Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Effect of Synthetic Method and Annealing Temperature on the Structure of Hollandite-Type Oxides Ifeoma Ebinumoliseh and Andrew P. Grosvenor*
Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on November 1, 2018 at 16:58:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada ABSTRACT: Hollandite is a class of metal oxide material with the general formula A2B8O16. Several methods have been used in the synthesis of this type of metal oxide, and the synthetic methods reported have typically employed high annealing temperatures between 1200 and 1300 °C. Appropriate synthetic methods must be employed to successfully synthesize these hollandite-type oxides at lower annealing temperatures. Hollandite compounds have been synthesized using ceramic (high annealing temperature only) and coprecipitation (high and low annealing temperatures) methods. Annealing temperatures ranging from 1200 to 700 °C have been employed in the thermal treatment process. Powder X-ray diffraction and X-ray absorption near-edge spectroscopy (XANES) were conducted on hollandite-type oxides (BaxAl2xTi8−2xO16−δ; x = 1.2; and BaxAlxFexTi8−2xO16−δ, BaxFe2xTi8−2xO16−δ; x = 1.16). Structural comparisons between materials annealed in the temperature range from 1200 to 800 °C were made, and an examination of the XANES spectra and powder X-ray diffraction patterns has provided confirmation of the absence of significant structural changes in these hollandite materials. This study has shown that hollandite-type materials can be formed using annealing temperatures as low as 700−800 °C when a coprecipitation method is used, with little change to the local and long-range structures being detected.
1. INTRODUCTION The development of nuclear power plants for electricity generation has led to the production of high-level nuclear waste (HLW). Efforts have been made to isolate these nuclear waste elements, and one sequestration technology that is currently employed involves the incorporation of waste elements into a glass matrix.1−3 The limitations of glassbased waste forms include low waste loading, low thermal stability, and low resistance to radiation-induced structural damage.4−9 These limitations have prompted the consideration of crystalline waste forms for the storage of these HLW elements. Radioactive cesium is one of the high-level waste elements of concern, which results from neutron-induced fission reactions of nuclear fuels.1 The Cs isotopes are βemitters, which can result in high levels of radiogenic heating, and one of the few crystalline matrices that are considered for its immobilization are hollandite-type oxides.10 Compounds adopting the hollandite-type structure crystallize in a tetragonal unit cell (I4/m).11 The hollandite structure, nominally A2B8O16, consists of a chain of B3+ octahedra linked via corner and edge-sharing, where the A- and B-site cations can be disordered over the two crystallographically distinct sites. The A-site cations occupy a box-shaped cavity, each coordinated by eight oxygen ions within tunnels parallel to the c-axis (Figure 1). The hollandite structure exhibits a high tolerance for cation substitution over the A (Ba2+, Cs+, Rb+, Sr2+) and B (Ti3+, Al3+, Fe3+, Mg2+,Ga3+, Cr3+, Sc3+) sites, suggesting the capability of materials adopting this structure type to incorporate multiple radionuclides into the structure.11−14 Hollandite can be found in SYNROC C (a mutliphase ceramic nuclear waste form) and serves as a host phase © XXXX American Chemical Society
Figure 1. Tetragonal crystal structure of Hollandite-type oxides (A = Ba and B= Al/Fe/Ti; space group: I4/m). The crystal structure is shown with the c axis directed into the page and was generated using the VESTA software program.55
for radioactive Cs.15 The ability of this phase to accommodate Ba2+, Cs+, Rb+, and Sr2+ into the crystallographic sites in this structure makes it applicable for the immobilization of 137 Cs.16−18 Barium titanate-type compounds that adopt the hollandite-type structure have also been proposed for use as oxidation catalysts, ion exchangers, solid ionic conductors, dielectric resonators, and battery materials.1,19,20 In the past, hollandite compounds have been synthesized by employing different methods which include solid-state (ceramic) high-temperature/high-pressure methods, sol−gel methods, and reflux methods.21,22 These synthesis methods have typically used high temperatures (1200−1300 °C) to Received: August 30, 2018
A
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
