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High resolution powder X-ray diffraction profiles o f eFe(C2 0^)g were

recorded at various temperatures between 5 K and 60 K on instrument BM16 at the ESRF in Grenoble. The sample was sealed in a glass capillary o f 1 mm internal diameter and the capillary mounted in the instrument's cryostat and aligned with the X-ray beam. The capillary was rotated to reduce the effects o f preferred orientation. As shown in Figure 3.3.1, the X-ray beam was monochromated by a double crystal sagitally focussing monochromator^. The wavelength was 0.800615 Â, calibrated with NBS silicon. The analyser comprises nine G e(l 1 1 ) crystals separated by ~ 2° intervals and data were recorded in continuous scanning mode.

optics hutch experimental hutch focussing mirror (opfonal) collimating mirror Bending M agi et 2-circle diftractometer 9 311 Sagittally- Focussing Monochromator

Figure 3 .3 .1 - Schem atic o f the BM 16 instrum ent at the ESRF in G renoble

Follow ing data collection, the counts from the nine detectors were rebinned and norm alised using local softw are to give the equivalent step scan betw een 1° - 21° tw o -th eta, w ith a step size o f 0.003°.

3.4

Extended X-ray Absorption Fine Structure (EXAFS)

T em perature dependent K -e d g e EX AFS spectra were recorded on Station 8.1 at the SRS in Daresbury. The instrum ent w as used in transm ission m ode w hich relies on the

m easurem ent o f photons using ion cham bers to m easure the incident ( Iq) and

transm itted (1J beam intensities, w here the absorption coefficient (Section 2.2), p(E) = I n i i y y . A double Si crystal m onochrom ator is em ployed to step scan the desired energy range as well as to allow the second crystal to be slightly detuned from the first,

to suppress any higher han n o n ics in the X -ray beam . A typical experim ental

double crystal monochromator

lo ion chamber It ion chamber

sam ple white

beam

Figure 3.4.1 - Typical arrangem ent for transm ission m ode EX AFS experim ent

The ion cham bers are filled to a pressure o f 1000 m bar with m ixtures o f Ar and Ne relevant to the edge being excited, the first cham ber, Iq, being filled with a m ixture o f gas to absorb 20 % o f the incident flux and the second cham ber 80 %. C alibration is carried out with a thin foil o f the elem ent o f interest.

The sam ple w as m ixed w ith the zeolite MCM 41 in an approxim ately 50;50 ratio to prevent excessive X -ray absorption, and the m ixture form ed into a 20 m m diam eter disc in a pellet press at a pressure o f 0.5 tonne for 30 s. T he prepared disc w as m ounted inside the instrum ent refrigerator and aligned in the X -ray beam. D ata were collected up to the m onocrom ator angular range corresponding to approxim ately 15 Â-*. Total counting tim es for each tem perature were around 45 min. The data were norm alised, background-subtracted and converted into w avevector vs absorption coefficient using the local program s ECALIB^ and EXBROOK*^f

3.5

Polarised Neutron Diffraction"

Polarised neutron diffraction experim ents were carried out on fully deuterated sam ples o f PPh^(d-20)FeFe(C 2O ^)3 and PPh^(d-20)M nFe(C 2O ^)g on the 0 7 instrum ent at the ILL in Grenoble. A schem atic o f the instrum ent is show n in figure 3.5.1.

Monochromator Unpjlanised beam Supermirtor bender polaas&r .— S upermirror be nde r analyser [Guide fieIÂ | Detectors Unpdarised monochromatic neutron beam Neutrons polarised b y supermirrors A A A A A A A A

Î

Some neutrons flipped b y sample

Uîîiîlî

Figure 3.5.1 - Schem atic o f the D7 instrum ent at the ILL in G renoble

As the figure show s, the w hite, unpolarised neutron beam is m onochrom ated and polarised before passing through a spin flipper w hich allow s the initial polarisation direction to be defined. The beam then passes through a chopper (used in tim e-of-flight experim ents) and through the sam ple. A set o f three H elm holtz coils (not show n)

around the sam ple position produce m agnetic fields in orthogonal directions. This

allow s the incident neutron spin direction to be defined. Six m easurem ents are m ade; spin-flip and non spin-flip for each o f the three m agnetic field directions generated by

allows separation o f the total diffraction pattern into its nuclear coherent, spin incoherent and magnetic coherent contributions.

Powder samples were accurately weighed into a cylindrical vanadium can and this was mounted inside the instrument's cryostat and aligned in the neutron beam. The same sample can was used for all samples. Data were collected over the range 0.15 < Q < 2.51 Â -f Full xyz polarisation analysis was carried out with a total count time at each temperature o f 12 hours for the PPh4(d-20)MnFe(C2O^)3 sample (4.9 g) and 24 hours for the PPh4(d-20)FeFe(C2O4)3 sample (3.58 g). In order to improve the statistics and reduce the count time, a larger sample o f PPh4(d-20)FeFe(C2O4)g (7.3 g) was synthesised (Section 3.1.4) and data recorded at more temperatures. It was found that statistics comparable to the smaller sample could be obtained in 16 hours but it was concluded that further improvement would not be possible as the limiting factor was now the instrument rather than the sample. The data were corrected for background and non-sample scattering and normalised using a calibration with a known amount o f vanadium.