Trevor R. Griffiths

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.

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.

A review of the high temperature oxidation of uranium oxides in molten salts and in the solid state to form alkali metal uranates, and their composition and properties

Abstract An extensive review of the literature on the high temperature reactions (both in melts and in the solid state) of uranium oxides (UO 2 , U 3 O 8 and UO 3 ) resulting in the formation of insoluble alkali metal (Li to Cs) uranates is presented. Their uranate(VI) and uranate(V) compounds are examined, together with mixed and oxygen-deficient uranates. The reactions of uranium oxides with carbonates, oxides, per- and superoxides, chlorides, sulfates, nitrates and nitrites under both oxidising and non-oxidising conditions are critically examined and systematised, and the established compositions of a range of uranate(VI) and (V) compounds formed are discussed. Alkali metal uranates(VI) are examined in detail and their structural, physical, thermodynamic and spectroscopic properties considered. Chemical properties of alkali metal uranates(VI), including various methods for their reduction, are also reported. Errors in the current theoretical treatment of uranate(VI) spectra are identified and the need to develop routes for the preparation of single crystals is stressed.

Vibrational spectra of alkali metal (Li, Na and K) uranates and consequent assignment of uranate ion site symmetry

Abstract Vibrational (Raman and infrared) spectra of nine alkali metal (lithium, sodium and potassium) uranates have been measured in the infrared range of 4000–250 cm −1 and the Raman shift range of 1100–50 cm −1 . The Raman spectra of sodium and potassium uranates, and lithium polyuranates are reported for the first time. From these spectra the site symmetries of intrinsic uranate groups are deduced by comparing the number of observed and resolved bands with the number predicted by the various site symmetries subgroups possible for each uranate crystal symmetry. The following could then be assigned: C 2 h for Li 2 UO 4 ; D 2 h for α-Na 2 UO 4 ; D 4 h for K 2 UO 4 ; D 2 for Na 2 U 2 O 7 and C 2 h for K 2 U 2 O 7 . For the lithium polyuranates, Li 2 O·1.6UO 3 , Li 2 O·1.75UO 3 , Li 2 U 2 O 7 and Li 2 U 3 O 10 , as their crystal symmetries are not fully known, no definite conclusions concerning uranate site symmetry could be drawn, except that primitive monoclinic symmetry point groups must be excluded.

The electronic spectra of alkali metal uranates and band assignments: an analysis of their diffuse reflectance spectra

The diffuse reflectance spectra of alkali metal (Li, Na, K, Rb and Cs) mono- and diuranates, as well as several additional lithium polyuranates(VI), have been measured at 25 and − 196°C, and up to 500°C for the potassium uranates. The spectra of the mono- and diuranates of caesium and sodium were similar as were the spectra of the uranates of rubidium and potassium, because of identical site symmetries. Spectra recorded at − 196°C exhibited enough features to enable their resolution into individual overlapping bands. Existing theoretical models of the uranate spectra have been critically examined. The recorded spectra were analysed using the MO diagram for the octahedral UO66− moiety. The spectra were interpreted in terms of the various expected uranate ion site symmetries and the results obtained for all the alkali metal mono- and diuranates are reported here.

A new method for determining oxygen solubility in molten carbonates and carbonate–chloride mixtures using the oxidation of UO2 to uranate reaction

One of the possible pyrochemical reprocessing procedures for spent ceramic nuclear fuels may involve the oxidation of UO2 in alkali metal carbonate and carbonate-based melts, and this is controlled by the level of dissolved oxygen in the melt. A quantitative relationship has been derived between the extent of UO2 oxidation and the concentration of oxygen (peroxide/superoxide) species formed upon oxygen dissolution in carbonates. A novel sensitive method for determining oxygen solubility in molten carbonates and carbonate-based melts has thus been developed. The concentrations of the alkali metal uranates(VI) formed can then be accurately determined without interference from unreacted UO2. Oxygen solubilities at temperatures from 450°C to 800°C have been determined. The solubility of oxygen in a range of carbonate–chloride melts was also determined and found to increase with decreasing radius of the cation of the alkali metal chloride added. Measurements at various partial pressures of oxygen allowed the determination of the predominant oxygen species formed in the melt, and preliminary experiments showed that in the ternary carbonate melt, at 450°C, oxygen dissolves forming mainly superoxide ions. The applicability of Henrys Law in this situation is examined.

