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An unconventional method for measuring the Tc L3-edge of technetium compounds

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aSchool of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia, bAustralian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia, and cAustralian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
*Correspondence e-mail: brendan.kennedy@sydney.edu.au

Edited by S. M. Heald, Argonne National Laboratory, USA (Received 12 February 2014; accepted 24 June 2014; online 1 October 2014)

Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc4+) and NH4TcO4 (tetrahedral Tc7+) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the pre-edge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc4+) and NH4TcO4 (Tc7+) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge.

1. Introduction

The 4d transition metal oxides continue to be one of the most exciting areas of solid-state chemistry, exhibiting a wide range of interesting magnetic and electronic properties. Of the 4d transition metal oxides, the structural and electronic properties of technetium oxides have remained somewhat elusive. This is not surprising given the absence of stable technetium isotopes and the radioactivity of the existing isotopes (Deutsch et al., 1983[Deutsch, E., Libson, K., Jurisson, S. & Lindoy, L. F. (1983). Prog. Inorg. Chem. 30, 75-139.]). One isotope in particular, 99Tc, is a common byproduct in nuclear fission that decays by β-emission with a relatively long half-life of 2.12 × 105 years. As such, there has been considerable effort to develop solid-state forms (e.g. pyrochlores) of technetium in order to sequester technetium from nuclear waste (Hartmann et al., 2011[Hartmann, T., Alaniz, A., Poineau, F., Weck, P. F., Valdez, J. A., Tang, M., Jarvinen, G. D., Czerwinski, K. R. & Sickafus, K. E. (2011). J. Nucl. Mater. 411, 60-71.], 2012[Hartmann, T., Alaniz, A. J. & Antonio, D. J. (2012). Proc. Chem. 7, 622-628.]). Technetium (5s24d5) is known to adopt a wide range of oxidation states analogous to manganese (4s23d5) and rhenium (6s25d5) (Kotegov et al., 1968[Kotegov, K. V., Pavlov, O. N. & Shvedov, V. P. (1968). Advances in Inorganic Chemistry and Radiochemistry, edited by H. J. Emeléus & A. G. Sharpe, pp. 1-90. New York: Academic Press.]). Technetium oxides are also known to exhibit magnetic properties that are unusual for 4d and 5d transition metal oxides (Rodriguez et al., 2011[Rodriguez, E. E., Poineau, F., Llobet, A., Kennedy, B. J., Avdeev, M., Thorogood, G. J., Carter, M. L., Seshadri, R., Singh, D. J. & Cheetham, A. K. (2011). Phys. Rev. Lett. 106, 067201.]; Avdeev et al., 2011[Avdeev, M., Thorogood, G. J., Carter, M. L., Kennedy, B. J., Ting, J., Singh, D. J. & Wallwork, K. S. (2011). J. Am. Chem. Soc. 133, 1654-1657.]). Recently the perovskites CaTcO3 and SrTcO3, which adopt an orthorhombic structure in space group Pnma, were shown to exhibit antiferromagnetic ordering with exceptionally high Néel temperatures of 800 and 1023 K, respectively (Rodriguez et al., 2011[Rodriguez, E. E., Poineau, F., Llobet, A., Kennedy, B. J., Avdeev, M., Thorogood, G. J., Carter, M. L., Seshadri, R., Singh, D. J. & Cheetham, A. K. (2011). Phys. Rev. Lett. 106, 067201.]; Avdeev et al., 2011[Avdeev, M., Thorogood, G. J., Carter, M. L., Kennedy, B. J., Ting, J., Singh, D. J. & Wallwork, K. S. (2011). J. Am. Chem. Soc. 133, 1654-1657.]). The magnetic moment on the Tc4+ cations in SrTcO3 was relatively unaffected by a high-temperature orthorhombic cubic phase transition at 800 K (Thorogood et al., 2011a[Thorogood, G. J., Avdeev, M., Carter, M. L., Kennedy, B. J., Ting, J. & Wallwork, K. S. (2011a). Dalton Trans. 40, 7228-7233.]). It has been suggested that the unusual magnetic ordering in these materials originates from the half-filling of the t2g band (Avdeev et al., 2011[Avdeev, M., Thorogood, G. J., Carter, M. L., Kennedy, B. J., Ting, J., Singh, D. J. & Wallwork, K. S. (2011). J. Am. Chem. Soc. 133, 1654-1657.]; Middey et al., 2012[Middey, S., Nandy, A. K., Pandey, S. K., Mahadevan, P. & Sarma, D. D. (2012). Phys. Rev. B, 86, 104406.]; Mravlje et al., 2012[Mravlje, J., Aichhorn, M. & Georges, A. (2012). Phys. Rev. Lett. 108, 197202.]).

