Hugh V.St.A. Hubbard

Some aspects of the scope and limitations of derivative spectroscopy

Abstract Synthesised spectra are used to illustrate discussion of some relationships between recorded absorption profiles and their second and fourth derivative spectra. Limitations arising from the fortuitous overlap of a derivative peak with a neighbouring wing, and the possibilities of resolving spectra into their overlapping bands are also considered. The combined use of second and fourth derivative spectra to ascertain the correct number of bands within an observed profile is described. It is suggested that the practice of computing a least-squares fit of overlapping bands to a spectral profile be changed, because the minimisation achieved often produces a result involving excessive or negative absorbances: the spectral profile should be regarded as a boundary limit and any unaccounted (positive) absorbance then assessed as possible evidence for an additional band. An example is given, concerning the resolution of the spectrum of a thin, single crystal of uranium(IV) oxide at 77 K superimposed on an absorption edge. A comparison of the difference between the observed spectrum and the sum of its resolution into twelve overlapping bands, plus a similar comparison of their fourth derivative spectra, reveals a thirteenth band.

Dielectric and pyroelectric properties of poly[2,3-bis(trifluoromethyl)norbornadiene]

The fluoropolymer poly[2,3-bis(trifluoromethyl)norbornadiene] (poly(BTFMND)) has been synthesized via ring-opening metathesis polymerization, using initiators based on W, Mo or Ru compounds or via variations of the Schrock-type initiator Mo(CHR)(N-2,6-C 6 H 3 -i-Pr 2 )(OR′) 2 (R = CMe 3 , CMe 2 Ph; R′ = CMe 3 , CMe(CF 3 ) 2 ), giving poly(BTFMND) with controllable proportions of trans -vinylene units. Analysis of the 13 C nuclear magnetic resonance fine structure establishes that the high- trans (> 98%) polymer is 92% tactic and the high- cis (> 98%) polymer is 75% tactic, but these methods cannot determine whether the material is predominantly isotactic or syndiotactic. Dielectric and thermally stimulated current measurements have been performed and give relaxed permittivities % strongly dependent on trans content, ranging from above 40 for the 98% trans material through 15 for the 54% trans polymer to 6 for 98% cis polymer. The significantly higher permittivity above T g for the high- trans material than for the 54% trans polymer and the significantly reduced value for the 98% cis polymer suggest that both the high- cis and high- trans materials have syndiotactic structures. The total polarization for free films of 98% trans poly(BTFMND) saturates at about 20 mC m −2 , with a pyroelectric coefficient approaching 6 μ C m −2 K −1 above poling fields of 200 MV m −1 . These usefully high values combined with low dielectric loss in its glassy state at ambient temperatures indicate an excellent material for potential use as a pyroelectric transducer, with a figure of merit comparable with or better than that of poly(vinylidene fluoride).

A new method for the determination of x in UO2 + x: Optical absorption spectroscopy measurements

Optical absorption spectra of single crystal UO2 + x have been measured between 0.5 and 2.2 eV, for various values of x between 0.0 and 0.06, and previous spectra up to 6 eV have been reassessed. A steady increase in absorbance at all wavelengths was observed and was linear with x increase. Thus from absorption spectra the value of x in a single crystal of UO2 + x may be determined. The sensitivity of optical absorption spectroscopy permits changes in x as small as 0.0002 to be reliably determined. As excess oxygen entered the lattice a small peak appeared at 0.78 eV, and is identified with the formation of U5+. The oxidation processes in single crystal UO2 + x may therefore be investigated dynamically in-situ and in considerable detail by optical absorption spectroscopy.

