Thermal Polymorphism in CsCB11H12

Thermal polymorphism in the alkali-metal salts incorporating the icosohedral monocarba-hydridoborate anion, CB₁₁H₁₂^-, results in intriguing dynamical properties leading to superionic conductivity for the lightest alkali-metal analogues, LiCB₁₁H₁₂ and NaCB₁₁H₁₂. As such, these two have been the focus of most recent CB₁₁H₁₂^- related studies, with less attention paid to the heavier alkali-metal salts, such as CsCB₁₁H₁₂. Nonetheless, it is of fundamental importance to compare the nature of the structural arrangements and interactions across the entire alkali-metal series. Thermal polymorphism in CsCB₁₁H₁₂ was investigated using a combination of techniques: X-ray powder diffraction; differential scanning calorimetry; Raman, infrared, and neutron spectroscopies; and ab initio calculations. The unexpected temperature-dependent structural behavior of anhydrous CsCB₁₁H₁₂ can be potentially justified assuming the existence of two polymorphs with similar free energies at room temperature: (i) a previously reported, ordered R3 polymorph stabilized upon drying and transforming first to R3c symmetry near 313 K and then to a similarly packed but disordered I43d polymorph near 353 K and (ii) a disordered Fm3 polymorph that initially appears from the disordered I43d polymorph near 513 K along with another disordered high-temperature P6₃mc polymorph. Quasielastic neutron scattering results indicate that the CB₁₁H₁₂^- anions in the disordered phase at 560 K are undergoing isotropic rotational diffusion, with a jump correlation frequency [1.19(9) × 10¹¹ s^-1] in line with those for the lighter-metal analogues.

Anion reorientations and cation diffusion in a carbon-substituted sodium nido-borate Na-7,9-C2B9H12: 1H and 23Na NMR studies

Polyhydroborate-based salts of lithium and sodium have attracted much recent interest as promising solid-state electrolytes for energy-related applications. A member of this family, sodium dicarba-nido-undecahydroborate Na-7,9-C2B9H12 exhibits superionic conductivity above its order-disorder phase transition temperature, ∼360 K. To investigate the dynamics of the anions and cations in this compound at the microscopic level, we have measured the 1H and 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 148–384 K. It has been found that the transition from the low-T ordered to the high-T disordered phase is accompanied by an abrupt, several-orders-of-magnitude acceleration of both the reorientational jump rate of the complex anions and the diffusive jump rate of Na+ cations. These results support the idea that reorientations of large [C2B9H12]− anions can facilitate cation diffusion and, thus, the ionic conductivity. The apparent activation energies for anion reorientations obtained from the 1H spin-lattice relaxation data are 314 meV for the ordered phase and 272 meV for the disordered phase. The activation energies for Na+ diffusive jumps derived from the 23Na spin-lattice relaxation data are 350 and 268 meV for the ordered and disordered phases, respectively.

Promoting Persistent Superionic Conductivity in Sodium Monocarba-closo-dodecaborate NaCB11H12 via Confinement within Nanoporous Silica

Superionic phases of bulk anhydrous salts based on large cluster-like polyhedral (carba)borate anions are generally stable only well above room temperature, rendering them unsuitable as solid-state electrolytes in energy-storage devices that typically operate at close to room temperature. To unlock their technological potential, strategies are needed to stabilize these superionic properties down to subambient temperatures. One such strategy involves altering the bulk properties by confinement within nanoporous insulators. In the current study, the unique structural and ion dynamical properties of an exemplary salt, NaCB₁₁H₁₂, nanodispersed within porous, high-surface-area silica via salt-solution infiltration were studied by differential scanning calorimetry, X-ray powder diffraction, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and impedance spectroscopy. Combined results hint at the formation of a nanoconfined phase that is reminiscent of the high-temperature superionic phase of bulk NaCB₁₁H₁₂, with dynamically disordered CB₁₁H₁₂ ^- anions exhibiting liquid-like reorientational mobilities. However, in contrast to this high-temperature bulk phase, the nanoconfined NaCB₁₁H₁₂ phase with rotationally fluid anions persists down to cryogenic temperatures. Moreover, the high anion mobilities promoted fast-cation diffusion, yielding Na⁺ superionic conductivities of ∼0.3 mS/cm at room temperature, with higher values likely attainable via future optimization. It is expected that this successful strategy for conductivity enhancement could be applied as well to other related polyhedral (carba)borate-based salts. Thus, these results present a new route to effectively utilize these types of superionic salts as solid-state electrolytes in future battery applications.