diameter filter paper. Samples were dried in air for 24 h and further dried at 110 °C. The dried material was ground with a mortar and pestle to a fine powder and then pressed into pellets at 6 MPa and annealed at temperatures ranging from 1200 to 650 °C, followed by quench cooling in air. Samples annealed at temperatures ranging from 1200 to 900 °C were annealed for 9 days, while samples annealed at temperatures ranging from 800 to 700 °C were annealed for 15 days. Samples annealed at 650 °C were annealed for more than 21 days, but the hollandite phase could not be formed at this temperature. All materials were ground and pelleted in 3 day intervals. 2.2. Powder X-ray Diffraction. Powder XRD experiments were performed to confirm the phase purity of the synthesized materials. Patterns from all samples were collected using a PANalytical Empyrean system (Co Kα1,2 X-ray source). Powder XRD patterns from the synthesized samples were collected at room temperature in the 2θ range of 20 ≤ 2θ ≤ 80° using a step size of 0.017°. Lattice constants were determined using the HighScore Plus software program.29 Rietveld refinement was performed on powder XRD patterns collected from select materials using a Cu Kα1,2 X-ray source over a 2θ range of 10° to 110° using a step size of 0.017°. The powder XRD patterns from select synthesized hollandite compounds were fitted and refined using a tetragonal structural model.30 The following parameters were refined: scale factor, zero shift, lattice constant, atomic coordinates, and overall thermal parameter (Bovl). 2.3. Scanning Electron Microscopy. SEM experiments were performed using a JEOL 840A scanning electron microscope. Dry samples were mounted on double sided carbon tape applied to 10 mm mounting stubs and sputter coated with approximately 200 Å of gold prior to the start of the experiment. Secondary electron micrograph images were acquired at 15 kV using a working distance of 15 mm. 2.4. XANES. 2.4.1. Ti and Fe K-Edge XANES. The Ti and Fe Kedge XANES spectra were collected using the Canadian Light Source (CLS) and the Advanced Photon Source (APS; Argonne National Laboratory) synchrotron radiation facilities. The Soft X-ray Microcharacterization Beamline (SXRMB; 06B1-1) located at the CLS was used to collect Ti K-edge XANES spectra, while the Sector 20 and Sector 9 bending magnet Beamlines (20-BM and 9-BM) located at the APS were used to collect both Ti and Fe K-edge XANES spectra.31,32 All three beamlines employed Si(111) monochromators which provided a photon flux of 1 × 1011 photons/seconds. The resolution of the spectra was 0.5 at 4996 eV when collected using the SXRMB beamline, while the resolution was 0.7 eV at 4996 eV (Ti Kedge) and 1.0 eV at 7112 eV (Fe K-edge) when the 20-BM beamline was used and 0.5 eV at 4996 eV (Ti K-edge) and 0.7 eV (Fe K-edge) when the 9-BM beamline was used. Spectra were collected from finely ground powders deposited on C tape (SXRMB) or sealed between layers of Kapton tape (9-BM and 20-BM). Spectra collected using SXRMB were collected in total electron yield mode. Transmission spectra collected using 9-BM and 20-BM were measured with He and N2 (80%:20%) filled ionization chambers. All spectra were collected using an energy step size of 0.15 eV/step through the absorption edge. Ti metal and Fe metal were used as references for calibrating the energies of the Ti and Fe K-edge XANES spectra by setting the peak maximum of the first derivative of the spectra to 4996 eV (Ti) or 7112 eV (Fe).33 2.4.2. Al L2,3-Edge XANES. Al L2,3-edge XANES spectra from the Ba1.2Al2.4Ti5.6O16−δ and Ba1.16Al1.16Fe1.16Ti5.68O16−δ materials were collected using the Variable Line Spacing Plane Grating Monochromator (VLS PGM, 11ID-2) beamline located at the CLS. Finely ground powders were deposited on C tape before being inserted into the vacuum chamber.34 Spectra were collected in fluorescence yield (FLY) mode using a 0.025 eV step size. Al metal foil was used as a reference for calibrating the energy by setting the peak maximum of the first derivative of the spectrum to 72.55 eV.33 The beamline provided a photon flux of 2 × 1011 photons/seconds and resolution of approximately 0.01 eV with a spot size of 500 μm × 500 μm [34]. All XANES spectra reported in this study were analyzed using the Athena software program.35
produce these materials.23,24 One area that this study seeks to address is to investigate the effect of using a coprecipitation method and lower annealing temperatures on the long-range order and local chemical environment of the hollandite-type materials. Forming these materials using lower annealing temperatures would likely result in lower production costs. Using a coprecipitation method allows for optimal mixing of the starting materials to be achieved, leading to a homogeneous mixture on the nm/atomic scale. This synthesis method involves the precipitation of metal hydroxides from metal ion solutions where precipitation can be carried out in either an alkaline or acidic medium.25,26 Only micron-scale mixing can be achieved when the starting materials are mixed by mechanical grinding, which is commonly employed as part of the solid-state/ceramic method, resulting in large diffusion path lengths. In contrast, using a coprecipitation method results in greater mixing of the starting materials and, therefore, shorter distances that the ions have to diffuse before reacting to form the desired compounds. This is an important characteristic of the coprecipitation synthesis method, which can result in the formation of the desired compound at lower annealing temperatures.27,28 The objective of this study was to demonstrate that hollandite-type materials annealed at high and low temperatures are comparable when formed using a coprecipitation method. The local and long-range structures of the synthesized Ba 1. 2 Al 2.4 Ti 5.8 O 16−δ , Ba 1.16 Al 1.16 Fe 1.16 Ti 5. 68 O 16−δ , and Ba1.16Fe2.32Ti5.68O16−δ hollandite-type materials were studied by use of powder X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES), while the particle size was investigated by use of scanning electron microscopy (SEM).