Because of the radioactive nature of technetium, there are relatively few tools available for analyzing the electronic properties of these materials compared with the other 4d transition metals. Tc K-edge X-ray absorption near-edge structure (XANES) has proven to be an effect tool for studying the oxidation state and coordination environment of Tc (Antonini et al., 1983[Antonini, M., Caprile, C., Merlini, A., Petiau, J. & Thornley, F. R. (1983). EXAFS and Near Edge Structure, edited by A. Bianconi, L. Incoccia & S. Stipcich, pp. 261-264. Berlin/Heidelberg: Springer.]; Almahamid et al., 1995[Almahamid, I., Bryan, J. C., Bucher, J. J., Burrell, A. K., Edelstein, N. M., Hudson, E. A., Kaltsoyannis, N., Lukens, W. W., Shuh, D. K., Nitsche, H. & Reich, T. (1995). Inorg. Chem. 34, 193-198.]; Mausolf et al., 2011[Mausolf, E., Poineau, F., Droessler, J. & Czerwinski, K. R. (2011). J. Radioanal. Nucl. Chem. 288, 723-728.]; Poineau et al., 2008[Poineau, F., Sattelberger, A. P., Conradson, S. D. & Czerwinski, K. R. (2008). Inorg. Chem. 47, 1991-1999.]; Ferrier et al., 2012[Ferrier, M., Weck, P. F., Poineau, F., Kim, E., Stebbins, A., Ma, L. Z., Sattelberger, A. P. & Czerwinski, K. R. (2012). Dalton Trans. 41, 6291-6298.]; Thorogood et al., 2011b[Thorogood, G. J., Zhang, Z. M., Hester, J. R., Kennedy, B. J., Ting, J., Glover, C. J. & Johannessen, B. (2011b). Dalton Trans. 40, 10924-10926.]). However, the Tc K-edge XANES spectrum, which predominantly corresponds to the dipole transition of 1s electrons into empty 5p states, is limited by a broad line shape and can only provide indirect information on the Tc 4d bonding states via the unoccupied p component in pd hybridized orbitals. We have previously shown that the L3-edge, which corresponds to the transition of 2p3/2 electrons into unoccupied 4d states, is a sensitive probe for directly analyzing the bonding states of 4d transition metal oxide with empty or partially filled d orbitals (Zhou et al., 2009[Zhou, Q. D., Kennedy, B. J., Zhang, Z. M., Jang, L. Y. & Aitken, J. B. (2009). Chem. Mater. 21, 4203-4209.]; Ting et al., 2010[Ting, J., Kennedy, B. J., Zhang, Z., Avdeev, M., Johannessen, B. & Jang, L. Y. (2010). Chem. Mater. 22, 1640-1646.]; Ricciardo et al., 2010[Ricciardo, R. A., Cuthbert, H. L., Woodward, P. M., Zhou, Q., Kennedy, B. J., Zhang, Z. M., Avdeev, M. & Jang, L. Y. (2010). Chem. Mater. 22, 3369-3382.]; Qasim et al., 2011[Qasim, I., Kennedy, B. J., Zhang, Z. M., Avdeev, M. & Jang, L. Y. (2011). J. Phys. Condens. Matter, 23, 435401.]; Blanchard et al., 2012[Blanchard, P. E. R., Clements, R., Kennedy, B. J., Ling, C. D., Reynolds, E., Avdeev, M., Stampfl, A. P. J., Zhang, Z. M. & Jang, L. Y. (2012). Inorg. Chem. 51, 13237-13244.]; Reynolds et al., 2013[Reynolds, E., Blanchard, P. E. R., Kennedy, B. J., Ling, C. D., Liu, S., Avdeev, M., Zhang, Z. M., Cuello, G. J., Tadich, A. & Jang, L. Y. (2013). Inorg. Chem. 52, 8409-8415.]). In these studies, we demonstrated that crystal field splitting can be extracted from the 4d transition metal L3-edge by fitting the spectra to component peaks (it is difficult to extract the crystal-field-splitting energy from the K-edge since the K-edge spectra do not directly probe the metal 4d states). To our knowledge, the Tc L-edge has never been measured. This is likely to be due to the radioactive nature of technetium (limiting the amount and form of technetium that can be analyzed at a synchrotron facility) and the inherent safety difficulties in conducting a Tc L-edge XANES experiment (the relatively weak excitation energy of the Tc L-edge at ∼2680 eV means that samples would have to be directly exposed to the incoming X-ray beam without any shielding).