Polymorphism of Calcium Decahydrido-closo-decaborate and Characterization of Its Hydrates

Metal closo-borates and their derivatives have shown promise in several fields of application from cancer therapy to solid-state electrolytes partly owing to their stability in aqueous solutions and high thermal stability. We report the synthesis and structural analysis of α- and β-CaB₁₀H₁₀, which are structurally and energetically similar, both showing a tetrahedral coordination of Ca²⁺ to four closo-borate cages. The main distinctions between the α- and β-polymorph are found in the crystal system (monoclinic or orthorhombic), topology (wurtzite or cag), and the degree of displacement of Ca²⁺ from the center of the coordination tetrahedron. Neutron vibrational spectroscopy measurements further revealed distinct perturbations in the cation-anion interactions arising from the different crystal structures. We also synthesized and structurally investigated five stoichiometric hydrates, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, and discovered an order-disorder polymorphic transition, α- to β-CaB₁₀H₁₀·6H₂O. The hydrates reveal a rich structural diversity with ordered structures, CaB₁₀H₁₀·xH₂O, x = 1, 4, 5, 6, and 7, as well as disordered structures, x = 6 and 8. The latter allow for a continuum of compositions within 7-8 molecules of crystal water. The DFT-optimized experimental crystal structures reveal complex networks of three types of hydrogen interactions: dihydrogen bonds, B-H^δ-···^+δH-O; hydrogen-hydrogen interactions, B-H···H-B; and hydrogen bonds, O-H^δ+···^-δO-H. A rather short B-H···H-B (2.14 Å) interaction is observed for CaB₁₀H₁₀·5H₂O, which is locally stabilized by four hydrogen bonds.

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

Complex metal hydrides are a fascinating and continuously expanding class of materials with many properties relevant for solid-state hydrogen and ammonia storage and solid-state electrolytes. The crystal structures are often investigated using powder X-ray diffraction (PXD), which can be ambiguous. Here, we revisit the crystal structure of Y(11BD4)3·3ND3 with the use of neutron diffraction, which, in comparison to previous PXD studies, provides accurate information about the D positions in the compound. Upon cooling to 10 K, the compound underwent a polymorphic transition, and a new monoclinic low-temperature polymorph denoted as α-Y(11BD4)3·3ND3 was discovered. Furthermore, the series of Y(11BH4)3·xNH3 (x = 0, 3, and 7) were also investigated with inelastic neutron scattering and infrared spectroscopy techniques, which provided information of the local coordination environment of the 11BH4- and NH3 groups and unique insights into the hydrogen dynamics. Partial deuteration using ND3 in Y(11BH4)3·xND3 (x = 3 and 7) allowed for an unambiguous assignment of the vibrational bands corresponding to the NH3 and 11BH4- in Y(11BH4)3·xNH3, due to the much larger neutron scattering cross section of H compared to D. The vibrational spectra of Y(11BH4)3·xNH3 could roughly be divided into three regions: (i) below 55 meV, containing mainly 11BH4- librational motions, (ii) 55-130 meV, containing mainly NH3 librational motions, and (iii) above 130 meV, containing 11B-H and N-H bending and stretching motions.

Structural and reorientational dynamics of tetrahydroborate (BH4-) and tetrahydrofuran (THF) in a Mg(BH4)2·3THF adduct: neutron-scattering characterization