2. EXPERIMENTAL SECTION 2.1. Synthesis: Ceramic and Coprecipitation Methods. Hollandite compounds (Ba1.2Al2.4Ti5.6O16−δ, Ba1.16Al1.16Fe1.16Ti5.68O16−δ and Ba1.16Fe2.32Ti5.682O16−δ) were synthesized using ceramic and coprecipitation methods. For the ceramic method, stoichiometric amounts of BaCO3 (VWR alfa, 99.95%), Fe2O3 (VWR alfa, 98%), Al2O3 (Sigma-Aldrich, 98%), and TiO2 (VWR alfa, 99.6%) were weighed out, and the powder was mixed in a mortar using a pestle. All three hollandite-type samples were synthesized and annealed at 1200 °C. An attempt was also made to synthesize Ba1.16Al1.16Fe1.16Ti5.68O16−δ at 800 °C using the ceramic method. All samples were pelleted at 6 MPa before being annealed at 1200 °C or 800 °C. The sample produced using the ceramic method and synthesized at 1200 °C was annealed for 9 days, while the sample produced using the ceramic method and synthesized at 800 °C was annealed for 30 days with grinding and pelleting occurring every 3 days. All samples were quench cooled in air. For the coprecipitation method, Ba(NO3)3·9H2O (Alfa Aeser, 99.99%), Al(NO3)3.9H2O (Alfa Aeser, 98−102%), Fe(NO3)3·9H2O (Acros Organic, 99+%), and Ti[OC(CH3)3]4 (Acros Organics, 99%) were weighed out in separate vials and dissolved in distilled water (10 mL). Individual solutions of the nitrates were transferred into a 250 mL beaker and stirred until a clear solution was achieved. Concentrated HNO3 (1 mL; 15.8 M) was added to the solution to prevent premature precipitation. Titanium tert-butoxide was then added to the mixture of nitrate solutions. Magnetic stirring of the mixture continued for approximately 30 min, while concentrated NH4OH (50 mL; 28−30%) was added dropwise to the solutions using a buret. White (Al-substituted hollandite) or brown (Fesubstituted hollandite) colored precipitates were observed to form when the pH reached 11.5, a pH attained after adding approximately 40 mL of concentrated NH4OH. The resultant precipitate was filtered by vacuum filtration using a Buchner funnel and a Whatman 55 mm B
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
ceramic method and annealed at 1200 °C (cf., Figure 2).15 Evaluation of the powder XRD pattern from Ba1.2Al2.4Ti5.6O16−δ annealed at 700 °C showed broad and weak reflections. These broad reflections (Figure 2a) can be attributed to poor crystallinity of the material when compared to the patterns obtained from samples that were annealed at temperatures ranging from 1200 to 800 °C. The hollandite samples synthesized using the coprecipitation method and annealed at 650 °C showed that the hollandite-type structure could not be formed at this temperature, as no diffraction peaks were observed. Figure 3 shows the powder XRD patterns
3. RESULTS AND DISCUSSION 3.1. Structure, Crystallite Size, and Particle Size Analysis. 3.1.1. Powder X-ray Diffraction. Powder XRD patterns were collected from the samples synthesized by the coprecipitation method and annealed at temperatures ranging from 1200 to 700 °C. Powder XRD patterns were also collected from samples synthesized by the ceramic method and annealed at 1200°C (see Figure 2). The Ba1.2Al2.4Ti5.6O16−δ, Ba1.16Al1.16Fe1.16Ti5.68O16−δ, and Ba1.16Fe2.32Ti5.68O16−δ samples annealed at temperatures ranging from 1200 to 800 °C were deemed to be phase pure due to the observed reflections that are characteristic of hollandite powder XRD patterns, which also compare well to the materials synthesized using the
Figure 3. Powder XRD patterns from Ba1.16Al1.16Fe1.16Ti5.68O16−δ synthesized using the ceramic and coprecipitation method and annealed at 800 °C are shown. The major reflections for the diffraction patterns from these hollandite-type materials are labeled.
from Ba1.16Al1.16Fe1.16Ti5.68O16−δ synthesized using the ceramic and coprecipitation methods and annealed at 800 °C. The diffraction pattern from the material synthesized using the ceramic method showed a weak/broad set of reflections after annealing the material for a total of 30 days, while the pattern from Ba1.16Al1.16Fe1.16Ti5.68O16−δ synthesized using the coprecipitation method indicated that this material was successfully formed after annealing at 800 °C for just 15 days. This observation demonstrates the ability of the coprecipitation method to form materials at much lower temperatures and in a quicker amount of time compared to the ceramic method. The refined powder XRD patterns obtained from the Ba1.16Al1.16Fe1.16Ti5.68O16−δ samples annealed at 800 and 1200°C are presented in Figure 4, and the results of the refinement are presented in Table 1. The lattice parameters and atomic positions determined by the refinement were found to be consistent with values reported previously for these materials.30 This further confirms that these hollandite-type oxides can be synthesized using lower annealing temperatures when the coprecipitation method is employed. 3.1.2. Crystallite size. The crystallite size was determined using the Scherrer equation:36 D=
Kλ B cosθ
(1)
where D is the crystallite size, K is the dimensionless shape factor, B is the full width at half-maximum (fwhm) of the peak, λ is the X-ray wavelength, and θ is the Bragg angle. The shape factor used in this calculation was 0.9, as spherical crystallites were assumed.37 Average crystallite sizes of the hollandite-type compounds were calculated by considering the full width at
Figure 2. Powder XRD patterns from the synthesized (a) Ba1.2Al2.4Ti5.6O16−δ, (b) Ba1.16 Al1.16Fe1.16Ti5.68 O16−δ, and (c) Ba1.16 Fe2.32Ti5.68 O16−δ materials formed by the ceramic or coprecipitation method and annealed using temperatures ranging from 1200 to 650 °C are presented. The major reflections for the diffraction patterns from these hollandite-type materials are labeled in (c). C
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. Refined powder XRD patterns from Ba1.16Al1.16Fe1.16Ti5.68O16−δ formed using the coprecipitation method and annealed at (a) 1200 and (b) 800 °C are shown.
