Tuesday 9 July 2013, Moorfoot & Kilsyth Rooms, 11:00-13:00
Electronic and nuclear materials
Point defect analysis in thin film solar cell absorber materials: A neutron diffraction study
S Schorr and C Stephan
Helmholtz Centre Berlin for Materials and Energy, Germany
Problems concerning enviromental and energy policy belong to the greatest challanges these days. For a long-term
solution of the energy complex of problems in Europe, the EU set the goal to produce 20% of total EU energy
consumption from renewable energy sources by 2020. Within the renewable energies photovoltaics, the direct
conversion of sunlight into electrical energy, plays a key role. Thereby thin film solar cells using compound
semiconductors as absorber material are foreseen as one of the most promising and cost-efficient technology. The
electronic properties of a polycrystalline heterojunction thin film solar cell are strongly influenced by the presence of
electrical active defects. Especially the native defects in the compound semiconductor, are crucial.
Neutron diffraction allows a non-destructive analysis of the crystal structure of photovoltaic absorber materials like
chalcopyrites and kesterites from the surface deep into the volume of the samples. Only with the use of neutrons a
differentiation between the electronic similar elements copper / gallium as well as copper / zinc in the crystal
structure is possible.
The presentation will focus on:
(i) The chalcopyrite type compounds Cu(In,Ga)(Se,S)2 used as absorber layers in high efficient thin film solar cells
generally shows a non-stoichiometric composition. Using neutron powder diffraction a detailed analysis of intrinsic
point defects within the crystal structure was possible.
(ii) Kesterites (Cu2ZnSn(S/Se)4) have newly attracted attention as absorber material in thin film solar cells. Using
neutron diffraction the cation order and disorder effects were revealed obtaining the Cu-Zn anti site defect
experimentally for the first time.
Lattice dynamics in thermoelectric nanocomposites
R Hermann
Jülich Centre for Neutron Science (JCNS), Germany
Optimization of the thermoelectric properties of a material requires simultaneous tuning of the electric conductivity
and Seebeck coefficient as well as a reduction of the thermal conductivity. Extensive research has been devoted to
the lattice dynamics of thermoelectric materials in order to grasp the mechanisms that reduce thermal conductivity.
Next to tuning the material's crystal structure control of the material's microstructures is a good handle to achieve
low thermal transport. Characterizing the lattice dynamics in nanocomposite materials is a task that inelastic
scattering of neutrons and x-rays can elegantly address. Insights in the specificity of lattice dynamics of
nanocomposite thermoelectric materials processed by spark plasma sintering will be presented, notably in Si [1],
Si/Ge, and Bi2Te3.
The Helmholtz Gemeinschaft Deutscher Forschungszentren is acknowledged for funding VH NG-407 “Lattice
dynamics in emerging functional materials”. The JCNS, ILL, and SINQ-PSI are acknowledged for provision of neutron
beam time; the ESRF and the APS for synchrotron radiation beam time at ID18 and ID22N, and at 6IDD,
respectively. The DFG is acknowledged for funding SPP1386 'Nanostructured thermoelectrics'.
[1]
Claudio T. et al., J. Mater. Sci.48, 2836-2845 (2013); DOI: 10.1007/s10853-012-6827-y.
ICNS 2013 International Conference on Neutron Scattering
Neutron powder diffraction as a tool to elucidate the experimental composition of half-Heusler Thermoelectrics
J-W Bos1, R Downie1, D MacLaren2 and R Smith3
1
Heriot-Watt University, UK, 2University of Glasgow, UK, 3ISIS Facility/Rutherford Appleton Laboratory, UK
Thermoelectric waste heat recovery can be used to increase the efficiency of heat generating processes and is
therefore of great interest for a sustainable energy future. Half-Heuslers are a prominent class of thermoelectric
materials whose thermoelectric performance is mainly limited by a large thermal conductivity [1].
We recently discovered that the half-Heusler TiNiSn is capable of accommodating a small amount of excess Ni by
occupying an interstitial site [2]. This leads to a dramatic reduction in the lattice thermal conductivity, and
thermoelectric figures of merit ZT = 0.5-0.6 for TiNiSn with 3-6% extra Ni (ZT = 0.2 for stoichiometric TiNiSn).