Metal borohydrides are considered promising materials for hydrogen storage applications due to their high volumetric and gravimetric hydrogen density. Recently, different Lewis bases have been complexed with Mg(BH4)2 in efforts to improve hydrogenation/dehydrogenation properties. Notably, Mg(BH4)2·xTHF adducts involving tetrahydrofuran (THF; C4H8O) have proven to be especially interesting. This work focuses on exploring the physicochemical properties of the THF-rich Mg(BH4)2·3THF adduct using neutron-scattering methods and molecular DFT calculations. Structural analysis, based on neutron diffraction measurements of Mg(11BH4)2·3TDF (D - deuterium), has confirmed a lowering of the symmetry upon cooling, from monoclinic C2/c to P1[combining macron] via a triclinic distortion. Vibrational properties are strongly influenced by the THF environment, showing a splitting in spectral features as a result of changes in the bond lengths, force constants, and lowering of the overall symmetry. Interestingly, the orientational mobilities of the BH4- anions obtained from quasielastic neutron scattering (QENS) are not particularly sensitive to the presence of THF and compare well with the mobilities of BH4- anions in unsolvated Mg(BH4)2. The QENS data point to uniaxial 180° jump reorientations of the BH4- anions around a preferred C2 anion symmetry axis. The THF rings are also found to be orientationally mobile, undergoing 180° reorientational jumps around their C2 molecular symmetry axis with jump frequencies about an order of magnitude lower than those for the BH4- anions. In contrast, no dynamical behavior of the THF rings is observed with QENS for a more THF-deficient 2Mg(BH4)2·THF adduct. This lack of comparable THF mobility may reflect a stronger Mg2+-THF bonding interaction for lower THF/Mg(BH4)2 stoichiometric ratios, which is consistent with DFT calculations showing a decrease in the binding energy with each additional THF ring in the adduct. Based on the combined experimental and computational results, we propose that combining THF and Mg(BH4)2 is beneficial to (i) preventing weakly bound THF from coming free from the Mg2+ cation and reducing the concentration of any unwanted impurity in the hydrogen and (ii) disrupting the stability of the crystalline phase, leading to a lower melting point and enhanced kinetics for any potential hydrogen storage applications.

Low-Temperature Rotational Tunneling of Tetrahydroborate Anions in Lithium Benzimidazolate-Borohydride Li2(bIm)BH4

To investigate the dynamical properties of the novel hybrid compound, lithium benzimidazolate-borohydride Li₂(bIm)BH₄ (where bIm denotes a benzimidazolate anion, C₇N₂H₅ ^-), we have used a set of complementary techniques: neutron powder diffraction, ab initio density functional theory calculations, neutron vibrational spectroscopy, nuclear magnetic resonance, neutron spin echo, and quasi-elastic neutron scattering. Our measurements performed over the temperature range from 1.5 to 385 K have revealed the exceptionally fast low-temperature reorientational motion of BH₄ ^- anions. This motion is facilitated by the unusual coordination of tetrahedral BH₄ ^- anions in Li₂(bIm)BH₄: each anion has one of its H atoms anchored within a nearly square hollow formed by four coplanar Li⁺ cations, while the remaining -BH₃ fragment extends into a relatively open space, being only loosely coordinated to other atoms. As a result, the energy barriers for reorientations of this fragment around the anchored B-H bond axis are very small, and at low temperatures, this motion can be described as rotational tunneling. The tunnel splitting derived from the neutron spin echo measurements at 3.6 K is 0.43(2) μeV. With increasing temperature, we have observed a gradual transition from the regime of low-temperature quantum dynamics to the regime of classical thermally activated jump reorientations. The jump rate of the uniaxial 3-fold reorientations reaches 5 × 10¹¹ s^-1 at 80 K. Nearer room temperature and above, both nuclear magnetic resonance and quasielastic neutron scattering measurements have revealed the second process of BH₄ ^- reorientations characterized by the activation energy of 261 meV. This process is several orders of magnitude slower than the uniaxial 3-fold reorientations; the corresponding jump rate reaches ~7 × 10⁸ s^-1 at 300 K.

Tracking the Progression of Anion Reorientational Behavior between α-Phase and β-Phase Alkali-Metal Silanides, MSiH3, by Quasielastic Neutron Scattering