Table 1. Rietveld Refinement Results for Ba1.16Al1.16Fe1.16Ti5.68O16−δ Formed by the Coprecipitation Method and Annealed at 800 and 1200 °C 800 °C lattice constants (Å) z(Ba) x(Ti/Al/Fe) Y(Ti/Al/Fe) x(O1) y(O1) x(O2) y(O2) BOVL goodness of fit Rexpected Rprofile weighted Rprofile Site Occupancy Ba Ti Fe Al
1200 °C
a = b = 10.0516(6); c = 2.9506(5); α = β = γ = 90° 0.1228 0.1478(1) 0.3325(6) 0.3463(6) 0.2993 0.3337(4) 0.0409(7) 0.50 3.46(3) 2.58 3.57(1) 4.80(3)
a = b = 10.0643(3); c = 2.9560(5); α = β = γ = 90° 0.1228 0.1466(1) 0.3311(1) 0.3463(6) 0.2993 0.3337(4) 0.0409(7) 0.50 5.97(6) 2.37(6) 4.05(8) 5.81
0.29 0.71 0.15 0.15
0.29 0.71 0.15 0.15
Figure 5. Variations in the crystallite size of (a) Ba1.2Al2.4Ti5.6O16−δ, (b) Ba1.16 Al1.16Fe1.16Ti5.68 O16−δ, and (c) Ba1.16 Fe2.32Ti5.68 O16−δ versus annealing temperature are shown. Error bars are presented for each data point, and the red line represents the line of best fit.
hollandite series, the crystallite size of the Ba1.2Al2.4Ti5.6O16−δ materials was the lowest, particularly when comparing the materials annealed at 700 °C. The higher crystallite size values observed for the Fe-substituted hollandite materials suggests that these materials are able to form at much lower annealing temperatures in comparison to the non-Fe substituted materials. The hollandite samples synthesized using the ceramic method and annealed at 1200 °C are comparable to materials synthesized using the coprecipitation method at the same annealing temperature (cf. Table 2). 3.1.3. Scanning Electron Microscopy. SEM micrographs from Ba1.2Al2.4Ti5.6O16−δ annealed at 1200 °C, 800 °C, and 700 °C are shown in Figure 6. The images from the materials annealed at 1200 and 800 °C show similar, large particle sizes, while the image from the material annealed at 700 °C indicated the presence of smaller and less defined particles.
half-maximum (fwhm) of at least nine of the individual peaks at similar 2θ positions for all compounds. The effect of instrumental broadening was considered by subtracting the instrumental line width (0.0559°) from the width of the observed diffraction peaks. Plots of the crystallite size versus annealing temperature are presented in Figure 5 and Table 2. All plots show a progressive decrease in crystallite size with decreasing annealing temperature. Compared to the other D
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 2. Average Crystallite Sizes of Ba1.2Al2.4Ti5.8 O16−δ, Ba1.16Al1.16Fe1.16Ti5.68O16−δ, and Ba1.16Fe2.32Ti5.68O16−δ crystallite size (μm) temperature (°C) 1200 (ceramic) 1200 (coprecipitation) 1100 1000 900 800 700
Ba1.2Al2.4Ti5.8 O16−δ 1.85 2.00 1.80 1.50 1.30 1.10 0.70
± ± ± ± ± ± ±
Ba1.16Al1.16Fe1.16Ti5.68O16−δ
4 5 4 3 5 4 2
4.29 4.40 4.20 3.70 3.02 2.62 1.97
± ± ± ± ± ± ±
4 3 2 3 3 4 3
Ba1.16Fe2.32Ti5.68O16−δ 2.25 2.50 2.20 1.80 1.50 1.20 1.02
± ± ± ± ± ± ±
4 2 2 3 1 2 3
environment of the B-site cations changed depending on composition, synthesis method, and annealing temperature. 3.2.1. Ti K-edge XANES. Ti K-edge XANES spectra were collected from hollandite-type compounds (Ba 1.2 Al 2.4 Ti 5.6 O 16−δ , Ba 1.16 Al 1.16 Fe 1.16 Ti 5.68 O 16−δ , and Ba1.16Fe2.32Ti5.68O16−δ) annealed at temperatures ranging from 1200 to 700 °C. The spectra are comprised of two features: the pre-edge region and the main-edge region. The pre-edge features result from quadrupolar, 1s → 3d transitions, while the main-edge features (B and C) result from dipolar, 1s → 4p transitions.38−41 The pre-edge region provides information on the coordination number of Ti in the material of interest.42,43 Based on the dipolar selection rule (Δl = +/− 1), the quadrupolar transition observed in the pre-edge region is forbidden, but it becomes allowed because of the overlap of 4p and 3d orbitals. An increase in the intensity of the pre-edge peak is observed as the coordination number (CN) decreases from 6 to 4 because of the overlap of 3d and 4p orbitals. The pre-edge peak intensity can therefore be used to infer the Ti CN.43−45 Examination of the Ti K-edge XANES spectra from the Ba1.16Al1.16Fe1.16Ti5.68O16−δ and Ba1.16Fe2.32Ti5.68O16−δ compounds synthesized using the coprecipitation method and annealed at temperatures ranging from 1200 to 700 °C indicates no apparent change