Herein, we report on a method for measuring the Tc L3-edge of technetium oxides that involves immobilizing powder samples in an epoxy resin. Although immobilizing samples in epoxy is a common practice in electron microscopy, it usually involves a much larger quantity than the allowed milligram quantity for Tc-containing samples. Furthermore, immobilizing powder samples in an epoxy resin is not common practice in soft X-ray experiments due to the relatively short penetration depth of soft X-rays compared with hard X-rays. It has previously been demonstrated that soft X-ray absorption spectroscopy experiments can be conducted on actinides (uranium- and thorium-based chlorides) by embedding the powder in a polystyrene matrix (Kozimor et al., 2009[Kozimor, S. A., Yang, P., Batista, E. R., Boland, K. S., Burns, C. J., Clark, D. L., Conradson, S. D., Martin, R. L., Wilkerson, M. P. & Wolfsberg, L. E. (2009). J. Am. Chem. Soc. 131, 12125-12136.]); in that work, the compounds of interest and the polystyrene were dissolved in a volatile solvent and the solid formed by evaporation of the solution. Since it is not possible to dissolve many condensed materials such as SrTcO3, an alternative approach has been employed to study the Tc L3-edge of SrTcO3 (Tc4+) and NH4TcO4 (Tc7+) using an epoxy resin to immobilize the samples rather than polystyrene. We will demonstrate that both fluorescence (FLY) and total electron (TEY) yield measurements can be collected using this sample preparation method. These spectra will be compared with their respective Tc K-edge XANES spectra as well as the L3-edge of other 4d transition metal oxides, specifically the neighboring Mo and Ru. The goals of this work are, firstly, to determine if spectra collected from epoxy-embedded samples are in fact representative of the bulk material and, secondly, to establish if information on the electronic and local structure of Tc cations can be obtained from the Tc L3-edge XANES spectra.

2. Experimental

Caution! 99Tc is a β emitter (Emax = 0.29 MeV). Appropriate shielding was employed during the synthesis and all manipulations. The purity of all starting materials was greater than 99%. NH4TcO4 was obtained from Oak Ridge National Laboratory and used as received. Although the sample was black in color, X-ray diffraction measurements indicated the sample to be phase pure. Polycrystalline samples of SrTcO3 were synthesized using solid-state methods described elsewhere (Thorogood et al., 2011a[Thorogood, G. J., Avdeev, M., Carter, M. L., Kennedy, B. J., Ting, J. & Wallwork, K. S. (2011a). Dalton Trans. 40, 7228-7233.]). Stoichiometric amounts of NH4TcO4 and Sr(NO3)2 were dry rolled in polyethylene vials for 16 h, to ensure complete mixing, and annealed at 973 K under Ar for 1 h. The samples were re-ground, wet ball-milled in cyclohexane for 16 h, air-dried, pressed into pellets and annealed under Ar at 1423 K for 4 h. The purity of the samples was confirmed by powder X-ray diffraction (see supporting information1).

All L3-edge XANES spectra were collected on the soft X-ray beamline at the Australian Synchrotron (Cowie et al., 2010[Cowie, B. C. C., Tadich, A. & Thomsen, L. (2010). AIP Conf. Proc. 1234, 307-310.]). In order to comply with the Australian Radiation Protection and Nuclear Safety Agency regulations, the amount of technetium used in these measurements was limited to ∼5 mg per sample. Technetium-containing powder samples were mixed with a minimum amount of industrial epoxy resin forming an extremely thick paste. The mixture was left to harden overnight by which time the technetium compounds were effectively immobilized. Swipe tests of the immobilized samples gave (beta) values of less than 100 counts s−1 A similar method was used to prepare epoxy-embedded CaRuO3 (Fig. 1[link]). Ru and Mo oxide standards were lightly dusted onto double-sided carbon tape (SPI Supplies).

[Figure 1]
Figure 1
(a) Diagram of a powder sample embedded in an epoxy resin mounted onto a sample stage and (b) an image of the CaRuO3 sample used in the experiment. The diameter of the epoxy disc is approximately 1.1 mm.

All samples were inserted into the vacuum chamber via a load lock. The pressure inside the analysis chamber was maintained at <10−9 mbar during the measurements. The epoxy encapsulated samples did not significantly degas. Spectra were collected from ∼30 eV below to ∼80 eV above the edge, with a step size of 0.2 eV using both TEY and FLY modes (except that the FLY signal at the Ru L3-edge had to be collected from another beamline, see §3). All spectra were recorded simultaneously with the TEY signal measured from Mo (Mo and Tc L3-edge) or Ru(OH)Cl3 (Ru L3-edge) reference samples positioned upstream in the beamline. The reference sample removed approximately 10% of the beam intensity. Mo L3-edge spectra were calibrated to the Mo L3-edge of Mo foil, with the maximum of the first derivate set to 2520 eV. Tc L3-edge spectra were calibrated to the Mo L2-edge of Mo foil, with the maximum of the first derivate set to 2625 eV. Due to the relatively weak edge jump, the Ru L3-edge spectra were calibrated against the maximum peak height of the Ru L3-edge XANES spectra of Ru(OH)Cl3 rather than the maximum of the first derivative. The maximum peak height of the Ru L3-edge XANES spectra of Ru(OH)Cl3 was at 2840.1 eV, based on the calibration against Ru metal with the maximum of the first derivate set to 2838 eV.

