Magnetic Excitations in the Ground State of Yb2Ti2O7\mathrm{Yb_2Ti_2O_7}

We report an extensive study on the zero field ground state of a powder sample of the pyrochlore $\mathrm{Yb_2Ti_2O_7}$. A sharp heat capacity anomaly that labels a low temperature phase transition in this material is observed at 280 mK. Neutron diffraction shows that a \emph{quasi-collinear} ferromagnetic order develops below $T_\mathrm{c}$ with a magnetic moment of $0.87(2)\mu_\mathrm{B}$. High resolution inelastic neutron scattering measurements show, below the phase transition temperature, sharp gapped low-lying magnetic excitations coexisting with a remnant quasielastic contribution likely associated with persistent spin fluctuations. Moreover, a broad inelastic continuum of excitations at $\sim0.6$ meV is observed from the lowest measured temperature up to at least 2.5 K. At 10 K, the continuum has vanished and a broad quasielastic conventional paramagnetic scattering takes place at the observed energy range. Finally, we show that the exchange parameters obtained within the framework of linear spin-wave theory do not accurately describe the observed zero field inelastic neutron scattering data.Comment: 11 pages, 9 figures, Phys. Rev. B. (accepted

Proton Diffusion Mechanism in Hydrated Barium Indate Oxides

We report on quasielastic neutron scattering (QENS) andab initiomolecular dynamics (AIMD) simulations of the mechanism of proton diffusionin the partially and fully hydrated barium indate oxide proton conductorsBa(2)In(2)O(5)(H2O)( x ) (x = 0.30 and 0.92). Structurally,these materials are featured by an intergrowth of cubic and "pseudo-cubic"layers of InO6 octahedra, wherein two distinct proton sites,H(1) and H(2), are present. We show that the main localized dynamicsof these protons can be described as rotational diffusion of O-H(1)species and H(2) proton transfers between neighboring oxygen atoms.The mean residence times of both processes are in the order of picosecondsin the two studied materials. For the fully hydrated material, Ba2In2O5(H2O)(0.92), we also reveal the presence of a third proton site, H(3), whichbecomes occupied upon increasing the temperature and serves as a saddlestate for the interexchange between H(1) and H(2) protons. Crucially,the occupation of the H(3) site enables long-range diffusion of protons,which is highly anisotropic in nature and occurs through a two-dimensionalpathway. For the partially hydrated material, Ba2In2O5(H2O)(0.30), the occupationof the H(3) site and subsequent long-range diffusion are not observed,which is rationalized by hindered dynamics of H(2) protons in thevicinity of oxygen vacancies. A comparison to state-of-the-art proton-conductingoxides, such as barium zirconate-based materials, suggests that thegenerally lower proton conductivity in Ba2In2O5(H2O)( x ) is dueto a large occupation of the H(1) and H(2) sites, which, in turn,means that there are few sites available for proton diffusion. Thisinsight suggests that the chemical substitution of indium by cationswith higher oxidation states offers a novel route toward higher protonconductivity because it reduces the proton site occupancy while preservingan oxygen-vacancy-free structure

Geometric frustration and concerted migration in the superionic conductor barium hydride

Authors would like to thank the ISIS Facility Development Studentship for funding this work. Additionally, I would like to thank ISIS Neutron and Muon Source for providing the beam time to collect all the scattering data presented in this paper. Finally, I would like to thank the Crockett Scholarship for supporting my studies. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) license to any Accepted Author Manuscript version arising.Ionic conductivity is a phenomenon of great interest, not least because of its application in advanced electrochemical devices such as batteries and fuel cells. While lithium, sodium, and oxide fast ion conductors have been the subjects of much study, the advent of hydride (H–) ion fast conductors opens up new windows in the understanding of fast ion conduction due to the fundamental simplicity of the H– ion consisting of just two electrons and one proton. Here we probe the nature of fast ion conduction in the hydride ion conductor, barium hydride (BaH2). Unusually for a fast ion conductor, this material has a structure based upon a close-packed hexagonal lattice, with important analogues such as BaF2 and Li2S. We elucidate how the structure of the high temperature phase of BaH2 results in a disordered hydride sublattice. Furthermore, using novel combined quasi-elastic neutron scattering (QENS) and electrochemical impedance spectroscopy (EIS) we show how the high energy ions interact to create a concerted migration that results in macroscopic superionic conductivity via an interstitialcy mechanism.Publisher PDFPeer reviewe

Dynamics in the ordered and disordered phases of barocaloric adamantane

High-entropy order-disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K-1 kg-1 . Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this behaviour to the contrast between an 'interlocked' state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials.Comment: 14 pages, 6 figure

A New Avenue to Relaxor-like Ferroelectric Behaviour Found by Probing the Structure and Dynamics of [NH3NH2]Mg(HCO2)3

The field of relaxor ferroelectrics has long been dominated by ceramic oxide materials exhibiting large polarisations with temperature and frequency dependence. Intriguingly, the dense metal-organic framework (MOF) [NH3NH2]Mg(HCO2)3 was reported as one of the first coordination frameworks to exhibit relaxor-like properties. This work clarifies the origin of these relaxor-like properties through re-examining its unusual phase transition using neutron single crystal diffraction, along with solid-state NMR and quasielastic neutron scattering studies. This reveals that the phase transition is caused by the partial re-orientation of NH3NH2 within the pores of the framework, from lying in the planes of the channel at lower temperatures to along the channel direction above the transition temperature. The transition occurs via a dynamic process such that the NH3NH2 cations can slowly interconvert between parallel and perpendicular orientations, with an estimated activation energy of 60 kJ mol-1. Furthermore these studies are consistent with proton hopping between the hydrazinium cations oriented along the channel direction via a proton site intermediate. This suggests the ferroelectric properties of [NH3NH2]Mg(HCO2)3 likely driven by a hydrogen bonding mechanism. The relaxor behaviour is proposed to be the result of polar regions, which likely fluctuate due to increased cation dynamics at high temperature. The combination of cation reorientation and proton hopping fully describes this material’s relaxor-like behaviour, suggesting a route to future design of non-oxide-based relaxor ferroelectrics

Experimental evidence for two different dynamical regimes in liquid rubidium

We present evidence for changes in the dynamics of liquid rubidium with rising temperature. The thermal expansion of this liquid alkali metal shows a changing derivative with temperature in a temperature range of about 400-500 K. With neutron scattering the amplitude at the structure factor maximum demonstrates a changing slope with increasing temperature. A derived averaged structural relaxation time can be understood that an additional relaxation process sets in upon cooling. The deduced generalized viscosity and high frequency shear modulus indicate a change in dynamics in the same temperature range. All these findings point to a change in dynamics of the equilibrium liquid metal state with a dynamical crossover from a viscous to a fluid-like liquid metal well above the melting point