Neutron powder diffraction played a key role in the discovery of the excess Ni in the crystal structure. The nonstoichiometry was initially observed in samples prepared by arc-melting and is due to kinetic constraints (poor
mixing of reactants) during the synthesis. Avoiding cooling from an incongruent melt by using solid state reactions
offers much more control over the final composition. We will present our latest results on TiNiSn samples with extra
transition metal prepared using conventional solid state reactions. This affords a novel route to systematically
control the thermoelectric properties of these materials. Neutron diffraction plays a vital role in linking the
composition and structure to the thermoelectric properties.
[1]
[2]
Sakurada et al., Appl. Phys. Lett. (2005) 86, 082105;
Downie et al., Chem. Comm. (2013) DOI:10.1039/c2cc37121a
Anion-ordered chains in a d1 perovskite oxynitride; NdVO2N
L Clark1, J Oró-Solé2, W Bonin2, A Fuertes2, P Attfield1
1
University of Edinburgh, UK, 2Institut de Ciència de Materials de Barcelona, Spain
Oxynitride perovskites are an important class of functional materials with a wide variety of interesting properties that
may depend on anion order. A recent study of the insulating SrMO2N perovskites revealed that strong covalent
effects favour a cis-coordination of the MO4N2 octahedra, which results in disordered M-N chains.1 Here we extend
these local anion ordering principles to a new d1 oxynitride phase, NdVO2N, and show that the formation of cisVO4N2 is stable in the presence of itinerant 3d electrons. Room temperature time-of-flight powder neutron diffraction
data were collected on a sample of NdVO2N on HRPD at ISIS. Refinement of the O:N occupancies within an
orthorhombic Pbnm model to the data gave values of 0.55(2):0.45 and 0.73(1):0.27 across the axial and
equatorial sites, respectively. This is in excellent agreement with the anion distributions observed in the SrMO 2N
perovskites, indicating that the same local anion order principles are obeyed. We also show that the true symmetry
of the system is lowered from orthorhombic Pbnm to monoclinic P1121/m as a result of the local anion order.2
Finally we extend this study to the RVO2+xN1-x (R = La, Pr) analogues to investigate the local anion order in nonstoichiometric samples by means of our variable temperature neutron diffraction study of these materials,
performed on the D2B powder diffractometer at the Institut Laue-Langevin (ILL) and show a coexistence of
orthorhombic and rhombohedral oxynitride perovskite phases in the La system.
[1]
[2]
M. Yang et al. Nat. Chem. 2011, 3, 47.
J. Oró-Solé et. al Chem. Comm., DOI:10.1039/C3CC38736D.
ICNS 2013 International Conference on Neutron Scattering
(Invited) Quantum oscillation of nitrogen atoms in uranium nitride
A Aczel1, G Granroth1, G MacDougall 1, W Buyers 2, D Abernathy1, G Samolyuk1, G Stocks1 and S Nagler1
1
Oak Ridge National Laboratory, USA, 2Chalk River Laboratories, Canada
The vibrational excitations of crystalline solids corresponding to acoustic or optic one phonon modes appear as
sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally
produce a complicated, weak, and featureless response. Here we present time-of-flight neutron scattering
measurements for the binary solid uranium nitride (UN), showing well-defined, equally-spaced, high energy
vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum
harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral
cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the
fundamental, exactly-solvable problems in quantum mechanics. There are also practical implications, as the
oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use UN as a fuel.
Microstructural characterization of activated materials with neutron and x-ray diffraction
B Clausen1, D Brown1, T Sisneros1, M Bourke1, L Balogh1 and J Almer2
1
Los Alamos National Laboratory, USA, 2Argonne National Laboratory, USA
Diffraction is well suited to the characterization of microstructures. Neutron diffraction and high energy (>60keV) xrays, in particular, have some undeniable advantages for studying activated samples. Both penetrate millimeters
into most materials, providing a statistically relevant probe of the microstructure in the bulk of the material.
Moreover, little or no hazardous and costly sample preparation is necessary, which enables repeat tests on the
same sample or even in-situ measurements under simulated operating conditions. This talk will use the example of
in-situ diffraction measurements irradiated HT-9 steel and Zr pressure tubes to highlight the opportunities
associated with these techniques, in particular, related to the study of activated materials. For example, diffraction
line profile analysis has been used to characterize the evolution of the dislocation density in both materials during
irradiation.
ICNS 2013 International Conference on Neutron Scattering