Quasielastic neutron scattering (QENS) measurements over a wide range of energy resolutions were used to probe the reorientational behavior of the pyramidal SiH₃ ^- anions in the monoalkali silanides (MSiH₃, where M = K, Rb, and Cs) within the low-temperature ordered β-phases, and for CsSiH₃, the high-temperature disordered α-phase and intervening hysteretic transition region. Maximum jump frequencies of the β-phase anions near the β-α transitions range from around 10⁹ s^-1 for β-KSiH₃ to 10¹⁰ s^-1 and higher for β-RbSiH₃ and β-CsSiH₃. The β-phase anions undergo uniaxial 3-fold rotational jumps around the anion quasi-C ₃ symmetry axis. CsSiH₃ was the focus of further studies to map out the evolving anion dynamical behavior at temperatures above the β-phase region. As in α-KSiH₃ and α-RbSiH₃, the highly mobile anions (with reorientational jump frequencies approaching and exceeding 10¹² s^-1) in the disordered α-CsSiH₃ are all adequately modeled by H jumps between 24 different locations distributed radially around the anion center of gravity, although even higher anion reorientational disorder cannot be ruled out. QENS data for CsSiH₃ in the transition region between the α- and β-phases corroborated the presence of dynamically distinct intermediate (i-) phase. The SiH₃ ^- anions within i-phase appear to undergo uniaxial small-angular-jump reorientations that are more akin to the lower-dimensional β-phase anion motions rather than to the multidimensional α-phase anion motions. Moreover, they possess orientational mobilities that are an order-of-magnitude lower than those for α-phase anions but also an order-of-magnitude higher than those for β-phase anions. Combined QENS and neutron powder diffraction results strongly suggest that this i-phase is associated chiefly with the more short-range-ordered, nanocrystalline portions (invisible to diffraction) that appear to dominate the CsSiH₃.

Glassy carbon, NIST Standard Reference Material (SRM 3600): hydrogen content, neutron vibrational density of states and heat capacity

Commercial glassy carbon plates being used as absolute intensity calibration standards in small-angle X-ray scattering applications (NIST SRM 3600) have been characterized in several recent publications. This contribution adds to the characterization by measuring the hydrogen content of a plate to be (4.8 ± 0.2) × 10^-4 (mol H)/(mol C), and by measuring the vibrational spectrum by neutron inelastic scattering. The spectrum bears a strong resemblance to published measurements on graphite, allowing the identification of several spectral features. The measured spectrum is used to calculate the heat capacity of low-hydrogen-content glassy carbon for comparison with measurements reported here from 20 to 295 K.

Structure-dependent vibrational dynamics of Mg(BH4)2 polymorphs probed with neutron vibrational spectroscopy and first-principles calculations

Neutron vibrational spectroscopy and DFT calculations are used in order to gain deeper insights into the structure-dependent vibrational properties of Mg(BH₄)₂ polymorphs.

Unparalleled Lithium and Sodium Superionic Conduction in Solid Electrolytes with Large Monovalent Cage-like Anions

Solid electrolytes with sufficiently high conductivities and stabilities are the elusive answer to the inherent shortcomings of organic liquid electrolytes prevalent in today's rechargeable batteries. We recently revealed a novel fast-ion-conducting sodium salt, Na₂B₁₂H₁₂, which contains large, icosahedral, divalent B₁₂H₁₂^2- anions that enable impressive superionic conductivity, albeit only above its 529 K phase transition. Its lithium congener, Li₂B₁₂H₁₂, possesses an even more technologically prohibitive transition temperature above 600 K. Here we show that the chemically related LiCB₁₁H₁₂ and NaCB₁₁H₁₂ salts, which contain icosahedral, monovalent CB₁₁H₁₂^- anions, both exhibit much lower transition temperatures near 400 K and 380 K, respectively, and truly stellar ionic conductivities (> 0.1 S cm^-1) unmatched by any other known polycrystalline materials at these temperatures. With proper modifications, we are confident that room-temperature-stabilized superionic salts incorporating such large polyhedral anion building blocks are attainable, thus enhancing their future prospects as practical electrolyte materials in next-generation, all-solid-state batteries.

Fast lithium-ionic conduction in a new complex hydride-sulphide crystalline phase

A new crystalline phase derived from a 90LiBH4:10P2S5 mixture displays high lithium-ionic conductivity of log(σ/S cm(-1)) = -3.0 at 300 K. It is stable up to 473 K and has both a wide potential window of 0-5 V and favorable mechanical properties for battery assembly. Its incorporation into a bulk-type all-solid-state TiS2/InLi battery enabled repeated battery operation at 300 K.

Exceptional superionic conductivity in disordered sodium decahydro-closo-decaborate

Na2 B10 H10 exhibits exceptional superionic conductivity above ca. 360 K (e.g., ca. 0.01 S cm(-1) at 383 K) concomitant with its transition from an ordered monoclinic structure to a face-centered-cubic arrangement of orientationally disordered B10 H10 (2-) anions harboring a vacancy-rich Na(+) cation sublattice. This discovery represents a major advancement for solid-state Na(+) fast-ion conduction at technologically relevant device temperatures.