in the average Ti CN when compared to the spectra from these materials produced using the ceramic method and annealed at 1200 °C (cf., Figures 7b,c). A similar observation can be made when examining the spectra from the Ba1.2Al2.4Ti5.6O16−δ materials synthesized using the coprecipitation method; however, only over an annealing temperature range of 1200 to 800 °C (Figure 7a). An evaluation of the pre-edge region from the Ti K-edge spectrum from Ba1.2Al2.4Ti5.6O16−δ annealed at 700 °C shows an increase intensity of this peak compared to the materials that were annealed at higher temperatures. This change indicates a lower average Ti CN, which suggests that the Ba1.2Al2.4Ti5.6O16−δ material may not have formed fully at 700 °C.46,47 The increase in the pre-edge peak intensity from Ba1.2Al2.4Ti5.6O16−δ annealed at 700 °C is in good agreement with the powder XRD results (Figure 2a) and suggests that an amorphous fraction is present in this material.48 Ti occupies an octahedral site in hollandite-type oxides when the material is fully crystalline; however, Ti is generally found in lower coordination environments when present in an amorphous form.49 Examination of the Ti K-edge XANES spectra confirm that the Ti coordination environment is maintained in all three materials studied regardless of the synthesis method used and when the material is annealed at temperatures as low as 800 °C (Ba1.2Al2.4Ti5.6O16−δ; Figure 7a) or 700 °C (Ba1.16Al1.16Fe1.16Ti5.68O16−δ, Ba1.16Fe2.32Ti5.68O16−δ; Figure 7b,c). These results also indicate the importance of the
Figure 6. SEM images from Ba1.2Al2.4Ti5.6O16−δ annealed at (a) 1200 °C (b) 800 °C, and (c) 700 °C are shown. The scalebar in each image is 1 micrometer.
This observation is consistent with the crystallite size analysis performed by examination of the powder XRD patterns and discussed above. 3.2. XANES. XANES spectra were collected at the Ti K-, Fe K-, and Al L2,3-edges to determine how the local chemical E
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 8. Fe K-edge XANES spectra from (a) Ba1.16Al1.16Fe1.16Ti5.68O16−δ and (b) Ba1.16Fe2.32Ti5.68O16−δ are shown.
Evaluation of the Fe K-edge XANES spectra from the Ba1.16Al1.16Fe1.16Ti5.68O16−δ and Ba1.16Fe2.32Ti5.68O16−δ materials did not show any changes in the line shape regardless of synthesis method or annealing temperature used to form these materials, indicating the absence of changes in the local chemical environment of Fe in these materials. 3.2.3. Al L2,3-edge XANES. Al L2,3-edge XANES spectra were collected to study the effect of annealing temperature on the local chemical environment of Al in the Al-substituted hollandite materials. These spectra have been determined previously to be very sensitive to changes in the Al coordination environment in Al-containing materials.51,52 The features shown in the Al L2,3-edge XANES spectra (Figure 9) are due to 2p → 3s/3d transitions.52 In crystalline compounds containing Al with a single CN (e.g., Ba1.2Al2.4Ti5.6O16−δ and Ba1.16Al1.16Fe1.16Ti5.68O16−δ), individual L2- and L3-edge features can be observed because of spin orbit splitting.50,53 These features become broader when Al is found in multiple coordination environments with the lowest energy feature being affected by the presence of Al in low-coordinate environments (e.g., CN = 4), while the higher energy feature is primarily affected by the presence of Al in high-coordinate environments (e.g., CN = 6). A decrease in the average Al CN is observed by an increase in the intensity of the lowest energy feature in these spectra, as a result of changes in the screening of the final state after the excitation of 2p electrons depending on the coordination environment.53,54 The Al L2,3-edge XANES spectra collected from the Ba1.2Al2.4Ti5.6O16−δ and Ba1.16Al1.16Fe1.16Ti5.68O16−δ materials were observed to be very similar when the annealing temperature was varied between 1200 and 800 °C (Figure 9). These observations indicate the absence of changes in the
Figure 7. Ti K-edge XANES spectra from (a) Ba1.2Al2.4Ti5.6O16−δ, (b) Ba1.16 Al1.16Fe1.16Ti5.68 O16−δ, and (c) Ba1.16Fe2.32Ti5.68 O16−δ are shown. The inset in (a) indicates the change in pre-edge peak intensity that occurs with decreasing annealing temperature.
