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1 This is the author s version of a work that was submitted/accepted for publication in the following source: Frost, Ray L., Erickson, Kristy L., Weier, Matt L., McKinnon, Adam, Williams, Peter, & Leverett, Peter (2004) Use of infrared spectroscopy for the determination of electronegativity of rare earth elements. Applied Spectroscopy, 58(7), pp This file was downloaded from: c Copyright 2004 Society for Applied Spectroscopy Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reflected in this document. For a definitive version of this work, please refer to the published source:

2 Use of infrared spectroscopy for the determination of electronegativity of rare earth elements Ray L. Frost 1, Kristy L. Erickson 1, Matt L Weier 1, Adam R. McKinnon 2, Peter A. Williams 2 and Peter Leverett 2 1 Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. 2 School of Science, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia. Abstract: Infrared spectroscopy has been used to study a series of synthetic agardite minerals. Four OH stretching bands are observed at around 3568, 3482, 3362 and 3296 cm -1. The first band is assigned to zeolitic, non-hydrogen bonded water. The band at 3296 cm -1 is assigned to strongly hydrogen bonded water with an H bond distance of 2.72 Å. The water in agardites is better described as structured water and not as zeolitic water. Two bands at around 999 and 975 cm -1 are assigned to OH deformation modes. Two sets of AsO symmetric stretching vibrations were found and assigned to the vibrational modes of AsO 4 and HAsO 4 units. Linear relationships between positions of infrared bands associated with bonding to the OH units and the electronegativity of the rare earth elements were derived, with a correlation coefficients > These linear functions were then used to calculate the electronegativity of Eu, for which a value of on the Pauling scale was found. Index headings: agardite, goudeyite, mixite, petersite, infrared spectroscopy, electronegativity INTRODUCTION Agardite is a member of the mixite group, ACu 6 (AsO 4 ) 2 (OH) 6.3H 2 O for the fully hydrated formula, with (A = REE 3+ ). Mixite (A = Bi), goudeyite (A = Al), zalesiite (A = Ca, with protonation of the lattice for charge compensation) and petersite-(y), the phosphate analogue of agardite-(y) are recognized isomorphous species in the group. Of the many possible rare earth congeners constituting the agardite group, only agardite-(y) and agardite-(la) are recognized as distinct species by the IMA. 1 Others have been reported in the literature 2-7 but their species status remains unresolved. It should be noted that formulae given above refer to ideal, endmember compositions; extensive solid solution in the A site and involving P for As is known for naturally occurring material. Water in the lattice is for the most part thought to be zeolitic in nature, as evidenced by single-crystal X-ray structure determinations The crystal structures of natural mixite and agardite compounds reveal a microporous framework structure. 4, 8 The mixite group of minerals has a microporous structure 4, 8 with a framework similar to that of zeolites 10. Dietrich et Author to whom correspondence should be addressed (r.frost@qut.edu.au) 1

3 al. proposed that the water in agardite was zeolitic 11. As such these types of minerals have the potential for catalytic applications. The mixite group consists of secondary minerals formed through crystallisation from aqueous solution. The conditions under which this crystallisation takes place, particularly relating to anion and cation concentration, ph, temperature and kinetics of crystallisation, determines the particular mineral that is formed. Recently, infrared spectroscopy has been used to gain an understanding of many properties of secondary minerals In particular, infrared spectroscopy has been used to determine paragenetic relationships between many closely related phases. In this paper we report an infrared spectroscopic analysis of a set of synthetic phases of the agardite structure and a relationship involving spectroscopic parameters and the electronegativities of the REE elements. Minerals: The minerals were checked for phase composition using X-ray diffraction and for chemical composition using the electron probe. Synthesis of agardites Pure end-members of the agardite group were synthesized in the following way. To 30 cm 3 of water in a 250 cm 3 Teflon-lined acid digestion bomb was added with stirring 6.0 mmol of Cu(NO 3 ) 2.2.5H 2 O, 3.0 mmol of Na 2 HAsO 4.7H 2 O, and 1.0 mmol of Y(NO 3 ) 3.6H 2 O. The ph of the mixture was adjusted to 6.6 by dropwise addition of 1.0 M aqueous NaOH. The bomb was sealed and the mixture heated at 180 o C for 48 hours, then cooled to room temperature. The product, agardite-(y), was filtered off, washed with water, then acetone and sucked dry at the pump (yield > 95%). Powder X-ray diffraction using a Phillips PW1825 X-ray diffractometer with Mo Kα radiation showed that a single crystalline phase was present. Separate pure samples of agardite-(la), -(Ce), -(Pr), -(Nd), -(Sm) and -(Eu) were prepared in identical fashion with similar yields by substituting the appropriate hydrated REE nitrate for Y(NO 3 ) 3.6H 2 O. Infrared spectroscopy Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000 to 525 cm -1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm -1 and a mirror velocity of cm/s. The Spectracalc software package GRAMS was used for data analysis. Band component analysis was undertaken using the Jandel Peakfit software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was carried out using a Gauss-Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at a value of 0.7 2

