Stability and Phase Formation in the (Li/Na)₆C₆₀–H Systems Studied by Neutron Scattering

Abstract

Small-angle neutron scattering (SANS) combined with powder neutron diffraction (PND) were employed to study phase formation upon absorption/desorption of hydrogen gas in Li-and Naintercalated fullerides. The comparative system analysis, addressing changes in the sample phase compositions and morphology of powders, revealed higher degree of complexity in the Na-based samples. Results also showed that disappearance of the crystalline lithium/sodium hydride during desorption was accompanied by an increased surface roughness in both systems, most likely due to formation of a local surface strain. Furthermore, increased specific internal surface of the studied materials upon subsequent deuterium absorption/desorption cycles was demonstrated.This could indicate moderate fracturing of crystallites. IntroductionUnusual physical properties of alkali and alkaline earth metal intercalated fullerides have resulted in extended studies of this group of compounds. 1-9 More recently, light metal-based systems, in particular Li-and Na-incorporating fullerenes, have gained additional interest due to their properties suitable for energy storage applications, e. g. for batteries 10-13 and hydrogen storage. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Correlation between structural properties of Na-and Li-based fullerides and their chemical and physical characteristics have been studied extensively by many techniques, including powder X-ray and neutron diffraction (PXD and PND, respectively), 15,17,[19][20][21][22][23][24][25][26][30][31][32][33] nuclear magnetic resonance (NMR), 26, 28, 34 differential scanning calorimetry (DSC), 20,22,26,29 infrared spectroscopy (IR). 17,19,[25][26] However, the published results still lead to contradictory conclusions about sample phase compositions and metal(M):C60 ratios. Comprehensive diffraction studies have been carried out for both LixC60 and NaxC60 systems in a wide range of metal atom concentrations. 9, 14, 19-26, 28, 30-33 The results demonstrate variations in structure types and symmetries among the formed intercalated fullerides, which were likely to originate from the applied synthesis methods and/or the M: C60 ratio.The small size of Li ions, with respect to a relatively large free volume of fullerenes, allowed for synthesis of a broad family of LixC60 compounds. 15, 19-20, 23, 26, 35-37 Li4C60 is one of the most studied compositions. It was reported to crystallize with a monoclinic (I2/m) polymeric structure at room temperature (RT) that transformed to a monomeric face-centered cubic (fcc) symmetry at higher temperatures. [36][37][38] Other LixC60 systems, with a higher x (≥ 6), formed monomeric phases with Li clusters inside the fullerene voids. 23,[39][40] Li6C60 forms the fcc structure with a small contraction of the unit cell volume, as compared to the pristine C60. 15,21,26,34 The reported LixC60, with x = 12 and 28, crystallized at RT as a cubic or pseudo-cubic LixC60 phase with small Li clusters residing at the available interstitial sites. 19-20, 23, 35 ...

In Situ Small-Angle Neutron Scattering Analysis of the Initial Deuteride Formation of Deuterium–Cerium Reactions under a Controlled Gas Flow

“…Beaucage model can represent the characteristics of the system with a fractal cluster structure, 37 which is often found in the hydrides. 38 Generally, the Beaucage model involves the Guinier regime and the power law regime, as expressed in eq 1 37…”

“…The scattering intensity I ( q ) is measured as a function of scattering vector: q = 4π sin θ/λ, where λ is the wavelength of the incident radiation and θ is half of the scattering angle. Beaucage model can represent the characteristics of the system with a fractal cluster structure, which is often found in the hydrides . Generally, the Beaucage model involves the Guinier regime and the power law regime, as expressed in eq where the first term denotes the Guinier regime representing the low q region of scattering curves, the second term of eq denotes the power law regime representing the mid/high q region of scattering curves, G and B are, respectively, the scaling of intensities for the Guinier regime and the power law regime and correlated with the number of scatterers and the contrast (Δρ) between the scatterers and the matrix, R g is the radius of gyration and related to the dimension of particles, BG is the background, erf is the error function, and α is the power law exponent and denotes the surface properties of particles in the sample.…”

“…The SANS method is generally used to measure deuterides. The measurement results of fully deuterated samples normally indicated a smooth surface, whereas the measurement results of partially deuterated samples indicated a rough surface. , Our experiment was carried out in the q -range of the power law regime in order to explore the initial deuterium reaction. Partially deuterated samples were adopted in the experiment.…”

Dynamics & Spectroscopy with Neutrons—Recent Developments & Emerging Opportunities

“…The full hydrogenation of the celebrated Buckminsterfullene molecule C , for example, would lead to record-breaking hydrogen-storage capacities around 8 wt.% [ 212 ]. Several strategies have been attempted to ensure the reversibility of this process, either by intercalation [ 213 , 214 , 215 , 216 , 217 , 218 , 219 , 220 , 221 , 222 , 223 , 224 , 225 , 226 , 227 , 228 , 229 , 230 , 231 , 232 ], or through the use of intermediate hydrogen carriers, like ammonia [ 233 , 234 , 235 ]. The already excellent sorption properties of alkali-doped fullerides can be further improved via decoration with transition-metal nanoparticles [ 227 , 236 ].…”