composition to the minimum temperature required to fully form these materials, as only the Fe-bearing materials could be fully formed when an annealing temperature as low as 700 °C was used. 3.2.2. Fe K-edge XANES. Fe K-edge XANES spectra were collected to study how the local Fe coordination environment was affected depending on the annealing temperature used to form the Ba1.16Al1.16Fe1.16Ti5.68O16−δ and Ba1.16Fe2.32Ti5.68O16−δ materials. The pre-edge (feature A in Figure 8) results from quadrupolar, 1s → 3d transitions, while the main-edge features (B and C in Figure 8) represent dipolar, 1s → 4p transitions.38−40 These features, particularly the pre-edge region, are sensitive to changes in the local chemical environment of Fe and can change because of variations in the coordination environment, as described above when the Ti K-edge XANES spectra were discussed (Section 3.2.1).49−51 F
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
pounds are more crystalline when lower annealing temperatures are employed. This study has demonstrated how lowtemperature methods can be used to form crystalline hollandite-type materials, which have multiple applications, including for the sequestration of high-level (nuclear) waste; however, further work will be needed to determine the optimum time and temperature conditions needed to produce large quantities of hollandite-type materials as efficiently and economical as possible.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Andrew P. Grosvenor: 0000-0003-0024-0138 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This project was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada through a discovery grant awarded to A.P.G. I.E. would like to thank the Government of Saskatchewan and the University of Saskatchewan for financial support. Mr. Tom Bonli (Department of Geological Sciences, University of Saskatchewan) is thanked for performing the scanning electron microscopy experiments. Ms Sarah McCaugherty, Dr. Xiaoxuan Guo, and Dr. Jeremiah Beam from the Department of Chemistry, University of Saskatchewan are thanked for help in the collection of the XANES spectra presented in this study. The CLS is supported by NSERC, the National Research Council of Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan. The Sector 9 and Sector 20 (CLS@APS) facilities located at the Advanced Photon Source are supported by the U.S. Department of Energy - Basic Energy Sciences, the Canadian Light Source and its funding partners, and the Advanced Photon Source. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under contract no. DE-AC0206CH11357.
Figure 9. Al L2,3-edge XANES spectra from (a) Ba1.2Al2.4Ti5.6O16−δ and (b) Ba1.16Al1.16Fe1.16Ti5.68O16−δ are shown.
local Al coordination environment in these materials over an annealing temperature range from 1200 to 800 °C. However, a comparison of the Al L2,3-edge XANES spectra from Ba1.2Al2.4Ti5.6O16−δ (Figure 9a) shows an increase in the intensity of the lower energy feature of the spectrum collected from the Ba1.2Al2.4Ti5.6O16−δ material that was annealed at 700 °C compared to the spectra from the materials annealed at higher temperatures. This difference may be attributed to the presence of an amorphous fraction, leading to the presence of Al in a lower average coordination environment in the Ba1.2Al2.4Ti5.6O16−δ material annealed at 700 °C. The spectral change observed from the Ba1.2Al2.4Ti5.6O16−δ material annealed at 700 °C sample has been corroborated by a corresponding change observed in the powder XRD patterns (Figure 2) and an increase in the pre-edge peak intensity of the Ti K-edge XANES spectrum (Figure 7). A change in the spectral line shape from the Ba1.16Al1.16 Fe1.16Ti5.68O 16−δ material annealed at 700 °C was also observed (Figure 9b), which may also indicate the presence of an amorphous fraction in this materials, albeit at a lower concentration than in the Ba1.2Al2.4Ti5.6O16−δ owing to the absence of significant changes in the Ti and Fe K-edge XANES spectra from these materials regardless of annealing temperature used (cf. Figures 7 and 8).
■
REFERENCES
(1) Raj, K.; Kaushik, C. P. Glass matrices for vitrification of radioactive waste − an Update on R & D Efforts. IOP Conf. Ser.: Mater. Sci. Eng. 2009, 2, 012002. (2) Ojovan, M. I.; Lee, W. E. Glassy Waste for Nuclear Waste Immobilization. Metall. Mater. Trans. A 2011, 42, 837−851. (3) Ojovan, M. I.; Juoi, J. M.; Lee, W. E. Application of glass composite materials for nuclear waste immobilization. J. Pak. Mater. Soc. 2008, 2, 72−76. (4) Martin, C.; Ribet, I.; Frugier, P.; Gin, S. Alteration Kinetics of the Glass-Ceramic Zirconolite and Role of the Alteration Film − Comparison with the SON68 Glass. J. Nucl. Mater. 2007, 366, 277. (5) Ewing, R. C.; Weber, W. J.; Clinard, F. W. Radiation Effects in Nuclear Waste Forms for High-level Radioactive waste. Prog. Nucl. Energy 1995, 29, 63−127. (6) Pace, S.; Cannillo, V.; Wu, J.; Boccaccini, D. N.; Seglem, S.; Boccaccini, A. R. Processing Glass-pyrochlore composite for nuclear waste encapsulation. J. Nucl. Mater. 2005, 341, 12−18.