4 and fitting was undertaken until reproducible results were obtained with squared regression coefficient of R 2 greater than The same fitting parameters were used through out the fitting of the bands. RESULTS AND DISCUSSION The infrared spectra of synthetic agardites are shown in Figures 1-3. Figure 1 displays the hydroxyl stretching region, Figure 2 the water HOH deformation region, and Figure 3 the low wavenumber region. The limits of the reflectance technique used (see experimental section) determines the lower limit of the infrared spectrum; in this case it is 600 cm -1. Results of the band component analyses are reported in Table 1. Infrared spectra of the hydroxyl stretching region for the rare earth element agardites show strong similarities. In REECu 6 (AsO 4 ) 2 (OH) 6.3H 2 O, OH units and water will contribute to the overall spectroscopic profile. Four OH stretching bands are observed at around 3568, 3482, 3362 and 3296 cm -1. The band at 3568 cm -1 is constant within experimental error for all congeners, but the band at 3296 cm -1 shows considerable variation in position. It has been shown that the water content of members of the mixite group can vary up to 3 moles per formula unit 10. The band at <3300 cm -1 is assigned to the OH stretching vibration of water. The position of this band suggests that the water is strongly hydrogen bonded. It may be reversibly removed from the lattice by heating, but it is certainly strongly structured water. Studies have shown a strong correlation between OH stretching frequencies and both O O bond distances and H O hydrogen bond distances Libowitzky (1999) based upon the hydroxyl stretching frequencies as determined by infrared spectroscopy, showed that a regression function can be employed relating the above correlations with regression coefficients better than Calculation shows that a wavelength of 3240 cm -1 corresponds to a hydrogen bond distance of about 2.72 Å. The O-H O bond distances in ice is 2.77 Å. In other words the hydrogen bonded water in the mixite channels for some congeners is more strongly hydrogen bonded than is the case with ice. In comparison, the band at ~3568 cm -1 is assigned to non-hydrogen bonded water and in this case calculations show the O-H O bond distance is about 3.07 Å. This type of water is space filling water. Confirmation of the concept of structured water in the mixite group is found by reference to the position of the HOH deformation mode (Figure 2). The band is observed at 1637 cm -1, a value that is higher than for hydrogen bonded water molecules (1630 cm -1 ). The two bands (numbered 2 and 3 in Table 1) show variation in position that appears to be cation-dependent. These bands are assigned to the OH stretching vibrations of the hydroxyl groups. The band around 999 cm -1 is assigned to the OH deformation mode of CuOH units. According to Mereiter and Preisinger [23] mixites have a framework structure based upon (M 3+ ) 1-x (M 2+ ) x Cu 6 (OH) 6 (AsO 4 ) 3-x (HAsO 4 ) x. According to this formulation, two types of hydroxyl units are found; firstly those associated with Cu and those with the AsO 3 OH units. The band at around 999 cm -1 is associated with the former. Infrared spectra of the 750 to 900 cm -1 region show a complex set of overlapping bands that can be attributed to the stretching vibrations of AsO 4 and AsO 3 OH units. This implies in turn that, on the spectroscopic time scale, protons may 3