Tc K-edge XANES spectra of SrTcO3 and NH4TcO4 were collected in transmission mode on the X-ray absorption spectroscopy beamline at the Australian Synchrotron (Glover et al., 2007[Glover, C., McKinlay, J., Clift, M., Barg, B., Boldeman, J., Ridgway, M., Foran, G., Garrett, R., Lay, P. & Broadbent, A. (2007). AIP Conf. Proc. 882, 884-886.]). Powder samples were mixed with an appropriate amount of BN, sandwiched between two Kapton tapes and positioned directly in front of the X-ray beam. Spectra were collected from ∼200 eV below to ∼1000 eV above the edge with a step size of 0.25 eV around the edge and a dwell time of 1 s. X-rays were monochromated using an Si(111) monochromator, which was detuned by 50% to reject higher harmonics. The Tc K-edge was calibrated against Mo foil with the maximum of the first derivative of the Mo K-edge set to 20000 eV.

All absorption-edge energies are reported from the maximum of the first derivative of their respective edges. All XANES data were analyzed using the Athena and CasaXPS software packages (Ravel & Newville, 2005[Ravel, B. & Newville, M. (2005). J. Synchrotron Rad. 12, 537-541.]; Fairley, 2012[Fairley, N. (2012). CasaXPS, version 2.3.17dev5.9n. Casa Software Ltd, Teignmouth, Devon, UK.]).

3. Results and discussion

Measurement of the Tc L-edge spectra utilized the ultra-high-vacuum soft X-ray beamline at the Australian Synchrotron. The essential safety requirements for the measurements were (i) the sample was encapsulated in such a way that no loose material could be inadvertently introduced into the beamline and (ii) the amount of radioactive material was within the regulated limits of a general user beamline. For high-energy X-ray experiments, X-ray transparent windows can be employed to contain the sample; however, these are unsuit­able for low-energy soft X-ray measurements. Our strategy was to form a solid pellet of the technetium compound using an epoxy host. Before measuring the Tc L3-edge, a proof-of-concept experiment was conducted to ensure that this sample preparation technique produces quality XANES spectra representative of the bulk material. For this, the Ru L3-edge XANES spectrum of CaRuO3 was collected. This was chosen because its absorption-edge energy is greater than that of the Tc L3-edge and is near the upper limit of the energy range of the soft X-ray spectroscopy beamline (Cowie et al., 2010[Cowie, B. C. C., Tadich, A. & Thomsen, L. (2010). AIP Conf. Proc. 1234, 307-310.]). Consequently, the X-ray signal at the Ru L3-edge would be much weaker than that at the Tc L3-edge.

The Ru L3-edge XANES spectra of CaRuO3, measured both as a powder dusted on carbon tape and as an epoxy-embedded sample, are shown in Fig. 2[link]. Overall, the Ru L3-edge spectra shown here are similar to those previously reported for CaRuO3 (Zhou et al., 2009[Zhou, Q. D., Kennedy, B. J., Zhang, Z. M., Jang, L. Y. & Aitken, J. B. (2009). Chem. Mater. 21, 4203-4209.]; Wu et al., 2003[Wu, H. H., Chen, S. W., Lin, B. N., Hsu, Y. Y., Lee, J. F., Jang, L. Y. & Ku, H. C. (2003). J. Low Temp. Phys. 131, 1193-1197.]; Bréard et al., 2007[Bréard, Y., Hardy, V., Raveau, B., Maignan, A., Lin, H. J., Jang, L. Y., Hsieh, H. H. & Chen, C. T. (2007). J. Phys. Condes. Matter, 19, 216212.]). The Ru L3-edge XANES spectrum of CaRuO3, where Ru occupies the slightly distorted octahedral site, consists of two features that correspond to the transition of a 2p3/2 electron into the unoccupied 4dt2g (lower energy peak) and 4deg (higher energy peak) states. Such features are typically observed in 4d transition metal L-edge XANES spectra (Ting et al., 2010[Ting, J., Kennedy, B. J., Zhang, Z., Avdeev, M., Johannessen, B. & Jang, L. Y. (2010). Chem. Mater. 22, 1640-1646.]; Qasim et al., 2011[Qasim, I., Kennedy, B. J., Zhang, Z. M., Avdeev, M. & Jang, L. Y. (2011). J. Phys. Condens. Matter, 23, 435401.]; Blanchard et al., 2012[Blanchard, P. E. R., Clements, R., Kennedy, B. J., Ling, C. D., Reynolds, E., Avdeev, M., Stampfl, A. P. J., Zhang, Z. M. & Jang, L. Y. (2012). Inorg. Chem. 51, 13237-13244.]; Reynolds et al., 2013[Reynolds, E., Blanchard, P. E. R., Kennedy, B. J., Ling, C. D., Liu, S., Avdeev, M., Zhang, Z. M., Cuello, G. J., Tadich, A. & Jang, L. Y. (2013). Inorg. Chem. 52, 8409-8415.]; Blanchard et al., 2013a[Blanchard, P. E. R., Liu, S., Kennedy, B. J., Ling, C. D., Zhang, Z. M., Avdeev, M., Cowie, B. C. C., Thomsen, L. & Jang, L. Y. (2013a). Dalton Trans. 42, 14875-14882.]). The peak corresponding to transitions to the t2g states appears as a lower energy shoulder due to the partial occupancy of the t2g states in Ru4+ (t2g4eg0). As shown in Fig. 2[link], the Ru L3-edge spectrum of the powder form of CaRuO3 is nearly identical to that of the epoxy-embedded form. The crystal field splitting, estimated from fitting the Ru L3-edge to component peaks corresponding to the t2g and eg peaks, was estimated to be ∼3.0 eV for both samples (more details later in the paper). Both spectra are similar to the FLY Ru L3-edge spectrum of CaRuO3 collected at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan (see supporting information), confirming that the TEY signal is representative of the bulk material and that XANES spectra can be obtained using this sample preparation technique. We were unable to collect the FLY spectra on the soft X-ray spectroscopy beamline at the Australian Synchrotron because the Ru L3-edge is outside the designed range of the FLY detector.

