Structural Phase Transitions in closo-Dicarbadodecaboranes C₂B₁₀H₁₂

Abstract

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo -dicarbadodecaboranes C 2 B 10 H 12 ( ortho , meta, and para ) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing ( ccp ) of rotating closo -molecules C 2 B 10 H 12 in the space group Fm 3̅. Phase II is the ccp of rotating closo -molecules C 2 B 10 H 12 in the cubic space group Pa 3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic ( meta ) or monoclinic ( ortho and para ) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho -isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C 2 B 10 H 12 and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C 2 B 10 H 12 molecules resist at least as well as the CB 11 H 12 – anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Na x (CB 11 H 12 ) x (C 2 B 10 H 12 ) 1 – x , where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

Li4B10H10B12H12 as solid electrolyte for solid-state lithium batteries

“…In C 2 B 10 H 12 , the transition between the ordered and disordered phases involves an intermediate, partially disordered phase ( Pa 3̄), which shows uniaxial rotation of the anion. 39 A similar transition with two high-temperature phases has been proposed for Li 2 B 10 H 10 in ref. 33, 37 and two phases have indeed been observed.…”

Role of Cation Size on Order-Disorder Phase Transition and Cation Hopping Mechanism based on LiCB11H12

“…Furthermore, the previous paragraph concluded that the larger cell size was in favor of high cationic diffusion and higher anionic reorientational motion, which could also reduce the T tran because of the larger interstitial space easing the anionic rotation. Thus, a guideline would be keeping smaller size cations in a larger cell, which could be achieved by partial adding of C 2 B 10 H 12 [72,73], where the effective anionic radius is expected to be larger because of higher charge in C, offering a larger cell volume compared to LiCB…”

“…Thus, a guideline for superior materials could be keeping a smaller-sized cation in a bigger simulation cell, as observed in Na x (CB 11 H 12 ) x (meta-C 2 B 10 H 12 ) 1 x [73]. However, in real materials both the cation sizes and the anionic arrangement play combined roles.…”

Polymorphism and solid solutions of trimethylammonium monocarboranes

“…C 2 B 10 H 12 , which both are very (electro-)chemically and thermally stable. 51–56 The monovalent charge of the [CB x H x +1 ] − anion as compared to the divalent [B x H x ] 2− also results in only half the amount of cations required for charge neutrality, effectively resulting in more available cation positions. Several solid solutions have been reported, e.g.…”

“…C 2 B 10 H 12 , which both are very (electro-) chemically and thermally stable. [51][52][53][54][55][56] H 14 ] x , but the nido-boranes usually have lower stability compared to the closo-boranes. 61,62 Well-defined solid solutions solved from single crystal X-ray diffraction data has also been reported for Pd[Br 1.133 Cl 0.867 (1,2-…”