5 transfer from lattice water molecules to arsenate ions. Raman spectra of tetrahedral anions in aqueous systems are well known. The symmetric stretching vibration of the arsenate anion (ν 1 ) is observed at 810 cm -1 and coincides with the position of the asymmetric stretching mode (ν 3 ). The symmetric bending mode (ν 2 ) is observed at 342 cm -1 and the out-of-plane bending modes (ν 4 ) is observed at 398 cm Of all tetrahedral oxyanion spectra, positions of the arsenate vibrations occur at lower wavenumbers than for any of the other naturally occurring mineral oxyanions. Thus the bands in the region 914 to 940 cm -1 are ascribed to OH deformations resulting from HAsO 4 units. Bands in the 865 to 877 cm -1 range are assigned to the ν 3 antisymmetric stretching vibrations of HAsO 4 units. Two bands observed at 847 and 844 cm -1 are of low intensity and are attributed to symmetric stretching vibrations of AsO 4 and AsO 3 OH units. The intense band in the 796 to 826 cm -1 region is assigned to the ν 3 antisymmetric stretching vibration of AsO 4. The positions of the fitted bands were plotted against the electronegativities of the rare earth elements. Electronegativity values are on the Pauling scale, obtained from the CRC Handbook of Chemistry and Physics. The value for Eu on this scale was not available. Of the 18 bands in the spectra, 6 showed a R 2 value greater than 0.9, indicating that those bands follow a linear relationship between wavenumber versus electronegativity. These graphs are shown in Figures 4-9. Figure 4 shows the relationship for the ~3486 cm -1 OH stretching vibration. The linear relationship illustrated in Figure 4 has an R 2 value of This graph shows a linear relationship between band centre and electronegativity such that with increasing electronegativity there is a decrease in band position. A similar result is evident for the second OH stretching vibration observed at around 3375 cm -1 (Figure 5). For this relationship R 2 = These graphs show that increase in electronegativity causes the OH bond to weaken as hydrogen bonding becomes more significant. There are several possible explanations for this. In the structure of agardite, the hydroxyl groups are bonded to Cu. As electronegativity increases, more electrons are withdrawn from Cu, causing the OH bond to strengthen. Another explanation is that higher electronegativity is linked to a smaller ionic radius. The decrease in radius results in a smaller unit cell size and strengthens the OH bond. Figure 6 shows the relationship between the band position of the band at around 995 cm -1. This band has been assigned to hydroxyl deformation modes associated with the Cu(OH) 6 structural unit. The correlation coefficient R 2 is now and the relationship has a positive slope. This means the higher wavenumber bands are associated with higher electronegativity. Thus the higher the electronegativity the stronger the bonding. It should be noted that the trends for the OH stretching vibration and the OH deformation modes have opposite senses. As the OH stretching frequency falls, the OH deformation frequency rises. The relationship for the ~925 cm -1 band is shown in Figure 7 and a correlation coefficient of was obtained. The trend in this Figure is similar to that shown in Figure 6. This supports the concept that this band is assignable to an OH deformation band. Figure 8 shows the relationship between the band position of the symmetric stretching mode of the AsO 4 units and electronegativity, with a correlation coefficient of Figure 9 shows the relationship between the band position of the antisymmetric stretching mode of the AsO 4 units and electronegativity. The correlation coefficient of is excellent. 4

6 The figures do not show the electronegativity of europium, but by use of the equations given in the Figures its electronegativity can be calculated, as shown in Table 2. Using equations of lines of best fit for bands that have R 2 >0.9 gives an average for the electronegativity of Eu of 1.18 on the Pauling scale. CONCLUSIONS A novel methodology for the determination of electronegativity values for rare earth elements based upon infrared spectroscopy of a series of synthesised rare earth minerals has been developed. This has enabled a value for the electronegativity of Eu 3+ of 1.18 to be obtained. Acknowledgment The financial and infrastructure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding. 5

7 References 1. J. W. Anthony, R. A. Bideaux, K. W. Bladh and M. C. Nichols, (2000) 2. F. Olmi, C. Sabelli and G. Brizzi, Mineralogical Record 19, 305 (1988) 3. R. S. W. Braithwaite and J. R. Knight, Mineralogical Magazine 54, 129 (1990) 4. H. Hess, Neues Jahrbuch fuer Mineralogie, Monatshefte 385 (1983) 5. E. A. Dunin-Barkovskaya, Miner. Uzb. 3, 25 (1976) 6. W. Krause, H. J. Bernhardt, G. Blass, H. Effenberger and H. W. Graf, Neues Jahrbuch fuer Mineralogie, Monatshefte 271 (1997) 7. K. Walenta, Neues Jahrbuch fuer Mineralogie, Monatshefte 223 (1960) 8. A. Aruga and I. Nakai, Acta Crystallographica, Section C: Crystal Structure Communications C41, 161 (1985) 9. P. Bayliss, L. J. Lawrence and D. Watson, Australian Journal of Science 29, 145 (1966) 10. R. Miletich, J. Zemann and M. Nowak, Physics and Chemistry of Minerals 24, 411 (1997) 11. J. E. Dietrich, M. Orliac and F. Permingeat, Bulletin de la Societe Francaise de Mineralogie et de Cristallographie 92, 420 (1969) 12. R. L. Frost, Z. Ding, W. N. Martens, T. E. Johnson and J. T. Kloprogge, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 59A, 321 (2003) 13. R. L. Frost, L. Duong and W. Martens, Neues Jahrbuch fuer Mineralogie, Monatshefte 223 (2003) 14. R. L. Frost, W. Martens, Z. Ding, J. T. Kloprogge and T. E. Johnson, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 59A, 291 (2003) 15. R. L. Frost, M. L. Weier, M. E. Clissold and P. A. Williams, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 59, 3313 (2003) 16. R. L. Frost, J. Yang and D. Zhe, Chinese Science Bulletin 48, 1844 (2003) 17. W. Martens and R. L. Frost, American Mineralogist 88, 37 (2003) 18. R. L. Frost, T. Kloprogge, P. A. Williams, W. Martens, T. E. Johnson and P. Leverett, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 58A, 2861 (2002) 19. R. L. Frost, W. Martens, P. A. Williams and J. T. Kloprogge, Mineralogical Magazine 66, 1063 (2002) 20. R. L. Frost, W. N. Martens, T. Kloprogge and P. A. Williams, Neues Jahrbuch fuer Mineralogie, Monatshefte 481 (2002) 21. J. Emsley, Chemical Society Reviews 9, 91 (1980) 22. H. Lutz, Structure and Bonding (Berlin, Germany) 82, 85 (1995) 23. W. Mikenda, Journal of Molecular Structure 147, 1 (1986) 24. A. Novak, Structure and Bonding (Berlin) 18, 177 (1974) 25. E. Libowitsky, Monatschefte für chemie 130, 1047 (1999) 26. R. L. Frost and J. T. Kloprogge, Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy 59A, 2797 (2003) 27. W. Martens, R. L. Frost and J. T. Kloprogge, Journal of Raman Spectroscopy 34, 90 (2003) 28. W. Martens, R. L. Frost and P. A. Williams, Journal of Raman Spectroscopy 34, 104 (2003) 6