Electron transfer reactions in non-stoichiometric ceria and urania

Abstract The fluorite oxides ceria and urania can be made non-stoichiometric by loss and gain, respectively, of oxygen atoms, thereby forming CeO2−x and UO2+x. The former now contains Ce3+ and Ce4+ ions and both single crystal and powder forms are blue and exhibit electron transfer in the form of intervalence spectra. The absorption spectra of pure and reduced ceria crystals are interpreted and used to help resolve electrical conductivity studies. Solid solutions of ceria and urania are blue-black and here electron transfer is of the form Ce(IV)+U(IV)→Ce(III)+U(V). The intervalence contribution to the absorption spectrum of ultra-thin single crystal wafers of UO2+x is only available as a difference spectrum, using the UO2 spectrum as reference. The U5+ ions formed and contributing to the intervalence spectra in freshly oxidised samples, quenched from UO2+x at high temperature, were found to be metastable at ambient temperature and slowly oxidised to U6+. The intervalence spectral profile changed significantly and slowly with time. Thus in aged crystals the electron transfer in the intervalence spectrum is principally due to U(IV)+U(VI)→U(V)+U(V). Monitoring these changes should yield information on the precipitation of interstitial clusters containing oxygen out of the urania matrix into aggregates and hence the growth as U3O7 on surfaces and at grain boundaries.

Absorption spectrum of single-crystal UO2: Identification of and effect of temperature on the peak positions of essentially all optical transitions in the visible to near infrared regions using derivative spectroscopy

The optical absorption spectrum of single-crystal UO2 has been measured over the range 600 to 2400 nm (2.2 to 0.5 eV) and between 85 and 1035 K. Much care was taken to ensure that the spectra were as accurate and reliable as possible. The digitised spectra were mathematically differentiated and optimised so that essentially all the transitions occurring within the absorption profile could be identified. Some 69 peaks were identified at 85 K, and this number decreased with increasing temperature. The effect of temperature on some of these maxima is reported. This is the first definitive compilation of peak maxima occurring within the UO2 spectrum from the near infrared to its absorption edge.

An investigation of defect structures in single-crystal UO2 +x by optical absorption spectroscopy

Optical absorption spectra of single-crystal UO2 +x(sample thicknesses ca. 20 µm) have been recorded in the visible and near-infrared regions as a function of excess oxygen, where x ranged between 0 and 0.06. As excess oxygen entered the lattice four main changes were identified in the spectra: (i) the absorption edge commencing around 2 eV shifted to higher energy, (ii) a reduction occurred in the contribution to the spectra due to U4+(5ƒ2) intracationic transitions, (iii) a small peak appeared at 0.78 eV and (iv) a very broad band arose, with a half-width > ca. 1 eV and a possible maximum around 2 eV. These changes are here attributed to specific changes in the charge state of the uranium sublattice associated with the formation of oxygen interstitial clusters. The origin of the main broad band is considered with reference to symmetric and unsymmetric intervalence transitions and their association with 2 : 1 : 2 and 2 : 2 : 2 defect clusters. The results are discussed in terms of the initial appearance of 2 : 1 : 2 clusters as excess oxygen enters the lattice, and that these only become 2 : 2 : 2 clusters when x in UO2 +x reaches ca. 0.1.

Spectroscopic studies on single crystals having the fluorite lattice. Part 1.—The fundamental absorption edge; Urbach's rule and the Debye temperature in CeO2

Measurements of the fundamental absorption edge of single crystal CeO2 up to absorption coefficients α of 120 cm–1 for 291 < T < 1235 K are presented. The results are analysed in terms of Urbachs rule, α=α0 exp[–σ(E0–hν)]/kT. Over this temperature range Urbachs rule is obeyed and its parameters are derived, viz., E0= 3.78 eV, α0= 7.6 × 107 cm–1, σ0= 0.90 and ħωp= 70.2 meV; σ increased with temperature more slowly than usual. Urbachs rule has not previously been obeyed up to such high temperatures or investigated for any fluorite lattice compound. Only optical photons are involved in the absorption tail formation. The characteristic Debye temperature for CeO2 was calculated as 630 and 606 K from specific heat and elastic data respectively. It was concluded that the ceria crystals retained their stoichiometry at all temperatures.