4. CONCLUSIONS This study has shown that lower annealing temperatures can be employed in the syntheses of hollandite-type Ba 1 . 2 Al 2 . 4 Ti 5 . 6 O 1 6− δ , Ba 1 . 16 Al 1.16 Fe 1.1 6 Ti 5 . 68 O 1 6− δ and Ba1.16Fe2.32Ti5.68O16−δ oxides using a coprecipitation method compared to the conventional ceramic method that is often used to form these materials. Examination of powder XRD patterns and XANES spectra has shown that the hollanditetype structure can be formed using annealing temperatures as low as 800 °C with few changes in the local- and long-range structures being detected. Results from this study have shown that compounds that have Fe substituted into the B crystallographic site in comparison to Al substituted comG
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (7) Boccaccini, A. R.; Atiq, S.; Grimes, R. W. Hot-pressed Glass Matrix Composites Containing Pyrochlore Phase Particles for Nuclear Waste Encapsulation. Adv. Eng. Mater. 2003, 5, 501−508. (8) McGann, O. J.; Bingham, P. A.; Hand, R. J.; Gandy, A. S.; Kavcic, M.; Zitnik, M.; Bucarb, K.; Edge, R.; Hyatt, N. C. The effects of γ-radiation on model vitreous wasteforms intended for the disposal of intermediate and high level radioactive wastes in the United Kingdom. J. Nucl. Mater. 2012, 429, 353−367. (9) Whittle, K. R.; Ashbrook, S. E.; Lumpkin, G. R.; Farnan, I. R.; Smith, I.; Redfern, S. A. T. The effect of caesium on barium hollandites studied by neutron diffraction and magic-angle spinning (MAS) nuclear magnetic resonance. J. Mater. Sci. 2007, 42, 9379− 9391. (10) McGann, O. J.; Gandy, A. S.; Bingham, P. A.; Hand, R. J.; Hyatt, N. C. The Effect of γ-radiation on Mechanical Properties of Model UK Nuclear Waste Glasses. MRS Online Proc. Libr. 2013, 1518, 41−46. (11) Aubin-Chevaldonnet, V.; Caurant, D.; Dannoux, A.; Gourier, D.; Charpentier, T.; Mazerolles, L.; Advocat, T. Preparation and Characterization of (Ba,Cs)(M,Ti)8O16 (M = Al3+, Fe3+, Ga3+, Cr3+, Sc3+, Mg2+) Hollandite Ceramics Developed for Radioactive Cesium Immobilization. J. Nucl. Mater. 2007, 366, 137−160. (12) Cheary, R. W. An Analysis of the Structural Characteristics of Hollandite Compounds. Acta Crystallogr., Sect. B: Struct. Sci. 1986, 42, 229−236. (13) Kesson, S. E.; White, T. J. BaxCsy [(Ti, Al)3+2x+y Ti4+8−2x−y] O16 Synroc-type hollandites - II. J. Solid State Chem. 1986, 63, 122− 125. (14) Xu, H.; Wu, L.; Zhu, J.; Navrostsky, A. Synthesis, characterization and thermochemistry of Cs-, Rb- and Sr-substituted barium aluminium titanate hollandites. J. Nucl. Mater. 2015, 459, 70−76. (15) Bart, F.; Rabiller, H.; Lecturcq, G.; Rigaud, D. Leaching behavior of hollandite ceramics for cesium immobilization. ATALANTE 2014, 2004, 1−4. (16) Costa, G. C. C.; Xu, H.; Navrotsky, A. Thermochemistry of Barium Hollandites. J. Am. Ceram. Soc. 2013, 96, 1554−1561. (17) Kensson, S. E. Radioactive Waste Management and the Nuclear Fuel Cycle. Nucl. Fuel Cycle. 1983, 4, 53−73. (18) Nguyen, N.; Ducouret, A.; Studer, F.; Aubin, V.; Caurant, D.; Gourier, D.; Constantini, J. M. Defects induced by electron irradiation in hollandite ceramics, specific radioactive cesium-host wasteforms: a 57 Fe Mössbauer study. Hyperfine Interact. 2005, 166, 489−493. (19) Chu, L. W.; Hsiue, G. H.; Chiang, Y. J.; Liu, K. S.; Lin, I. N. microwave dielectric materials synthesized by chemical process. J. Eur. Ceram. Soc. 2004, 24, 1781−1785. (20) Guignot, N.; Andrault, D. Equations of state of Na−K−Al host phases and implications for MORB density in the lower mantle. Phys. Earth Planet. Inter. 2004, 143−144, 107−128. (21) DeGuzman, R. N.; Shen, Y. F.; Neth, E. J.; Suib, S. L.; O’Young, C. L.; Levine, S.; Newsam, J. M. Synthesis and characterization of octahedral molecular sieves (OMS-2) having the hollandite structure. Chem. Mater. 1994, 6, 815−821. (22) Chen, X.; Suib, S. L.; Shen, Y. F.; O’Young, C. L. Characterization of Manganese Oxide Octahedral Molecular Sieve (M− OMS-2) Materials with Different Metal Cation Dopants. Chem. Mater. 2002, 14, 940−948. (23) Shlyk, L.; Niewa, R. Z. Crystal Structure and Magnetic Properties of the Novel Hollandite Ba1.3Co1.3Ti6.7O16. Z. Naturforsch., B: J. Chem. Sci. 2011, 66, 1097−1100. (24) Li, L.; King, D. L. Synthesis and Characterization of Silver Hollandite and Its Application in Emission Control. Chem. Mater. 2005, 17, 4335−4343. (25) Ahn, T.; Kim, J. H.; Yang, H.; Lee, J. W.; Kim, J. D. Formation pathways of magnetite nanoparticles by coprecipitation method. J. Phys. Chem. C 2012, 116, 6069−6076. (26) Khalil, M. L. Co-precipitation in aqueous solution synthesis of magnetite nanoparticles using iron (III) salts as precursors. Arabian J. Chem. 2015, 8, 279−284.