[Figure 2]
Figure 2
Ru L3-edge XANES spectra of powder (black line) and epoxy-embedded (red line) CaRuO3. The difference plot (green line) shows that the spectra are essentially identical. Both spectra were collected in TEY mode.

Our preliminary experiment with CaRuO3 suggests that embedding powder samples in an epoxy resin is a feasible method for measuring the Tc L3-edge. Consequently, we collected the Tc L3-edge spectra from ∼5 mg SrTcO3 and NH4TcO4 samples embedded in epoxy, and compared them with their respective K-edge XANES spectra (Fig. 3[link]). Absorption-edge energies are tabulated in Table 1[link]. Both Tc L3-edges presented in Fig. 3[link] were collected in TEY mode. As shown in Fig. 4[link], the FLY signal is considerably weaker than the TEY signal. The FLY signal has a similar line shape to the TEY signal, confirming that the TEY signal of the Tc L3-edge is representative of the bulk material and suitable for this analysis. The K and L edges have advantages and disadvantages when it comes to characterizing the electronic and structural properties of Tc oxides. Analysis of the Tc K-edge absorption-edge energy is particularly useful in identifying Tc4+ and Tc7+ species (Mausolf et al., 2011[Mausolf, E., Poineau, F., Droessler, J. & Czerwinski, K. R. (2011). J. Radioanal. Nucl. Chem. 288, 723-728.]; Ferrier et al., 2012[Ferrier, M., Weck, P. F., Poineau, F., Kim, E., Stebbins, A., Ma, L. Z., Sattelberger, A. P. & Czerwinski, K. R. (2012). Dalton Trans. 41, 6291-6298.]). The Tc L3-edge absorption-edge energy of SrTcO3 (2677.0 eV) is less than that of NH4TcO4 (2678.5 eV), but the difference between them (1.5 eV) is much less than the difference of 4.5 eV observed at the Tc K-edge (SrTcO3, 21058.2 eV; NH4TcO4, 21062.7 eV). Both energy shifts are consistent with the Tc cations in SrTcO3 having the lower oxidation state.

Table 1
Absorption-edge energies for the various 4d transition metal oxides

Sample Oxidation state Coordination number Electronic configuration Absorption edge Absorption edge energy (eV)
(NH4)TcO4 Tc7+ 4 [Kr] 4d0 Tc L3-edge 2678.5
        Tc K-edge 21062.7
SrTcO3 Tc4+ 6 [Kr] 4d3 Tc L3-edge 2677.0
        Tc K-edge 21058.2
Bi2MoO6 (LT) Mo6+ 6 [Kr] 4d0 Mo L3-edge 2523.0
Bi2MoO6 (HT) Mo6+ 4 [Kr] 4d0 Mo L3-edge 2523.0
MoO2 Mo4+ 6 [Kr] 4d2 Mo L3-edge 2521.4
Sr2RuYO6 Ru5+ 6 [Kr] 4d3 Ru L3-edge 2839.1
SrRuO3 Ru4+ 6 [Kr] 4d4 Ru L3-edge 2838.2
CaRuO3 Ru4+ 6 [Kr] 4d4 Ru L3-edge 2838.1
[Figure 3]
Figure 3
(a) Tc L3-edge (collected in TEY mode) and (b) K-edge (collected in transmission mode) XANES spectra of SrTcO3 (octahedral Tc4+) and NH4TcO4 (tetrahedral Tc7+). Dashed lines correspond to the absorption-edge energies of SrTcO3 (black line) and NH4TcO4 (red line).
[Figure 4]
Figure 4
Tc L3-edge XANES spectra of SrTcO3 collected in TEY (black line) and FLY (red line) mode, respectively.