Increased oxidation of UO2in molten alkali-metal carbonate based mixtures by increasing oxygen solubility and by controlled generation of superoxide ions, and evidence for a new sodium uranate

The oxidation of uranium dioxide to uranates in the ternary alkali-metal carbonate melt (Li–Na–K)2CO3 containing added chlorides or sulfates has been studied in the range 723–1023 K and a variety of uranium(VI) species was obtained. Increased oxygen solubility in fused carbonates was achieved by adding alkali-metal chloride or sulfate. The yield of uranates in chloride-containing melts decreased with increasing radius of the alkali-metal chloride cation, Li Na > K > Cs. When UO2 was oxidised in alkali chloride containing carbonate melts no intermediate uranium chloride complexes were observed. The temperature required for complete oxidation of UO2 can be lowered by 100–150 K by the addition of alkali-metal chlorides to carbonate melts. The addition of chloride and aluminium ions to form AlCl4− effected UO2 oxidation by a different mechanism, through intermediate formation of uranyl complexes, but the yield of uranates was not significantly altered. Attempts at oxidation by direct addition of potassium superoxide at 723 K were inefficient owing to thermal instability of the reagent, but UO2 oxidation was enhanced, by more than 10%, by superoxide formed insitu by the reaction of peroxide and nitrate, compared with oxidation by peroxide or nitrate individually. Optimum conditions for complete oxidation at 723 K required a peroxide:nitrate mole ratio of ca. 3. The major reaction pathway in melts containing nitrate and peroxide, at various concentrations of these reactants, was determined. At a mole ratio of oxidiser to UO2 of 0.3, the percentage of UO2 oxidised and the yield of uranates, increased in the order Na2O2 ≈ KO2 < KNO2 < KNO3 < KNO3 + Na2O2. Evidence has been obtained for a new sodium uranate, Na2O(UO3−y)x (1 < x <2 ; y < 0.02).

Oxidation of UO2 in molten alkali-metal carbonate mixtures: formation of uranates and diuranates

The oxidation of UO2 in a variety of alkali-metal carbonate melts has been studied in the temperature range 723–1123 K. UO2 reacts with carbonate in the presence of oxygen to produce a mixture of mono- and di-uranates. The monouranate is favoured at high temperature. In the presence of alkali-metal nitrate salts the reaction rate is increased markedly: 100% oxidation of UO2 is possible at 723 K in 1 h. Oxygen stirring of the melt also increases the rate of oxidation over air stirring and simple stirring. The mechanism of oxidation of UO2 is dependent upon generation of peroxide and superoxide ions in the carbonate melt.

Some applications of computer techniques to absorption spectra. Thiocyanate-cobalt(II) interactions in dimethyl sulphoxide-1 Elucidation of the octahedral-tetrahedral equilibria

Abstract The effect of temperature on the octahedral equilibria which occur in solutions of thiocyanate and cobalt(II) ions in dimethyl sulphoxide at various thiocyanate/cobalt mole ratios has been studied spectrophotometrically. With increasing temperature, or thiocyanate/cobalt mole ratio, the equilibrium between [Co(SCN)(DMSO)5]+ and [Co(NCS)4]2− favours the latter species, but at temperatures in excess of around 100°C [Co(NCS)3DMSO]− appears. This was established by conventional arguments and confirmed by computing the fourth derivative function of the spectra recorded at a thiocyanate/cobalt mole ratio of two. Trace amounts of [Co(NCS)3DMSO]−1 where also thereby identified at temperatures where it could not normally be detected. Details of the various digitizing procedures and computer programmes are given. It is concluded that computer calculations on selected spectra can yield the same, and often more, chemical information than that obtainable from an extensive experimental study.

Formation of lanthanide phosphates in molten salts and evaluation for nuclear waste treatment

The formation of phosphates of the lighter lanthanides (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Dy) by reaction with alkali metal phosphates has been studied in a LiCl–KCl eutectic melt between 450 and 650 °C and in a NaCl–KCl equimolar melt at 750 °C and under an inert atmosphere. Alkali metal ortho-, meta- and pyrophosphate precursors have been employed. The reaction results in the formation of single lanthanide or double alkali metal–lanthanide orthophosphates, the former favoured in LiCl–KCl melts and the latter in NaCl–KCl melts. The mean crystallite size of precipitated phosphates was evaluated from X-ray powder diffraction patterns, and found to be within 300–450 A. Increasing the melt temperature results in increasing the crystallite size of phosphate phases. This technique offers an attractive means of removing lanthanide fission products from chloride melts arising from pyrochemical reprocessing of spent nuclear fuels. LiCl–KCl based melts are recommended because the bulk of phosphate waste precipitated is smaller since normal rather than double phosphates are formed therein.