Research Guides

Department of Chemistry University of Oxford

Professor M.A. Hayward

Solid State Chemistry

Research in the group focuses on the synthesis and study of novel solids which exhibit unusual electronic and magnetic behaviour. A particular area of interest is the utilization of unusual synthetic techniques and conditions to increase the degree of control and selectivity exhibited by solid state reactions, with a view to undertaking the directed synthesis of ‘designed’ solid-state compounds.

Low Temperature Solid State Synthesis

In general, current preparative methods employ high temperatures to overcome the considerable diffusion barriers present in most solid-solid reactions. These high temperature treatments generally result in the thermodynamic selection of reaction products (they form the most thermodynamically stable products), precluding the formation of a large number of metastable compounds which have low thermal stability. Therefore there is a need to develop synthetic techniques that will allow much lower synthetic temperatures to be used, allowing us access to reaction conditions where kinetic control can be exerted on product selection.

One approach to this problem is to utilize the observation that in many extended solid systems, one type of ion is considerably more mobile than the rest of the species which make up the phase. This mobility difference allows us to selectively insert, or remove, these more mobile ions into, or out of, a phase, while the ‘host’ lattice remains essentially unchanged. By modifying solid-state systems in this way we can prepare highly metastable materials which are not easily prepared by conventional high-temperature synthesis routes.

Topochemical reduction using metal hydrides

Metal hydrides are extensively used as bases and reducing agents in organic chemistry. They can also be used as powerful low temperature reducing agents in the solid state to bring about the reductive deintercalation (removal) of oxide ions from complex transition metal oxides.  These processes extract oxide ions from the structures of complex materials whilst leaving the other atoms and ions in place. As a result materials with novel structures containing transition metals in highly unusual oxidation states and local coordination geometries can be prepared.

 

For example, the reaction between SrFe0.5Ru0.5O3 and CaH2 yields SrFe0.5Ru0.5O2, the first observation of Ru2+ in an extended oxide phase.

Hydride reduction reactions can also be applied to mixed anion systems, such as the oxide-chloride Sr3Fe2O5Cl2 which on reduction forms Sr3Fe2O4Cl2 containing Fe(II) centres in square-planar coordination sites.

 

 

Transition metal oxide-hydride phases

In addition to topochemical reductions, binary metal hydrides can bring about anion-exchange reactions to yield oxide-hydride phases. For example, reaction between SrVO3 and CaH2 yield SrVO2H, the first stoichiometric, anion-ordered, transition metal oxide-hydride phase reported. The oxide-hydride anion order in this phase means the structure of SrVO2H consists of V3+ cations located within square-planes of oxide ions. These V3+O4 units share corners to form infinite VO2 sheets directly analogous the CuO2 planes observed in Sr1-xCaxCuO2, the parent phase of the high-Tc superconducting cuprates. However unlike Sr1-xCaxCuO2 the VO2 sheets in SrVO2H are connected via the hydride ions which occupy the remaining two coordination sites around each V3+ center.

Despite the formal 6-fold VO4H2 coordination of the vanadium centers in SrVO2H, there is a direct structural and electronic analogy between this phase and the infinite-layer ABO2 phases. This is because the 1s valence orbitals of the hydride ions have strict sigma-type symmetry with respect to the vanadium cations and are thus orthogonal to the pi symmetry dxz, dyz and dxy orbitals from which the HOMO (a degenerate (dxz, dyz)2 pair) and LUMO (the dxy orbital) of the local VO4H2 unit are derived.

 

Thus to a first approximation the d-electrons in SrVO2H only ‘see’ the infinite layer V-O framework, as the sigma-type vanadium d-orbitals which do interact with the H 1s orbitals are empty and energetically remote from the filled orbitals. As a result SrVO2H can be considered directly analogous to a d2 infinite-layer system, such as the hypothetical phase ‘KVO2’ in which the SrH layers are replaced by layers of potassium cations.

 

 

Cation Ordered Phases

The coordination preferences of different metal cations can be utilized to encourage entropically disfavoured, cation ordered structures to form. Anion vacancies within the structure of the Ba4CaFe3O9.5 lead to a unique ordered arrangement of Ca2+ and Fe3+ cations within this phase which breaks the inversion symmetry of the cubic perovskite host lattice. This allows the material to exhibit properties such as second-harmonic generation (SHG) and piezoelectric behaviour forbidden to centric phases. In addition the phase exhibits magnetic order, making materials of this type good candidates for magnetoelectric and multiferroic behaviour. 

 

SrFe0.5Ru0.5O2: Square-planar Ru2+ in an extended oxide
F. Denis Romero, S. J. Burr, J. E. McGrady, D. Gianolio, G. Cibin and M. A. Hayward,
Journal of the American Chemical Society, 135 (2013) 1838.

The Topotactic Reduction of Sr3Fe2O5Cl2 – square planar Fe(II) in an extended oxyhalide
E. Dixon and M.A. Hayward,
Inorganic Chemistry, 49 (2010) 9649.

Strontium vanadium oxide-hydrides: ‘square-planar’ two-electron phases.
F. Denis Romero, A. Leach, J. S. Möller, F. Foronda, S. J. Blundell and M. A. Hayward,
Angewandte Chemie
, 53 (2014) 7556.

Noncentrosymmetric cation order in the cubic perovskite Ba4CaFe3O9.5
S.A. Turp, J. Hargreaves, J. Baek, P.S. Halasyamani and M.A. Hayward,
Chemistry of Materials, 22 (2010) 5580.