to fundamental systems [see Andreani et al., Adv. Phys. 54 (2005) p.377] A lot has happened since then, and this review intends to capture such progress in the field. With both expert and novice in mind, we start by presenting the general principles underpinning the technique and discuss recent conceptual and methodological developments. We emphasise the increasing use of the technique as a non-invasive spectroscopic probe with intrinsic mass selectivity, as well as the concurrent use of neutron diffraction and first-principles computational materials modelling to guide and interpret experiments. To illustrate the state of the art, we discuss in detail a number of recent exemplars, chosen to highlight the use of electron-volt neutron spectroscopy across physics, chemistry, biology, and materials science. These include: hydrides and proton conductors for energy applications; protons, deuterons, and oxygen atoms in bulk water; aqueous protons confined in nanoporous silicas, carbon nanotubes, and graphene-related materials; hydrated water in proteins and DNA; and the uptake of molecular hydrogen by soft nanostructured media, promising materials for energy-storage applications. For the primary benefit of the novice, this last case study is presented in a pedagogical and question-driven fashion, in the hope that it will stimulate further work into uncharted territory by newcomers to the field. All along, we emphasise the increasing (and much-needed) synergy between experiments using electron-volt neutrons and contemporary condensed matter theory and materials modelling to compute and ultimately understand neutron-scattering observables, as well as their relation to materials properties not amenable to scrutiny using other experimental probes

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T he structure and dynamics of liquid water are directly influenced by quantum mechanics, not only in terms of the electronic structure and chemical bonding but also at the level of the nuclear motion. So-called nuclear quantum effects (NQEs) include zero-point energy, tunnelling, isotope effects in the thermodynamic properties, and, what is most relevant to the present work, large deviations from the classical, Maxwell− Boltzmann behavior of both the average nuclear kinetic energy ⟨E K ⟩ and the momentum distribution n(p).Even though NQEs are very large (the zero-point energy content of an O−H stretching vibration is in excess of 200 meV), it is often the case that their net effect on macroscopic properties is relatively small. For instance, the melting temperatures of light and heavy water differ by less than 4 K, and the boiling temperatures differ by just 1 K. Recent theoretical analyses 1,2 have suggested that this could stem from a partial cancellation between quantum effects in the intra-and intermolecular components of the hydrogen bond, so that the net effect is small even if the individual contributions are large. In particular, the competition between quantum effects can be seen very clearly when decomposing the changes in the quantum kinetic energy of protons and deuterons along different molecular axes. 3,4 The mechanism that underlies the competition between changes in the different components of the quantum kinetic energy can be understood by considering as an analogy a twolevel quantum system with an environment-dependent offdiagonal coupling β. A small change in the coupling Δβ, arising from a phase transition or some other change in the environment of the system, will shift its eigenvalues by the same amount proportional to Δβ, but in opposite directions. Even though this picture is clearly oversimplified, it is consistent with a diabatic state model of the hydrogen bond, 5 it demonstrates that the notion of competing quantum effects is nothing exotic, and explains why it returns in many circumstances in the study of water and other hydrogenbonded systems.Competing quantum effects have in fact been identified in a diverse variety of simulations, 1−4 and it seems entirely plausible

The all-inorganic perovskite barium zirconate, BaZrO 3 , is a widely used material in a range of different technological applications. However, fundamental questions surrounding the crystal structure of BaZrO 3 , especially in regard to its ground-state structure, remain. While diffraction techniques indicate a cubic structure all the way down to T = 0 K, several first-principles phonon calculation studies based on density functional theory indicate an imaginary (unstable) phonon mode due to the appearance of an antiferrodistortive transition associated with rigid rotations of ZrO 6 octahedra. The first-principles calculations are highly sensitive to the choice of exchange-correlation functional and, using six well-established functional approximations, we show that a correct description about the ground-state structure of BaZrO 3 requires the use of hybrid functionals. The ground-state structure of BaZrO 3 is found to be cubic, which is corroborated by experimental results obtained from neutron powder diffraction, inelastic neutron scattering, and neutron Compton scattering experiments.

The VESUVIO spectrometer at the ISIS pulsed neutron and muon source is a unique instrument amongst those available at neutron facilities. This is the only inverted-geometry neutron spectrometer accessing values of energy and wavevector transfer above tens of eV and , respectively, and where deep inelastic neutron scattering experiments are routinely performed. As such, the procedure at the base of the technique has been previously described in an article published by this journal (Mayers and Reiter 2012 Meas. Sci. Technol. 23 045902). The instrument has recently witnessed an upsurge of interest due to a new trend to accommodate, within a single experiment, neutron diffraction and transmission measurements in addition to deep inelastic neutron scattering. This work presents a broader description of the instrument following these recent developments. In particular, we assess the absolute intensity and two-dimensional profile of the incident neutron beam and the capabilities of the backscattering diffraction banks. All results are discussed in the light of recent changes to the moderator viewed by the instrument. We find that VESUVIO has to be considered a high-resolution diffractometer as much as other diffractometers at ISIS, with a resolution as high as in backscattering. Also, we describe the extension of the wavelength range of the instrument to include lower neutron energies for diffraction measurements, an upgrade that could be readily applied to other neutron instruments as well.