The most striking feature of the Tc L3-edge is the line shape. The Tc L3-edge line shape of NH4TcO4 consists of two features (labeled A and B in Fig. 3[link]a). As previously discussed, features in 4d transition metal L-edge spectra typically result from the crystal field splitting of the 4d states. The Tc7+ cations have a tetrahedral coordination environment in NH4TcO4 and the features in the L3-edge correspond to transitions to the empty e (feature A) and t2 (feature B) states. Similar features corresponding to transitions to the partially occupied t2g and eg states are observed in SrTcO3 with the eg states undergoing further splitting due to distortion of the TcO6 octahedra. This will be discussed further below. Evidence of crystal field splitting is not obvious in the Tc K-edge spectra, which is expected as the K-edge spectra do not probe the 4d states directly. Instead the Tc K-edge spectrum consists of a pre-edge feature which corresponds to the dipole forbidden transition of a 1s electron into 4d states via pd hybridization (Kettle, 2008[Kettle, S. F. A. (2008). Symmetry and Structure: Readable Group Theory for Chemists. New York: Wiley.]). In the Tc K-edge spectrum of NH4TcO4, the pre-edge contains an intense peak because the tetrahedral site occupied by the Tc7+ cation has Td symmetry which allows the hybridization of metal 4d and 5p states (Kettle, 2008[Kettle, S. F. A. (2008). Symmetry and Structure: Readable Group Theory for Chemists. New York: Wiley.]). In contrast, the Tc K-edge spectrum of SrTcO3, where the Tc4+ cation occupies a nearly centrosymmetric octahedral site (Oh), does not contain an obvious pre-edge feature due to the lack of dp hybridization (as there are no irreducible representations of Oh to which both p and d orbitals belong) (Kettle, 2008[Kettle, S. F. A. (2008). Symmetry and Structure: Readable Group Theory for Chemists. New York: Wiley.]). A pre-edge feature at the K-edge can manifest in highly distorted octahedral systems because the reduced symmetry would allow a certain degree of pd mixing (Yamamoto, 2008[Yamamoto, T. (2008). X-ray Spectrom. 37, 572-584.]). Since the pre-edge feature results from a transition to the p component in dp hybridized orbitals (not d-orbitals directly), it is difficult to extract information on crystal field splitting from 4d transition metal K-edge XANES spectra. The features at the Tc K-edge are also broader than those at the L-edge due to lifetime broadening. In general, the L-edge spectra appear to be more sensitive to changes in crystal field splitting, therefore having a greater sensitivity to changes in the local structure.

Since these appear to be the first measurements at the Tc L3-edge, the experimental XANES spectra of SrTcO3 (Tc4+) and NH4TcO4 (Tc7+) were compared with appropriate Mo and Ru L3-edge standards (Fig. 5[link]) measured on the same beamline. The difference in the Tc L3-edge absorption edge energy between the Tc4+ and Tc7+ species (1.5 eV) is similar to that observed at the Mo L3-edge between the Mo6+ and Mo4+ species (∼1.6 eV; Veith et al., 2005[Veith, G. M., Greenblatt, M., Croft, M., Ramanujachary, K. V., Hattrick-Simpers, J., Lofland, S. E. & Nowik, I. (2005). Chem. Mater. 17, 2562-2567.]) and larger than that at the Ru L3-edge between Ru4+ and Ru5+ oxides (0.9 eV). The energy shift in the Tc L3-edge is somewhat smaller than expected for such a large change in oxidation state. Although there is no difference in the Mo L3-edge absorption-edge energy of the high-temperature (tetrahedral Mo6+) and low-temperature (octahedral Mo6+) polymorphs of Bi2MoO6 (2523 eV), we cannot confirm from this study that the coordination environment of Tc has no effect on the Tc L3-edge absorption-edge energy. Regardless, the Tc L3-edge absorption-edge energy does appear to be sensitive to the oxidation state of Tc. The line shape of the Tc L3-edge shares similarities to that of the Mo and Ru L3-edge spectra. In general, features in XANES L3-edge spectra of 4d0 transition metals correspond to the crystal field splitting of the 4d states. This is well illustrated by comparing the Mo L3-edge spectra of the high-temperature and low-temperature polymorphs of Bi2MoO6 (Sankar et al., 1995[Sankar, G., Roberts, M. A., Thomas, J. M., Kulkarni, G. U., Rangavittal, N. & Rao, C. N. R. (1995). J. Solid State Chem. 119, 210-215.]; Buttrey et al., 1994[Buttrey, D. J., Vogt, T., Wildgruber, U. & Robinson, W. R. (1994). J. Solid State Chem. 111, 118-127.]; Laarif et al., 1984[Laarif, A., Theobald, F. R., Vivier, H. & Hewat, A. W. (1984). Z. Kristallogr. 167, 117-124.]). The Mo L3-edge spectra of both forms of Bi2MoO6 consist of two features (labeled A and B in Fig. 5[link]a), which correspond to transitions to the doubly degenerate e and triply degenerate t2 states in tetrahedral systems, and to the triply degenerate t2g and doubly degenerate eg states in octahedral systems, respectively (Bare et al., 1993[Bare, S. R., Mitchell, G. E., Maj, J. J., Vrieland, G. E. & Gland, J. L. (1993). J. Phys. Chem. 97, 6048-6053.]; Hu et al., 1995[Hu, H. C., Wachs, I. E. & Bare, S. R. (1995). J. Phys. Chem. 99, 10897-10910.]; Lede et al., 2002[Lede, E. J., Requejo, F. G., Pawelec, B. & Fierro, J. L. G. (2002). J. Phys. Chem. B, 106, 7824-7831.]). The difference in intensity of the two features reflects the reversal of the orbital energies and the appropriate degeneracy of these (3:2 in octahedral systems versus 2:3 in tetrahedral systems, under simplified crystal field theory). The Tc L3-edge of NH4TcO4 has a similar line shape to the high-temperature form of Bi2MoO6, consistent with the presence of a tetrahedral 4d0 transition metal.

