Localized Proton Motions in Acceptor-Doped Barium Zirconates

Acceptor-doped barium zirconates are currently accumulating considerable interest because of their high proton conductivity, especially in the intermediate-temperature range targeted for next-generation solid oxide fuel cells, combined with their excellent chemical stability. However, fundamental questions surrounding the proton conduction mechanism in these materials remain, for instance, regarding the nature of localized proton motions and how they depend on the local structural properties of the material. Here we investigate the nature of localized proton motions in the two acceptor-doped proton-conducting perovskites BaZr_0.9M_0.1O_2.95 with M = Y and Sc, using quasielastic neutron scattering. We show the presence of pronounced localized proton dynamics, with mean residence periods on the time-scale of 1–30 ps and an activation energy of ∼100 meV for both materials. In view of first-principles calculations as reported elsewhere the experimentally established dynamics could comprise footprints from proton transfers as well as O–H rotational motions in several different types of proton sites due to a range of various local proton sites present in both materials

Dynamics of Pyramidal SiH₃^– Ions in ASiH₃ (A = K and Rb) Investigated with Quasielastic Neutron Scattering

The two alkali silanides ASiH₃ (A = K and Rb) were investigated by means of quasielastic neutron scattering, both below and above the order–disorder phase transition occurring at around 275–300 K. Measurements upon heating show that there is a large change in the dynamics on going through the phase transition, whereas measurements upon cooling reveal a strong hysteresis due to undercooling of the disordered phase. The results show that the dynamics is associated with rotational diffusion of SiH₃^– anions, adequately modeled by H-jumps among 24 different jump locations radially distributed around the Si atom. The average relaxation time between successive jumps is of the order of subpicoseconds and exhibits a weak temperature dependence with a small difference in activation energy between the two materials, 39(1) meV for KSiH₃ and 33(1) meV for RbSiH₃. The pronounced SiH₃^– dynamics explains the high entropy observed in the disordered phase resulting in the low entropy variation for hydrogen absorption/desorption and hence the origin of these materials’ favorable hydrogen storage properties

Structural and Vibrational Properties of Silyl (SiH₃^–) Anions in KSiH₃ and RbSiH₃: New Insight into Si–H Interactions

The alkali metal silyl hydrides <i>A</i>SiH₃ (<i>A</i> = K, Rb) and their deuteride analogues were prepared from the Zintl phases <i>A</i>Si. The crystal structures of <i>A</i>SiH₃ consist of metal cations and pyramidal SiH₃^– ions. At room temperature SiH₃^– moieties are randomly oriented (α modifications). At temperatures below 200 K <i>A</i>SiH₃ exist as ordered low-temperature (β) modifications. Structural and vibrational properties of SiH₃^– in <i>A</i>SiH₃ were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-<i>A</i>SiH₃ SiH₃^– ions relate closely to freely rotating moieties with <i>C</i>_3<i>v</i> symmetry (Si–H bond length = 1.52 Å; H–Si–H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm^–1 (ν₁, A₁), 1883/1872 cm^–1 (ν₃, E), 988/986 cm^–1 (ν₄, E), and 897/894 cm^–1 (ν₂, A₁) for <i>A</i> = K/Rb. In ordered β-<i>A</i>SiH₃ silyl anions are slightly distorted with respect to their ideal <i>C</i>_3<i>v</i> symmetry. Compared to α-<i>A</i>SiH₃ the molar volume is by about 15% smaller and the Si–H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH₃^– anions in α-<i>A</i>SiH₃. Si–H stretching force constants for SiH₃^– moieties in various environments fall in a range from 1.9 to 2.05 N cm^–1. These values are considerably smaller compared to silane, SiH₄ (2.77 N cm^–1). The reason for the drastic reduction of bond strength in SiH₃^– remains to be explored

Understanding the Interactions between Vibrational Modes and Excited State Relaxation in Y_3–<i>x</i>Ce_<i>x</i>Al₅O₁₂: Design Principles for Phosphors Based on 5<i>d</i>–4<i>f</i> Transitions