Sodium superionic conduction in Na2B12H12

Impedance measurements indicate that Na2B12H12 exhibits dramatic Na(+) conductivity (on the order of 0.1 S cm(-1)) above its order-disorder phase-transition at ≈529 K, rivaling that of current, solid-state, ceramic-based, Na-battery electrolytes. Superionicity may be aided by the large size, quasispherical shape, and high rotational mobility of the B12H12(2-) anions.

Low-temperature tunneling and rotational dynamics of the ammonium cations in (NH4)2B12H12

Low-temperature neutron scattering spectra of diammonium dodecahydro-closo-dodecaborate [(NH(4))(2)B(12)H(12)] reveal two NH(4)(+) rotational tunneling peaks (e.g., 18.5 μeV and 37 μeV at 4 K), consistent with the tetrahedral symmetry and environment of the cations. The tunneling peaks persist between 4 K and 40 K. An estimate was made for the tunnel splitting of the first NH(4)(+) librational state from a fit of the observed ground-state tunnel splitting as a function of temperature. At temperatures of 50 K-70 K, classical neutron quasi-elastic scattering appears to dominate the spectra and is attributed to NH(4)(+) cation jump reorientation about the four C(3) axes defined by the N-H bonds. A reorientational activation energy of 8.1 ± 0.6 meV (0.79 ± 0.06 kJ/mol) is determined from the behavior of the quasi-elastic linewidths in this temperature regime. This activation energy is in accord with a change in NH(4)(+) dynamical behavior above 70 K. A low-temperature inelastic neutron scattering feature at 7.8 meV is assigned to a NH(4)(+) librational mode. At increased temperatures, this feature drops in intensity, having shifted entirely to higher energies by 200 K, suggesting the onset of quasi-free NH(4)(+) rotation. This is consistent with neutron-diffraction-based model refinements, which derive very large thermal ellipsoids for the ammonium-ion hydrogen atoms at room temperature in the direction of reorientation.

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

The vibrational spectrum of hydrogen and the parameters of H jump motion in the rhombohedral Th(2)Zn(17)-type compound Ce(2)Fe(17)H(5) have been studied by means of inelastic and quasielastic neutron scattering. It is found that hydrogen atoms occupying interstitial Ce(2)Fe(2) sites participate in the fast localized jump motion over the hexagons formed by these tetrahedral sites. The H jump rate τ(-1) of this localized motion is found to change from 3.9 × 10(9) s(-1) at T = 140 K to 4.9 × 10(11) s(-1) at T = 350 K, and the temperature dependence of τ(-1) in the range 140-350 K is well described by the Arrhenius law with the activation energy of 103±3 meV. Our results suggest that the hydrogen jump rate in Th(2)Zn(17)-type compounds strongly increases with decreasing nearest-neighbor distance between the tetrahedral sites within the hexagons. Since each such hexagon in Ce(2)Fe(17)H(5) is populated by two hydrogen atoms, the jump motions of H atoms on the same hexagon should be correlated.

Inelastic Neutron Scattering Studies of Nonlinear Optical Materials: p-Nitroaniline Adsorbed in Alpo-5

Inelastic neutron scattering has been used to characterize the vibrational spectroscopy below 220 meV of para-nitroaniline adsorbed in the molecular sieve ALPO-5. Samples at loadings of both 3 and 13 weight %, which represent the onset of and the maximum in the nonlinear optical properties respectively, were studied. The torsional vibration of the amino (NH2) group has been identified at ca. 50 meV. The splitting and structure of this mode is sensitive to the loading level. This can be related to differences in the nature of the hydrogen bonding in these materials.

A neutron scattering study of hydrogen dynamics in coarse-grained and nanostructured ZrCr₂H₃

The vibrational spectra of hydrogen and parameters of H diffusion in the coarse-grained C15-type system ZrCr(2)H(3) and in nanostructured ZrCr(2)H(3) have been studied by means of inelastic and quasielastic neutron scattering. It is found that the diffusive motion of hydrogen in coarse-grained ZrCr(2)H(3) can be described in terms of at least two jump processes: a fast localized H motion with the jump rate τ(l)( - 1) over the hexagons formed by interstitial Zr(2)Cr(2) sites and a slower process with the rate τ(d)( - 1) associated with H jumps leading to long-range diffusion. While τ(d)( - 1)(T) in the range 250-380 K follows the Arrhenius law with the activation energy of 142 ± 4 meV, the temperature dependence of τ(l)( - 1) deviates from Arrhenius behavior. The nanostructured ZrCr(2)H(3) samples prepared by ball milling consist of C15-type grains and strongly distorted (amorphous-like) regions. H atoms in the strongly distorted regions are found to be immobile on the time scale of our experiments. The microscopic picture of H jump motion in the C15-type grains of the nanostructured samples is similar to that in coarse-grained ZrCr(2)H(3); however, the ball milling leads to a considerable decrease in the jump rate τ(d)( - 1).

