Comparison of anion and cation dynamics in a carbon-substituted closo-hydroborate salt: 1H and 23Na NMR studies of solid-solution Na2(CB9H10)(CB11H12)

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

“…It should be noted that the progression of the relaxation rate curve at lower temperatures shown in Figure suggests the coexistence of an additional faster reorientational jump process, not unexpected based on the molecular symmetry of the dipolar CB 11 H 12 – anion. Indeed, a similar relaxation rate behavior reflecting coexisting jump processes was also observed for superionic Na 2 (CB 9 H 10 )­(CB 11 H 12 ), with the maximum in R 1 H occurring near 220 K for the orientationally disordered CB 9 H 10 – and CB 11 H 12 – anions in the hexagonal structure. , This behavior for nanoconfined NaCB 11 H 12 is in sharp contrast to that for bulk NaCB 11 H 12 , where R 1 H ( T ) does not reach a maximum even upon heating to 376 K . Instead, the proton relaxation rate of bulk NaCB 11 H 12 exhibits an abrupt two-orders-of-magnitude drop at 376 K, which relates to the order–disorder phase transition, leading to the strong acceleration of the reorientational motion .…”

“…Indeed, a similar relaxation rate behavior reflecting coexisting jump processes was also observed for superionic Na 2 (CB 9 H 10 )(CB 11 H 12 ), with the maximum in R 1 H occurring near 220 K for the orientationally disordered CB 9 H 10 − and CB 11 H 12 − anions in the hexagonal structure. 10,30 This behavior for nanoconfined NaCB 11 H 12 is in sharp contrast to that for bulk NaCB 11 H 12 , where R 1 H (T) does not reach a maximum even upon heating to 376 K. 31 Instead, the proton relaxation rate of bulk NaCB 11 H 12 exhibits an abrupt two-orders-of-magnitude drop at 376 K, which relates and 428 K: gray) from a previous study. 3 The Bragg positions of the bulk ordered orthorhombic phase are marked with black circles, whereas those for the various known disordered fcc, hcp, hexagonal, and bcc polymorphs are marked by black squares, diamonds, triangles, and stars, respectively.…”

“…10 Like the closo-anions in this latter mixed-anion compound, the reorientational jump rate frequencies of the CB 11 H 12 − anions in NaCB 11 H 12 @SiO 2 in the temperature-transition region are of the order of 10 10 jumps/s, which is only an order of magnitude or so higher than the translational jump frequencies of the Na + cations. 30,31,36 Thus, although still significantly higher than the cation translational jump frequencies in this region, it is likely that further below these temperatures, the anion reorientational jump frequencies become too low and will no longer have such a marked synergistic dynamical effect 34 on the cation diffusion, as is the case at higher temperatures. This will ultimately lead to a change in the ratelimiting step, as reflected in the observed increase in E c to 567 meV.…”

Understanding Superionic Conductivity in Lithium and Sodium Salts of Weakly Coordinating Closo-Hexahalocarbaborate Anions

“…Rational design of properties in closo -polyborate-based conductors requires carefully weighing these factors to achieve the desired effect. Another successful approach for lowering the transition temperature involves mixing of different anion types in the same material. ,,,,,− The resulting intrinsic disorder in the crystal lattice has been shown to result in a lower transition temperature and higher ionic conductivity, possibly by enhancing some of the frustration effects discussed above. Another strategy is to nanoconfine such solid electrolytes inside mesoporous oxides, which has been shown to dramatically reduce the order–disorder transition temperature. − …”

“…Given the relative complexity of the closo -polyborate anions, a wealth of chemical modifications is possible, allowing for tailoring of the onset temperature for ionic conduction . Currently, the transition temperatures of sodium closo -borate-based systems range from well below room temperature for Na 2 (CB 9 H 10 )­(CB 11 H 12 ) to around 570 °C for Na 2 B 12 I 12 . − However, it is still not fully understood how specific chemical and structural modifications of the compounds alter the polymorphic transition temperature, making it challenging to rationally design materials with optimal properties. Trends derived from the growing family of metal closo -polyborates and derivatives suggest several factors that can affect the transition temperature and ionic conductivity of these compounds, including the crystal structure, coordination strength between anions and cations, lattice symmetry, particle size, anion mixing, and the moment of inertia of the anion. ,− In this context, the closo -7,8,9,10,11,12-hexahalocarbaborate anions [HCB 11 H 5 X 6 ] − (X = Cl, Br) are of great interest since they are extraordinarily weakly coordinating, even more so than [CB 11 H 12 ] − …”