The oxide garnet Y₃Al₅O₁₂ (YAG), when a few percent of the activator ions Ce³⁺ substitutes for Y³⁺, is a luminescent material widely used in phosphor-converted white lighting. However, fundamental questions surrounding the defect chemistry and luminescent performance of this material remain, especially in regard to the nature and role of vibrational dynamics. Here, we provide a complete phonon assignment of YAG and establish the general spectral trends upon variation of the Ce³⁺ dopant concentration and temperature, which are shown to correlate with the macroscopic luminescence properties of Y_3–<i>x</i>Ce_<i>x</i>Al₅O₁₂. Increasing the Ce³⁺ concentration and/or temperature leads to a red-shift of the emitted light, as a result of increased crystal-field splitting due to a larger tetragonal distortion of the CeO₈ moieties. Decreasing the Ce³⁺ concentration or cosubstitution of smaller and/or lighter atoms on the Y sites creates the potential to suppress thermal quenching of luminescence because the frequencies of phonon modes important for nonradiative relaxation mechanisms are upward-shifted and hence less readily activated. It follows that design principles for finding new Ce³⁺-doped oxide phosphors emitting at longer wavelengths require tetragonally distorted environments around the CeO₈ moieties and a sufficiently rigid host structure and/or low activator-ion concentration to avoid thermal quenching of luminescence

Structure and Conductivity of Epitaxial Thin Films of In-Doped BaZrO₃‑Based Proton Conductors

Epitaxial thin films of the proton-conducting perovskite BaZr_0.53In_0.47O_3−δH_0.47–2δ, grown by pulsed laser deposition, were investigated in their hydrated and dehydrated conditions through a multitechnique approach with the aim to study the structure and proton concentration depth profile and their relationship to proton conductivity. The techniques used were X-ray diffraction, X-ray and neutron reflectivity, nuclear reaction analysis, and Rutherford backscattering, together with impedance spectroscopy. The obtained proton conductivity and activation energy are comparable to literature values for the bulk conductivity of similar materials, thus showing that grain-boundary conductivity is negligible due to the high crystallinity of the film. The results reveal an uneven proton concentration depth profile, with the presence of a 3–4 nm thick, proton-rich layer with altered composition, likely characterized by cationic deficiency. While this surface layer either retains or reobtains protons after desorption and cooling to room temperature, the bulk of the film absorbs and desorbs protons in the expected manner. It is suggested that the protons in the near-surface, proton-rich region are located in proton sites characterized by relatively strong O–H bonds due to weak hydrogen-bond interactions to neighboring oxygen atoms and that the mobility of protons in these sites is generally lower than in proton sites associated with stronger hydrogen bonds. It follows that strongly hydrogen-bonding configurations are important for high proton mobility

Investigation of the Order–Disorder Rotator Phase Transition in KSiH₃ and RbSiH₃

The β–α (order–disorder) transition in the silanides ASiH₃ (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (<i>C</i>_<i>p</i>), solid-state ²H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH₃ corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH₃^– moieties. At temperatures below 200 K ASiH₃ exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH₃ and KSiH₃, respectively. The transition is accompanied by a large molar volume increase of about 14%. The <i>C</i>_<i>p</i>(<i>T</i>) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH₃^– rotation, is also seen in the evolution of atomic displacement parameters and the broadening and eventual disappearance of libration modes in the Raman spectra. In α-ASiH₃, the SiH₃^– anions undergo rotational diffusion with average relaxation times of 0.2–0.3 ps between successive H jumps. The first-order reconstructive phase transition is characterized by a large hysteresis (20–40 K). ²H NMR revealed that the α-form can coexist, presumably as 2–4 nm (sub-Bragg) sized domains, with the β-phase below the phase transition temperatures established from <i>C</i>_<i>p</i> measurements. The reorientational mobility of H atoms in undercooled α-phase is reduced, with relaxation times on the order of picoseconds. The occurrence of rotator phases α-ASiH₃ near room temperature and the presence of dynamical disorder even in the low-temperature β-phases imply that SiH₃^– ions are only weakly coordinated in an environment of A⁺ cations. The orientational flexibility of SiH₃^– can be attributed to the simultaneous presence of a lone pair and (weakly) hydridic hydrogen ligands, leading to an ambidentate coordination behavior toward metal cations