The nature of deuterium arrangements in YD3 and other rare-earth trideuterides

The efficacy of different structural models for describing the observed neutron-powder-diffraction (NPD) measurements of bulk polycrystalline YD3 as well as other hexagonal rare-earth (i.e., Nd, Tb, Dy, Ho, Er, and Tm) trideuteride powders has been investigated via Rietveld refinement. Between the two possible structural configurations, centrosymmetric P-3c1 and noncentrosymmetric P63 cm, the latter can be excluded due to very high correlations found between the positions of the D sites. Hence, the true “diffraction-average” structure for YD3 and all other rare-earth deuterides studied is centrosymmetric (P-3c1). This seems to contrast with the prior evidence from first-principles calculations and various spectroscopic probes suggesting that the true local symmetry is not P-3c1, but rather, noncentrosymmetric. A possible way to reconcile the apparently conflicting conclusions from NPD and spectroscopic measurements is by assuming that the real structure is a twinned arrangement of nanosized, noncentrosymmetric configurations. For example, we demonstrate that the diffraction-average centrosymmetric P-3c1 structure can result from a superposition of individual, noncentrosymmetric P3c1 twins. A comparison of neutron vibrational spectra for YH3 and YD3 confirms that both compounds share similar structural arrangements.

Size effects on the hydrogen storage properties of nanoscaffolded Li3BN2H8

The use of Li3BN2H8 complex hydride as a practical hydrogen storage material is limited by its high desorption temperature and poor reversibility. While certain catalysts have been shown to decrease the dehydrogenation temperature, no significant improvement in reversibility has been reported thus far. In this study, we demonstrated that tuning the particle size to the nanometer scale by infiltration into nanoporous carbon scaffolds leads to dramatic improvements in the reversibility of Li3BN2H8. Possible changes in the dehydrogenation path were also observed in the nanoscaffolded hydride.

Quasielastic neutron scattering study of hydrogen motion in NbC(0.71)H(0.28)

In order to study the mechanism and parameters of H jump motion in the nonstoichiometric Nb carbides, we have performed quasielastic neutron scattering (QENS) measurements for NbC(0.71)H(0.28) over the temperature range 11- 475 K. Our results indicate that about 30% of H atoms in this system participate in a fast diffusive motion. The temperature dependence of the corresponding H jump rate in the range 298-475 K follows the Arrhenius law with an activation energy of 328 ± 9 meV. The Q dependence of the QENS data suggests that the observed jump motion corresponds to long-range diffusion of H atoms along chains of the off-centre sites in carbon vacancies.

Structure of the novel ternary hydrides Li4 Tt 2D (Tt = Si and Ge)

The crystal structures of newly discovered Li4Ge2D and Li4Si2D ternary phases were solved by direct methods using neutron powder diffraction data. Both structures can be described using a Cmmm orthorhombic cell with all hydrogen atoms occupying Li6-octahedral interstices. The overall crystal structure and the geometry of these interstices are compared with those of other related phases, and the stabilization of this novel class of ternary hydrides is discussed.

The inverse relationship between protein dynamics and thermal stability

Protein powders that are dehydrated or mixed with a glassy compound are known to have improved thermal stability. We present elastic and quasielastic neutron scattering measurements of the global dynamics of lysozyme and ribonuclease A powders. In the absence of solvation water, both protein powders exhibit largely harmonic motions on the timescale of the measurements. Upon partial hydration, quasielastic scattering indicative of relaxational processes appears at sufficiently high temperature. When the scattering spectrum are analyzed with the Kohlrausch-Williams-Watts formalism, the exponent beta decreases with increasing temperature, suggesting that multiple relaxation modes are emerging. When lysozyme was mixed with glycerol, its beta values were higher than the hydrated sample at comparable temperatures, reflecting the viscosity and stabilizing effects of glycerol.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).