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OnderzoekZernike (ZIAM)Nanostructures of Functional OxidesBlake Group

Research

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Nanostructured thermoelectric materials

Thermoelectric materials are of much current interest in the field of sustainable energy due to their potential in the conversion of waste heat to electrical power, as well as their use in solid-state cooling systems. The performance of thermoelectric materials can be expressed by the dimensionless “figure of merit”, ZT = α2T / ρκ. The general characteristics of a good thermoelectric material are a high Seebeck coefficient (α), low electrical resistivity (ρ) and low thermal conductivity (κ). The highest reported thermoelectric figures of merit are currently ~2.0 for bulk materials, while ZT > 3 is considered necessary for widespread commercial use.

Our objective is to enhance the ZT of existing high-temperature thermoelectrics by controlling their nanostructures to minimise thermal conductivity, and by chemical doping to engineer their band structures.

Left: Cubic crystal structure of (GeTe)x(AgSbTe2)1-x at high temperature. The structure transforms to a rhombohedral phase (middle) on cooling, which gives rise to a complex nanostructure (right). This helps to lower the thermal conductivity, which is advantageous for a good thermoelectric material.
Left: Cubic crystal structure of (GeTe)x(AgSbTe2)1-x at high temperature. The structure transforms to a rhombohedral phase (middle) on cooling, which gives rise to a complex nanostructure (right). This helps to lower the thermal conductivity, which is advantageous for a good thermoelectric material.
Top left: molecular orbital diagram for superoxide O2-. Top right: room temperature crystal structure of CsO2. Bottom left: Ordering of half-occupied πx* and πy* orbitals below 70 K. An antiferromagnetic spin chain is formed (shown by red arrows) due to superexchange via Cs pz orbitals (bottom right).
Top left: molecular orbital diagram for superoxide O2-. Top right: room temperature crystal structure of CsO2. Bottom left: Ordering of half-occupied πx* and πy* orbitals below 70 K. An antiferromagnetic spin chain is formed (shown by red arrows) due to superexchange via Cs pz orbitals (bottom right).

Anionogenic magnetism

We study crystalline materials containing the magnetic superoxide (O 2 - ) anion. The motivation is to investigate p-electron-based magnetism, which has been relatively little studied. It is becoming clear that superoxide-based magnetic materials are strongly correlated electron systems, similar to transition metal compounds. For example, we have found that when CsO 2 (with a rocksalt structure) is cooled below 70 K, it undergoes a structural phase transition associated with orbital ordering. This phase transition is analogous to the Jahn-Teller effect in transition metal systems, because O 2 - is an orbitally degenerate species. The orbital ordering pattern in turn induces an antiferromagnetic spin chain. Magnetic superexchange between superoxide anions can only occur along one crystal direction, mediated by the 5p orbitals of the Cs + cations. Therefore a three-dimensional crystal structure (rocksalt) can give rise to one-dimensional magnetism. We also investigate other superoxide-based systems, including the possibility of inducing mixed-valency by incorporating peroxide (O 2 2- ) anions.

Crystal structure of (CH3NH3)PbI3 at 200 K, showing four-fold disordered orientation of the methylammonium molecule and three-fold twinning of the crystal structure by rotation around the [201] direction.
Crystal structure of (CH3NH3)PbI3 at 200 K, showing four-fold disordered orientation of the methylammonium molecule and three-fold twinning of the crystal structure by rotation around the [201] direction.

X-ray crystallography of hybrid perovskite solar cell materials

Organic-inorganic hybrid perovskite solar cell materials, in particular methylammonium lead iodide, are currently attracting much attention as highly efficient absorbers in solar cells with efficiencies of over 20%. However, the reasons for this exceptional performance are not yet fully understood. We use X-ray diffraction on both single crystal and polycrystalline samples to obtain detailed knowledge of the crystal structures of these materials, which is important in order to understand their physical properties. They often exhibit a series of temperature-induced structural phase transitions driven both by aspects of the inorganic component and by orientational degrees of freedom of the organic molecules.

Read article in Advanced Functional Materials.

Neutron scattering techniques

We are involved in the LARMOR project, coordinated by TU Delft and partially funded by an "NWO Groot" grant which involves the construction and utilization of a new neutron scattering instrument at the UK neutron source ISIS. This is a competitive, future-proof small-angle neutron scattering instrument that is fully paid, developed and run by ISIS. The Dutch contribution will significantly increase its functionality by exploiting a broad range of Larmor labelling methods in the space and time domain. We will also be regular users of the upgraded suite of neutron scattering instruments currently being developed at TU Delft, for example the PEARL diffractometer (see also the new PEARL site).

Neutron scatering facilities lab
Neutron scatering facilities lab
Laatst gewijzigd:08 mei 2018 13:42