Benjamin J. Morgan

Small polarons in Nb- and Ta-doped rutile and anatase TiO2

Experimental studies of thin-film Nb- and Ta-doped TiO2 have reported that doped anatase is highly conductive, yet doped rutile is semiconducting. Standard DFT functionals (LDA, GGA) predict that for doped anatase TiO2 the excess charge occupies the bottom of the conduction band, and is delocalised over all the Ti atoms. This has previously been proposed as the source of the experimentally observed high conductivity. GGA predicts a similar metallic system for Nb-doped rutile, however, in contradiction with experimental data that characterise doped rutile as a semiconductor with a localised gap state. This demonstrates that standard DFT functionals cannot explain the difference in experimental behaviour between polymorphs. Supplementing GGA with a “+ U” on-site Coulomb correction recovers an electronic structure for Nb-doped rutile TiO2 that is in agreement with the experimental data; a localised gap state is seen, corresponding to a small polaron on a single Ti site. GGA + U also predicts a small-polaronic Ti3+ gap state within a semiconducting system for {Nb,Ta}-doped anatase. On this basis we suggest the experimental variance between polymorphs in doped thin films is not an inherent property of the bulk crystals, but is due to other factors, e.g. additional defects or sample morphology, dependent on the synthesis history. For both anatase and rutile the defect feature is found to be insensitive to the identity of the dopant, and similar Ti3+ polarons are expected generally for doping where electrons are donated to the Ti lattice.

Understanding conductivity anomalies in CuI-based delafossite transparent conducting oxides: Theoretical insights

The Cu(I)-based delafossite structure, Cu(I)M(III)O(2), can accommodate a wide range of rare earth and transition metal cations on the M(III) site. Substitutional doping of divalent ions for these trivalent metals is known to produce higher p-type conductivity than that occurring in the undoped materials. However, an explanation of the conductivity anomalies observed in these p-type materials, as the trivalent metal is varied, is still lacking. In this article, we examine the electronic structure of Cu(I)M(III)O(2) (M(III)=Al,Cr,Sc,Y) using density functional theory corrected for on-site Coulomb interactions in strongly correlated systems (GGA+U) and discuss the unusual experimental trends. The importance of covalent interactions between the M(III) cation and oxygen for improving conductivity in the delafossite structure is highlighted, with the covalency trends found to perfectly match the conductivity trends. We also show that calculating the natural band offsets and the effective masses of the valence band maxima is not an ideal method to classify the conduction properties of these ternary materials.

The origin of the enhanced oxygen storage capacity of Ce1−x(Pd/Pt)xO2

Doping CeO(2) with Pd or Pt increases the oxygen storage capacity (OSC) and catalytic activity of this environmentally important material. To date, however, an understanding of the mechanism underlying this improvement has been lacking. We present a density functional theory analysis of Pd- and Pt-doped CeO(2), and demonstrate that the increased OSC is due to a large displacement of the dopant ions from the Ce lattice site. Pd(II)/Pt(II) (in a d(8) configuration) moves by ∼1.2 Å to adopt a square-planar coordination due to crystal field effects. This leaves three three-coordinate oxygen atoms that are easier to remove, and which are the source of the increased OSC. These results highlight the importance of rationalizing the preferred coordination environments of both dopants and host cations when choosing suitable dopants for next generation catalysts.

A GGA+U study of the reduction of ceria surfaces and their partial reoxidation through NO2 adsorption

We present the structure and energetics of surface reduction and NO2 adsorption on the reduced (1 1 1), (1 1 0) and (1 0 0) surfaces of ceria using density functional theory with the generalized gradient approximations (GGA) corrected for on-site Coulomb interactions, GGA+U. Vacancy formation at the surfaces of the ceria show reduction of two neighbouring Ce atoms to Ce(III) and gap states in the electronic density of states (EDOS). This gives rise to relaxation of the surface with elongated Ce–O distances around the reduced sites. Reduction is the easiest on the (1 1 0) surface which displays two energetically similar structures with reduction of a sub-surface cerium ion in one of the cases. NO2 adsorbs strongly on the surface resulting in an asymmetric molecule with significant expansion of the N–O bonds for the oxygen that fills the vacant site. This activation of the molecule is the weakest on the (1 1 0) surface. Analysis of the electronic structure and spin density distributions demonstrates that one Ce(III) has been re-oxidised to Ce(IV), with the formation of an adsorbed species. These results allow a rationalisation of experimental findings and demonstrate the applicability of the GGA+U approach to the study of systems in which reduced ceria surfaces play a role.

Reversible magnesium and aluminium ions insertion in cation-deficient anatase TiO2

In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg2+ and Al3+ into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg2+ and Al3+ insertion in anatase TiO2, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO2. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.

Understanding conductivity in SrCu2O2:Stability, geometry and electronic structure of intrinsic defects from first principles

Density functional theory calculations have been performed on stoichiometric and intrinsically defective p-type transparent conducting oxide SrCu2O2, using GGA corrected for on-site Coulombic interactions (GGA + U). Analysis of the absorption spectrum of SrCu2O2 indicates that the fundamental direct band gap could be as much as ∼0.5 eV smaller than the optical band gap. Our results indicate that the defects that cause p-type conductivity are favoured under all conditions, with defects that cause n-type conductivity having significantly higher formation energies. We show conclusively that the most stable defects are copper and strontium vacancies. Copper vacancies introduce a distinct acceptor single particle level above the valence band maximum, consistent with the experimentally known activated hopping mechanism.

Chemical bonding in copper-based transparent conducting oxides: CuMO2 (M = In, Ga, Sc)

The geometry and electronic structure of copper-based p-type delafossite transparent conducting oxides, CuMO(2) (M = In, Ga, Sc), are studied using the generalized gradient approximation (GGA) corrected for on-site Coulomb interactions (GGA + U). The bonding and valence band compositions of these materials are investigated, and the origins of changes in the valence band features between group 3 and group 13 cations are discussed. Analysis of the effective masses at the valence and conduction band edge explains the experimentally reported conductivity trends.

Sparse cyclic excitations explain the low ionic conductivity of stoichiometric Li7La3Zr2O12

We have performed long time scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion electrolyte Li_{7}La_{3}Zr_{2}O_{12} (LLZO), using a first-principles parametrized interatomic potential. Collective lithium transport was analyzed by identifying dynamical excitations: persistent ion displacements over distances comparable to the separation between lithium sites, and stringlike clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO (cubic) are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO (tetragonal) are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.

A molecular dynamics study of structural relaxation in tetrahedrally coordinated nanocrystals

The reorganisation of nanocrystals in order to reduce their surface energies has been examined in computer simulations. The relaxation takes a qualitatively different path for sphalerite- and wurtzite-structured particles. The surfaces of the sphalerite particles reconstruct into hexagonal nets, but the interior remains identifiable as sphalerite-like, whereas wurtzite particles form facetted, hexagonal nanorods by virtue of a reorganisation of the whole particle which involves the creation of a low energy internal interface between oppositely oriented domains. Despite the reorganisation, the diffraction patterns remain compatible with a wurtzite structure with some internal strain. The dipole moments of thermalized wurtzite particles are compared with experimental results for CdSe.

Sparse Cyclic Excitations Explain the Low Ionic Conductivity of Stoichiometric Li

We have performed long time scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion electrolyte Li_{7}La_{3}Zr_{2}O_{12} (LLZO), using a first-principles parametrized interatomic potential. Collective lithium transport was analyzed by identifying dynamical excitations: persistent ion displacements over distances comparable to the separation between lithium sites, and stringlike clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO (cubic) are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO (tetragonal) are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.