Información del Suelo

The applied methodology for the work of this thesis has been in accordance with established scientific empirical methods within solid-state chemistry.

Samples have been synthesised and analyzed with various characterization techniques and structural modelling has been undertaken to establish the chemical structure. Electrochemical impedance measurements have been used to evaluate the conductivities of the materials and this data has then been linked to the cell parameters. The result has been a model that considers the cell parameters to explain the physical characteristics that various P-Si compositions exhibit.

As much as has been possible, various characterizations have been done on individual samples taken from the same batch. If this is not the case (for example, if an extra firing was done between different types of measurements) then this is mentioned either in the text or in diagrams or both.

3.1.1  Methodology  for  diffraction  

In order to characterise crystals, which by definition are repetitive arrays of atoms, a very commonly used method is diffraction. What is taken advantage of here is the electromagnetic waves’ interaction with the atomic planes of the crystal.

Most commonly used in chemistry labs is X-ray diffraction, although electron diffraction uses the same principles, however interacts much weaker and can be used to study surfaces in conjunction with microscopy. X-rays go deeper and since they interact with the electron shells of the atoms, atomic positions etc will be a result of this interaction. Neutrons can also be used for diffraction, and in this case the interaction is with the atomic nuclei – yielding more accurate positions than with X- ray diffraction (XRD). Neutrons are also useful for mapping magnetic moments, for distinguishing between neighbouring elements and for mapping light elements like hydrogen.

θ  =  incident  angle  =  refracted  angle   dhkl  =  lattice  spacing   λ  =  X-­‐ray  wavelength   n  =  integer  (usually  1)   Equation 1: nλ = 2dhklsinθ where

If the incident rays (provided they are parallel and monochromatic) fall in with angle

θ and Bragg’s law is fulfilled then constructive interference will occur resulting in

diffraction. Bragg’s law is fulfilled if the angle of incidence equals the angle of reflection when atoms from different planes of the lattice scatter in phase. There will be different intensities as a function of the angle (due to constructive and destructive interference) and this is what produces the diffraction pattern.

In XRD the sample is placed in a sample holder, usually made from stainless steel. Powder is placed in the centre of the sample holder and caution is taken to make sure the surface of the powder is absolutely flat and in line with the rest of the holder. When the sample is mounted in the diffractometer and the doors to the machine are closed, the shutter will open and the monochromatic X-ray beam will interact with the atoms and molecules of the sample and diffract. During this whole time the sample holder will rotate in its own plane (in order to statistically spread any error).

Not all of the beams that diffract from the sample will reach the detector, however a sufficient amount will. Each intensity that the detector registers will be recorded in the computer as a function of the angle of incidence. This is what produces the diffractogram and for solids (crystals) it becomes a structural fingerprint.

If the structure changes, but the elements remain the same, there will be a change in the type of pattern, i.e. a visibly different diffractogram. If however, one element is substituted by another and it is the same structure, then it will be harder to see a

difference in the diffractogram4.

3.1.2  Rietveld  refinement  methodology  for  neutron  data  

Manual refinements for the neutron data were done in two series shown in the logs in

4 However, examining the peak positions usually will reveal differences – if e.g. all the peaks shift a little to lower two-theta then one can conclude that the unit cell has expanded.

Appendix B. The refinement data presented in Chapter 4 is for the latter series (some fractional occupancies were varied, however the same XYZ positions retained in both). Tables with summaries of the results can be found in Appendix A1 and Appendix A2, the latter also containing data from the refinements where the occupancies were all fixed.

More fixed occupancy data is also given in Appendix E1 and Appendix E2, where bond lengths and bond angles are given as a function of temperature for the parent

compositions, both untreated and hydrothermally treated with D2O.

Figure 9 shows an example of two plots from the refinements of the untreated parent

composition, Si5O(PO4)6, at room temperature (from bank 2, top, and from bank 4,

bottom). Note that the calculated model (following space group R -3) matches the

observed neutron data, further that peaks occur at the predicted position. This shows that the suggested model for this system displays high agreement with the measured data.

Figure   9  Refinement result from the untreated parent composition from bank 2 (top) and bank 4 (bottom). Note the correlation between the calculated pattern and the observed data. Additional plots are found in Appendix G for this sample and the

hydrothermally treated