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6.1 Which technique would you use to determine the following information?
(a) The composition of the product(s) obtained from the reaction.
CaO + ReO2
Quantitative information of the Ca and Re content of the product could be obtained using large area analysis EDX in a scanning electron microscope. Oxygen content may also be obtained in such a system but may be of low accuracy for this light element. Note that Re2O7, which is highly volatile, could be lost in the reaction so the Ca:Re ratio would not be defined by the reaction stoichiometry
If the product is single phase then the product stoichiometry would reflect that of the reactant composition and analysis across a sample or at a series of different points in the sample would give the same results. If the sample consisted of multiple phases, for example Ca2ReOn in one part and CaRe2Om in another, then this would also be apparent in the observed variation of the EDX spectrum across the sample.
If results were unclear from EDX, for example it might be difficult to distinguish phases in the system Ca2Re(IV)2O6 to Ca2Re(V)2O7 , then a measure of the rhenium oxidation state could be used to determine the oxygen content more accurately; this could be undertaken using XPS where the Re absorption edge position would be characteristic of either Re(IV) or Re(V).
(b) The silicon to oxygen distance in a silica glass.
In a noncrystalline glass diffraction techniques would be of limited use in determining distances (though PDF analysis, Section 2.22 could be used). Silicate glasses are commonly studied using Si K-edge EXAFS and Si-O distances determined as 1.6 – 1.65 Å ±0.02 Å
(c) The compounds formed on the surface of a sample calcium exposed to air.
Possible compounds formed on the surface include calcium oxide, calcium carbonate and calcium hydroxide. XPS using Al Kα X-rays (1486.6 eV) could be used to identify and distinguish these species. The Ca 3p binding energies would distinguish Ca metal and Ca2+ (in CaO, CaCO3 and Ca(OH)2 , the presence of [CO3]2− would be confirmed by characteristic C1s binding energies (at around 289 eV). The oxygen 1s binding energy is also dependent on which chemical species is present with characteristic energies from literature data as; O2− at 531 eV, OH− at 532.3 eV and [CO3]2− at 532.9 eV. Depth profiling using Ar ions could be used to monitor the levels of these ions through a surface film.
(d) In a sample of Nd-doped yttrium aluminium garnet Y3Al5O12 whether the neodymium ions occupy cubic (8-fold coordination), octahedral (6-fold coordination) or tetrahedral (4-fold coordination sites.
Nd-edge EXAFS would distinguish these sites in terms of the number of nearest back-scattering neighbours. In addition the Nd-O shell distance would increase with increasing coordination number. Octahedral neodymium has typically Nd-O distances at 2.40 Å while cubic 8-coordinate Nd-O distances are typically 2.50-2.60 Å.
6.2. Figure 6.22 shows the Br K-edge FT-EXAFS of molecular Br2O. Explain the form of this spectrum and calculate a value for the Br-O-Br bond angle.
There are two clear shells around bromine. The first shell corresponds to a Br-O distance and the second to Br…..Br, note that while the intensity of the 3.07 Å peak might be expected to be greater than that of the 1.85 Å peak, due to the higher atomic weight and electron count of bromine, the greater relative atomic displacement parameters of the bromine attenuate the signal significantly.
The Br-O-Br bond angle, θ, can be calculated using simple trigonometry in the triangle
So that sin (θ/2) = (3.07/2)/1.85 giving θ = 112°
6.3 An XPS study of rhenium metal and its oxides showed binding energies (eV) for the rhenium 4f electrons as follows
Re metal 42.3 ReO2 45.0 ReO3 47.2 Re2O7 49.5
Interpret these data.
Heating a sample of ReO2 deposited on platinum under hydrogen produces a material showing in its XPS spectrum a peak at 43.5 eV. Discuss a likely composition of the phase formed under these conditions.
The Re 4f electron binding energies will become greater with increasing rhenium oxidation number; that is they are becoming more tightly held. Increasing rhenium oxidation number is a result of removing initially 5d electrons and eventually 6s electrons. These shield the Re 4f electrons to some extent so that their removal monotonically increases the 4f electron binding energies. The XPS peak energy at 43.5 eV in the reduced ReO2 film implies an oxidation state between that of Re(0) metal and Re(IV)O2 and is roughly midway so that a likely oxidation state is Re(II). A reasonable interpretation is that ReO is formed under these conditions.
6.4 XANES spectra of titanium(IV) compounds show a pre-edge feature whose positions depend on the coordination of the titanium atom as follows 4-coordinate Ti 4969.6 eV, 5-coordinate Ti 4970.5 eV, and 6-coordinate Ti 4971.4 eV. Explain the origin of the pre-edge feature in these data and suggest a reason as to why its position depends upon the titanium coordination number.
These pre-edge features in the XANES spectrum are the result of transitions from the core level (1s for a K-edge) to a higher energy unoccupied orbital such as the 3d orbitals on Ti though this will interact with, and be mixed with, 2p orbitals on oxygen, and potentially the 4p on Ti. The combination of these interactions, which have been calculated theoretically, is such that in in 6 coordinate Ti(IV) the energy of the excited state is slightly higher than for 5 coordinate and this higher than four coordinate . (Francois Farges Gordon E. Brown, Jr and J. J. Rehr Phys Rev B 56 1809 ( 1997)). These energies and the intensity of the pre edge feature are diagnostic in distinguishing 4, 5 and 6 coordinated Ti(IV).