[Figure 5]
Figure 5
(a) Mo and (b) Ru L3-edge XANES spectra of various Mo and Ru oxide standards. All spectra were collected in TEY mode.

The line shape of the Tc L3-edge spectrum of SrTcO3 (Tc4+) is similar to that of other octahedral 4d transition metals with partially occupied 4d states, such as MoO2 (Mo4+ d2) and SrRuO3 (Ru4+ d4). As mentioned earlier, the intensity of feature A is weaker than that of feature B due to partial filling of the t2g state. The splitting between the t2g and eg orbitals depends on the ligand field strength, which typically increases as the oxidation state increases (with similar bond distances). The transition into the t2g states appears as a discrete peak (labeled A in Fig. 5[link]b) in the L3-edge spectra of Sr2RuVYO6 but only as a low-energy shoulder in Sr(Ca)RuIVO3 and SrTcIVO3, consistent with the higher Ru oxidation state in Sr2RuYO6. In octahedral systems, feature B, which corresponds to the eg states, shows asymmetry towards higher energy. This asymmetry has been observed in the Zr L3-edge of pyrochlores Ln2Zr2O7 (Blanchard et al., 2012[Blanchard, P. E. R., Clements, R., Kennedy, B. J., Ling, C. D., Reynolds, E., Avdeev, M., Stampfl, A. P. J., Zhang, Z. M. & Jang, L. Y. (2012). Inorg. Chem. 51, 13237-13244.]), and is believed to be a consequence of the sensitivity of the eg states to local distortions due to the overlap of dz2 and dx2-y2 states with ligand states. Such distortions can result in broadening and splitting of the eg peak (Schneller et al., 2008[Schneller, T., Kohlstedt, H., Petraru, A., Waser, R., Guo, J., Denlinger, J., Learmonth, T., Glans, P. A. & Smith, K. E. (2008). J. Sol-Gel Sci. Technol. 48, 239-252.]).

