The dynamic Jahn–Teller effect in Cu(II) doped MgO

M(II) effect on encapsulation of guests into a series of M3L2 chiral cages: enantio-recognition

Self-assembly of M(ClO4)2 (M2+ = Ni2+, Cu2+, and Zn2+) with (1S,1'S,1''S,2R,2'R,2''R)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl) trinicotinate (s,r-L) and the corresponding enantiomer (r,s-L) as a pair of chiral tridentate donors gives rise to the chiral cage pairs [M3(s,r- and r,s-L)2](ClO4)6. For the two pairs of [(Me2CO)(H2O)@M3(r,-s and s,r-L)2](ClO4)6 (M2+ = Ni2+ and Zn2+), the inner cavity is occupied by both an acetone and a single water molecule, whereas for the copper(II) pair of [Me2CO@Cu3(r,s- and s,r-L)2](ClO4)6 under the same conditions, the cavity is filled by only one acetone molecule. Thus, the encapsulation of guest molecules into the cages during self-assembly shows significant metal(II) ion effects. These chiral cages are effective for the enantio-recognition of chiral (S)-2-butanol and (R)-2-butanol via the shifts of the electrochemical oxidation potentials obtained by the linear sweep voltammetry (LSV) technique, density functional theory (DFT) calculations, and the chiral 2-butanol adsorption in the single-crystal-to-single-crystal (SCSC) mode.

Lattice strain induced d-band centre engineering enabled pseudocapacitive energy storage in 2D hypo-hyper electronic V-NiCo2O4 for asymmetric supercapacitors

Understanding the role of fundamental structural engineering of materials in unravelling the underlying rudimentary electronic structure-dependent charge storage mechanisms is crucial for developing new strategic approaches toward high-performance electrochemical energy storage devices. Here, we demonstrate the role of strain engineering by V doping-induced lattice contraction in NiCo2O4 for increasing the energy density and power density of aqueous asymmetric hybrid supercapacitors. For application in energy storage, we demonstrate the influence of electron-deficient V4+/5+ doping in electron-rich Ni2+ sites, which has been found to result in the formation of a hypo-hyper electronically coupled cation pair causing a shift in the d-band and O 2p band centres and distortion of CoO6 octahedra. Optimization of V doping to 3 mol%, achieved by a binder-free one-step hydrothermal method, has yielded a 96% increase in specific capacitance of up to 2316 F g-1 from 1193 F g-1 in pristine materials at 1 A g-1 in a three-electrode configuration with a coulombic efficiency (η%) of 94% and a 24% increase in rate capacity. A two-fold increase in specific capacitance in the pouch cell device, fabricated with a functionalized carbon nanosphere positive electrode, has been observed for the V-doped samples at 1 A g-1 with a η% of 97% and a maximum energy density of 96.3 W h g-1 and a maximum power density of 8733.6 W g-1 which are 41% and 24.3% higher than the pristine device, respectively. Excellent cycling stability of 95.4% capacitance retention has been observed after 6000 cycles. DFT calculations have been carried out to understand the previously unexplored effect of lattice strain on charge transport and quantum capacitance, and ultimately its effect on the transition state kinetics of energy storage faradaic reaction mechanisms. The aim of this work is to establish a fresh perspective on developing a deep understanding of the fundamental electronic and structural properties of materials by drawing in concepts from descriptor models in electrocatalysis to reveal the role of lattice strain and d-band centre tailoring in enabling pseudocapacitive energy storage.

Pressure Effects on 3d n (n=4, 9) Insulating Compounds: Long Axis Switch in Na 3 MnF 6 not Due to the Jahn‐Teller Effect

The pressure‐induced switch of the long axis of MnF₆³⁻ units in the monoclinic Na₃MnF₆ compound and Mn³⁺‐doped Na₃FeF₆ is explored with the help of first principles calculations. Although the switch phenomenon is usually related to the Jahn‐Teller effect, we show that, due to symmetry reasons, it cannot take place in 3dⁿ (n=4, 9) systems displaying a static Jahn‐Teller effect. By contrast, we prove that in Na₃MnF₆ the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF₆³⁻ unit at ambient pressure corresponds to the Mn³⁺−F₃⁻ direction, close to the crystal c axis, at 2.79 GPa the c axis is reduced by 0.29 Å while b is unmodified. This fact is shown to force a change of the HOMO wavefunction favoring that the long axis becomes the Mn³⁺−F₂⁻ direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d‐d transitions observed for Na₃MnF₆ and CrF₂ at ambient pressure is also discussed together with changes induced by pressure in Na₃MnF₆. The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d⁴ or d⁹ ions.