8 29. W. N. Martens, R. L. Frost, J. T. Kloprogge and P. A. Williams, American Mineralogist 88, 501 (2003) 7

9 List of Figures Figure 1 Infrared spectra of the hydroxyl stretching region of rare earth agardites Figure 2 Infrared spectra of the water bending region of rare earth agardites Figure 3 Infrared spectra of the hydroxyl deformation and AsO 4 stretching region of rare earth agardites Figure 4 Variation of the ~ 3486 cm -1 band centre of the OH stretching vibration with electronegativity Figure 5 Variation of the ~ 3375 cm -1 band centre of the OH stretching vibration with electronegativity. Figure 6 Variation of the ~ 995 cm -1 band centre of the OH deformation vibration with electronegativity Figure 7 Variation of the ~ 925 cm -1 band centre of the OH deformation vibration with electronegativity Figure 8 Variation of the ~ 810 cm -1 band centre of the AsO 4 stretching vibration with electronegativity Figure 9 Variation of the ~ 695 cm -1 band centre with electronegativity List of Tables Table 1 Table 2 Results of the infrared spectral analysis of synthetic rare earth agardites. Calculation of electronegativity (X) of Eu 8

10 Table 1 Results of the infrared spectral analysis of synthetic rare earth agardites **. Band Y Band Centre (cm -1 ) / Intensity (%) / / / / La Band Centre (cm -1 ) / Intensity (%) 3565 / / / / Ce Band Centre (cm -1 ) / Intensity (%) 3568 / / / / Pr Band Centre (cm -1 ) / Intensity (%) 3568 / / / / Sm Band Centre (cm -1 ) / Intensity (%) 3567 / / / / Eu Band Centre (cm -1 ) / Intensity (%) 3567 / / / / Suggested assignments Zeolitic water OH stretch OH stretch Structured water Water HOH deformation 1637 / / / / / / / / / / / 0.68 Combination bands / / / / / / / / / 2.35 CuOH bending 940 / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / 1.54 symmetric and antisymmetric stretching vibrations of the AsO4 and AsO3 units **Bands which fit the electonegativity relationship with a R 2 > 0.90 are shown in italics 9

11 Table 2 Calculation of electronegativity (X) of Eu Band Equation R 2 Eu Band Centre (cm -1 ) 2 y = x y = x y = x y = x y = x y = x X (Eu) Average ± Value is / based on the best 4 bands 10

12 Figure 1 Eu Sm Pr Infrared absorbance Ce La Y Wavenumber /cm -1 11

13 Figure 2 Eu Sm Infrared absorbance Pr Ce La Y Wavenumber /cm -1 12

14 Figure 3 Eu Sm Pr Infrared absorbance Ce La Y Wavenumber /cm -1 13

15 Figure La Ce Pr Wavenumber /cm y = x R 2 = Sm Y Electronegativity 14

16 Figure La 3385 Ce Pr Wavenumber /cm y = x R 2 = Sm 3365 Y Electronegativity 15

17 Figure Y 997 Wavenumber /cm Ce Pr Sm y = x R 2 = La Electronegativity 16

18 Figure Y 935 Wavenumber /cm Pr Sm y = x R 2 = La Ce Electronegativity 17

19 Figure y = x R 2 = Y Wavenumber /cm Pr Sm La Ce Electronegativity 18

20 Figure Y 700 Wavenumber /cm Pr Sm y = x R 2 = La Ce Electronegativity 19

21 20