Further information on the local structure can be obtained by fitting the Tc L3-edge to component peaks. Such analysis has previously been employed to study the local structure of zirconate and hafnate pyrochlores (Qasim et al., 2011[Qasim, I., Kennedy, B. J., Zhang, Z. M., Avdeev, M. & Jang, L. Y. (2011). J. Phys. Condens. Matter, 23, 435401.]; Blanchard et al., 2012[Blanchard, P. E. R., Clements, R., Kennedy, B. J., Ling, C. D., Reynolds, E., Avdeev, M., Stampfl, A. P. J., Zhang, Z. M. & Jang, L. Y. (2012). Inorg. Chem. 51, 13237-13244.], 2013b[Blanchard, P. E. R., Liu, S., Kennedy, B. J., Ling, C. D., Avdeev, M., Aitken, J. B., Cowie, B. C. C. & Tadich, A. (2013b). J. Phys. Chem. C, 117, 2266-2273.]; Reynolds et al., 2013[Reynolds, E., Blanchard, P. E. R., Kennedy, B. J., Ling, C. D., Liu, S., Avdeev, M., Zhang, Z. M., Cuello, G. J., Tadich, A. & Jang, L. Y. (2013). Inorg. Chem. 52, 8409-8415.]). As shown in Fig. 6[link], the spectra were fitted to pseudo-Voigt peaks with an arctan background. Fitted spectra of the Ru and Mo L3-edge XANES spectra can be found in the supporting information (Fig. S2 of the supporting information). Results of the fitting are presented in Table 2[link]. In all compounds listed, feature A was fitted to a single component peak. The number of components required to fit feature B varied depending on the coordination environment. In compounds where the metal cation occupies a tetrahedral coordination environment, only one component peak was required to fit feature B. In compounds where the metal is six-coordinate, two component peaks (labeled B1 and B2) were required to fit feature B. This is likely to be due to localized distortions of the metal octahedra (Ray et al., 2011[Ray, S. C., Hsueh, H. C., Wu, C. H., Pao, C. W., Asokan, K., Liu, M. T., Tsai, H. M., Chuang, C. H., Pong, W. F., Chiou, J. W., Tsai, M. H., Lee, J. M., Jang, L. Y., Chen, J. M. & Lee, J. F. (2011). Appl. Phys. Lett. 99, 042909.]; Ikeno et al., 2013[Ikeno, H., Krause, M., Hoche, T., Patzig, C., Hu, Y. F., Gawronski, A., Tanaka, I. & Russel, C. (2013). J. Phys. Condens. Matter, 25, 165505.]). The splitting of feature B into two peaks can be confirmed by analysing the first derivative XANES spectrum. For example, the first derivative Tc L3-edge XANES spectrum of SrTcO3 (see supporting information) has three peak maxima which corresponds to three unique peaks. By comparison, the first derivative Tc L3-edge XANES spectrum of NH4TcO7 has only two peak maxima. In order to reduce the number of free parameters, peak widths were constrained so that all peaks in a spectrum had the same width. Crystal-field-splitting energy (ΔE) can be taken as the energy difference between peaks A and B. For octahedral systems, ΔE is taken as the energy difference between EA (energy of peak A) and the weighted average of EB1 and EB2 (energies of peaks B1 and B2, respectively), i.e. EB = (EB1IB1 + EB2IB2)/(IB1 + IB2). The ΔE value of NH4TcO4 (2.5 eV) is less than that of SrTcO3 (2.9 eV). This is to be expected because tetrahedral systems have less crystal-field-splitting ΔE (in the simplified crystal field theory ΔTet ∼ 4/9ΔOct assuming the same oxidation state and distances). This reduction is evident when comparing the spectra of the low (oct) and high (tet) temperature forms of Bi2MoO6 (3.1 versus 2.0 eV). The ΔE value also scales with the oxidation state of the cation. For example, ΔE for SrRuIVO3 (2.8 eV) is less than that of Sr2RuVYO6 (3.1 eV). In the present technetium oxides these two opposing factors result in a larger splitting in the octahedral Tc4+ oxide compared (2.9 eV) with the tetrahedral Tc7+ oxide (2.6 eV).

Table 2
Fitting parameters to the 4d transition metal L3-edges

Ei is the energy of peak i and Ii is the intensity of peak i estimated from peak fitting. The width of the peaks and the estimated splitting between peaks A and B1 (ΔE) are also given.

Sample EA (eV) EB1 (eV) EB2 (eV) IA IB1 IB2 Peak width ΔE (eV)
(NH4)TcO4 2679.5 2682.0 N/A 2.682 3.201 N/A 2.7 2.5
SrTcO3 2678.0 2680.0 2682.3 4.265 9.283 6.136 2.7 2.9
Bi2MoO6 (LT) 2524.0 2526.7 2529.0 10.534 9.37 2.19 2.5 3.1
Bi2MoO6 (HT) 2523.6 2525.9 N/A 3.766 11.377 N/A 2.4 2.0
MoO2 2523.0 2525.0 2527.4 4.887 8.168 5.046 3.3 3.2
Sr2RuYO6 2840.2 2843.1 2846.2 33.213 45.249 3.278 2.8 3.1
SrRuO3 2839.2 2841.2 2843.9 20.964 29.95 12.38 2.9 2.8
CaRuO3 2839.6 2841.9 2845.1 27.436 38.139 8.186 3.4 3.1
[Figure 6]
Figure 6
Fitted Tc L3-edge XANES spectra of (a) NH4TcO4 and (b) SrTcO3.

4. Conclusion

In conclusion, we have successfully demonstrated that the Tc L3-edge can be measured by embedding a very small amount of technetium-containing powder samples (∼5 mg) in an epoxy resin. We have shown that the L-edge XANES spectra of the early 4d transition metals (Zr, Nb, Mo, Tc and Ru) are extremely sensitive to changes in both the oxidation state and coordination environment, providing information not only on the formal oxidation state of the cation but also on the crystal field splitting and coordination environment. Since the 4d orbitals are not fully occupied, the Tc L3-edge spectrum directly probes the 4d bonding states, which can provide more accurate information on the local structure of Tc cations than the Tc K-edge spectrum. Crystal-field-splitting energies can be deduced from the Tc L3-edge XANES spectra by fitting the spectra with component peaks. The methodology used here can easily be extended to other technetium species of various oxidation states and coordination environments, an important step in exploring this relatively unknown area of X-ray absorption spectroscopy.

Supporting information


Footnotes

1Supporting information for this paper is available from the IUCr electronic archives (Reference: HF5261 ).

Acknowledgements

This work was performed at the soft X-ray and X-ray absorption spectroscopy beamlines at the Australian Synchrotron. We thank Drs Chris Glover and Bernt Johannessen for their assistance with the K-edge measurements. We acknowledge the Australian Research Council for financial support.

References

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