Nuclear Magnetic Resonance Study of Atomic Motion in A₂B₁₂H₁₂ (A = Na, K, Rb, Cs): Anion Reorientations and Na⁺ Mobility

Abstract

To study the reorientational motion of icosahedral [B12H12]2– anions in A2B12H12 (A = Na, K, Rb, Cs) and the translational diffusion of Na+ cations in Na2B12H12, we have measured the 1H, 11B, and 23Na NMR spectra and spin–lattice relaxation rates in these compounds over the temperature range of 170–580 K. For cubic compounds K2B12H12, Rb2B12H12, and Cs2B12H12, the measured 1H and 11B spin–lattice relaxation rates are governed by thermally activated reorientations of the [B12H12]2– anions. The activation energy of this reorientational motion is found to decrease with increasing cation radius, changing from 800 meV for K2B12H12 to 549 meV for Rb2B12H12 and 427 meV for Cs2B12H12. For Na2B12H12, the first-order transition from the low-temperature monoclinic to the high-temperature cubic phase near 520 K is accompanied by a 2 orders of magnitude increase in the reorientational jump rate, and the corresponding activation energy changes from 770 meV for the low-T phase to 270 meV for the high-T phase. Measurements of the 23Na NMR spectra and spin–lattice relaxation rates show that the transition from the low-T to the high-T phase of Na2B12H12 is also accompanied by the onset of the fast translational diffusion of Na+ ions. Just above the transition point, the lower limit of the Na+ jump rate estimated from the 23Na spin–lattice relaxation data is 2 × 108 s–1, and the corresponding activation energy for Na+ diffusion is about 410 meV.

Formation of novel transition metal hydride complexes with ninefold hydrogen coordination

“…This type of motion has never been observed in the complex transition metal hydrides, but has often been discussed in relation to the occurrence of fast ion conduction in alkali metal dodecahydro- closo -dodecaborates such as Na 2 B 12 H 12 . In this material, the first-order phase transition from the low-temperature monoclinic to the high-temperature cubic phase occurs near 520 K, which is accompanied by an increase of two orders of magnitude in the rate of reorientational jumps of the icosahedral [B 12 H 12 ] 2− anions with the fast translational diffusion of Na + ions35. Although we observed no obvious translational diffusion of Li + ions in our FPMD calculations because of the limited simulation time (12 ps) and the unit cell size (136 atoms), the significant rotational motions of [MoH 9 ] 3− ions, as observed in Fig.…”

Bi-functional Li2B12H12 for energy storage and conversion applications: solid-state electrolyte and luminescent down-conversion dye

“…Previous NMR, quasielastic neutron scattering (QENS), and electrochemical measurements of boron-based superhalogens have suggested there are highly mobile alkali cations (Li + , Na + ) and complex anion species (BH 4 -, B 10 H 10 -2 , B 12 H 12 -2 ) in alkali metal borane salts. [8][9][10][11][12][13][14][15][16][17] These studies also showed that various structural phase transitions occur in these materials that lead to more atomic disorder in the lattice at elevated temperatures. T hese transitions create vacancies that facilitate the enhanced cation and anion mobility.…”

Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites

“…B-H vibrations are readily studied by vibrational spectroscopy while the much lower energy transfers of stochastic reorientations are investigated by means of quasi-elastic neutron scattering. The structural dynamics of monometallic borohydrides have been extensively investigated by vibrational spectroscopy [28][29][30], solid state NMR spectroscopy [31][32][33][34][35], inelastic x-ray scattering IXS [36], inelastic neutron scattering INS [37], QENS [38][39][40] and recently by molecular dynamics assisted density functional theory (MD-DFT) [41,42]. The importance of the vibrational density of states for the stabilities of different polymorphs has also been discussed [43].…”