Heavy-atom tunnelling in Cu(ii)N6 complexes: theoretical predictions and experimental manifestation

The degenerate rearrangement on Jahn-Teller distorted metal complexes is a promising reaction for the observation of significant heavy atom quantum mechanical tunnelling. Herein, a family of Cu(ii)-N₆ complexes are theoretically proven to exhibit rapid dynamical Jahn-Teller tunneling even close to the absolute zero. The manifestation of our predictions apparently appeared in solid state EPR experimental measurements on [Cu(en)₃]SO₄ more than 40 years ago, without the authors realizing that it was a quantum outcome.

Colour due to Cr3+ ions in oxides: a study of the model system MgO:Cr3+

Seeking to understand why the cubic centre in MgO:Cr(3+) has the same 10Dq value as emerald, ab initio cluster and periodic supercell calculations have been performed. It is found that the equilibrium Cr(3+)-O(2-) distance, R, in MgO:Cr(3+) is equal to 2.03 Å and thus 0.06 Å higher than that measured for the emerald. Calculations carried out on the isolated CrO(6)(9-) complex at R = 2.03 Å give 10Dq = 14,510 cm(-1), which is 10% smaller than the experimental figure for MgO:Cr(3+). Nevertheless, when the internal electric field, ER(r), due to the rest of the lattice ions is also taken into account, the calculated 10Dq = 16,210 cm(-1) coincides with the experimental value. Accordingly, the colour shift for different oxides doped with Cr(3+) can be well understood on the basis of this extrinsic contribution to 10Dq usually ignored in a ligand field description. The calculated electrostatic potential, VR(r), related to ER(r), is found to be attractive when the electronic density is lying along <110> directions and |r| > 1 Å driven by the first shell of twelve Mg(2+) ions. The action of VR(r) upon the CrO(6)(9-) complex slightly decreases the energy of t2g(xy,xz,yz) orbitals with respect to that for eg(3z(2) - r(2),x(2) - y(2)) orbitals, thus enhancing the 10Dq value by 0.2 eV. However, the addition of VR(r) induces very small changes in the electronic density, a relevant fact that is related to the (2)E(t(2g)(3)) −> (4)A(2)(t(2g)(3)) emission energy being nearly independent of the host lattice along the series of Cr(3+)-doped oxides.

Magnetic Dipole Origins of the 3A2g→3T2g Transition in Ni(II) Doped MgO

Magnetic circular dichroism and magnetic linear dichroism spectroscopy have been used to study the near infrared 3A2g → 3T2g transition of Ni(ii) doped MgO. Two sharp electronic origins are observed and it is show that their polarisation behaviour follows that expected from calculations in detail for magnetic dipole allowed transitions in octahedral symmetry. Not all transitions to the spin-orbit components of the 3T2g state are observed. We put forward an explanation for why some components are missing, even though they are expected to be of comparable intensity, in terms of the different coupling to the Jahn–Teller active tetragonal distortion in the excited state. The two lower energy spin-orbit split components are nearly independent of this distortion, while the two higher energy components are strongly dependent. We also examine the double quantum transition in the electron paramagnetic resonance spectrum and estimate the magnitude of the random strain of tetragonal symmetry in the MgO lattice.

Tunneling splitting of Jahn-Teller ions in oxides

The magnitude of tunneling splitting, 3Gamma, that governs decoherence in quantum systems under symmetric potentials, is frequently unknown. Using first-principles calculations, we have obtained 3Gamma for a number of E[symbol:see text]e Jahn-Teller impurities in solids. Calculated values span 6 orders of magnitude on passing from KCl:Ag{2+} to MgO:Cu{2+}. Values associated with Cu2+- and Ag2+-doped MgO are 2 orders of magnitude larger than those previously assumed and consistent with the existence of a dynamic Jahn-Teller effect. The origin and enhancement with pressure of these high 3Gamma values is discussed in detail.