(27) Carter, M. L.; Gillen, A. L.; Olufson, K.; Vance, E. R. HIPed Tailored Hollandite Waste Forms for the Immobilization of Radioactive Cs and Sr. J. Am. Ceram. Soc. 2009, 92, 1112−1117. (28) Leinekegel-le-Cocq, A. Y.; Deniard, P.; Jobic, S.; Gautier, E.; Evain, M.; Aubin, V.; Bart, F. Structural characterization of the hollandite host lattice for the confinement of radioactive cesium: Quantification of the amorphous phase taking into account the incommensurate modulated character of the crystallized part. J. Solid State Chem. 2007, 180, 322−330. (29) Degen, T.; Sadki, M.; Bron, E.; Konig, U.; Nénert, G. The HighScore suite, Powder Diffraction. Powder Diffr. 2014, 29, S13− S18. (30) Aubin-Chevaldonnet, V.; Deniard, P.; Evain, M.; LeinekugelLe-Cocq-Errien, A.; Jobic, S.; Caurant, D.; Petrícek, V.; Advocat, T. Incommensurate modulations in a hollandite phase Bax(Al, Fe)2x Ti8−2xO16 intended for the storage of radioactive wastes: A (3 + 1) dimension structure determination. Z. Kristallogr. 2007, 222, 383− 390. (31) Heald, S. M.; Brewe, D. L.; Stern, E. A.; Kim, K. H.; Brown, F. C.; Jiang, D. T.; Crozier, E. D.; Gordon, R. A. XAFS and micro-XAFS at the PNC-CAT beamlines. J. Synchrotron Radiat. 1999, 6, 347−349. (32) Hu, Y. F.; Coulthard, I.; Chevrier, D.; Wright, G.; Igarashi, R.; Stinikov, A.; Yates, B. W.; Hallin, E. L.; Sham, T. K.; Reininger, R.; et al. Preliminary Commissioning and Performance of the Soft X-ray Micro-characterization Beamline at the Canadian Light Source. AIP Conf. Proc. 2009, 1234, 343. (33) Thompson, A.; Attwood, D.; Gullikson, E.; Howells, M.; Kim, K.- J.; Kirz, J.; Kortright, J.; Lindau, I.; Pianetta, P.; Robinson, A.; Scofield, J.; Underwood, J.; Vaughan, D.; Williams, G.; Winick, H. Xray Data Booklet, 3rd ed.; Lawrence Berkeley National Laboratory: Berkeley, CA, 2009. (34) Hu, Y. F.; Zuin, L.; Reininger, R.; Sham, T. K. VLS-PGM Beamline at the Canadian Light Source. AIP Conf. Proc. 2006, 879, 535−538. (35) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (36) Langford, J. I.; Wilson, A. J. C. Scherrer after sixty years: A survey and some new results in the determination of crystallite size. J. Appl. Crystallogr. 1978, 11, 102−113. (37) Zalilah, U.; Mahmoodian, R. Comparative study on microstructure, crystallite size and lattice strain of as-deposited and thermal treatment silver silicon nitride coating on Ti6Al4V alloy. IOP Conf. Ser.: Mater. Sci. Eng. 2017, 210, 012074. (38) Huynh, L. T.; Eger, S. B.; Walker, J. D. S.; Hayes, J. R.; Gaultois, M. W.; Grosvenor, A. P. How temperature influences the stoichiometry of CeTi2O6. Solid State Sci. 2012, 14, 761−767. (39) Aluri, E. R.; Grosvenor, A. P. A study of the electronic structure and structural stability of Gd2Ti2O7based glass-ceramic composites. RSC Adv. 2015, 5, 80939−80949. (40) Paknahad, E.; Grosvenor, A. P. Investigation of CeTi2O6- and CaZrTi2O7-containing glass−ceramic composite materials. Can. J. Chem. 2017, 95, 1110−1121. (41) Cabaret, D.; Joly, Y.; Renevier, H.; Natoli, C. R. Pre-edge structure analysis of Ti K-edge polarized X-ray absorption spectra in TiO2 by full-potential XANES calculations. J. Synchrotron Radiat. 1999, 6, 258. (42) Gaultois, M. W.; Grosvenor, A. P. XANES and XPS investigations of (TiO2)x(SiO2)1−x: the contribution of final-state relaxation to shifts in absorption and binding energies. J. Mater. Chem. 2011, 21, 1829−1836. (43) Farges, F.; Brown, G. E.; Rehr, J.; Farges, F.; Brown, G. E.; Rehr, J. Ti K-edge XANES studies of Ti coordination and disorder in oxide compounds: Comparison between theory and experiment. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 1809. (44) Farges, F. Fivefold-coordinated Ti4+ in metamict zirconolite and titanite: A new occurrence shown by Ti K-edge XANES spectroscopy. Am. Mineral. 1997, 82, 44. H
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (45) Joly, Y.; Cabaret, D.; Renevier, H.; Natoli, C. R. Electron Population Analysis by Full-Potential X-Ray Absorption Simulations. Phys. Rev. Lett. 1999, 82, 2398. (46) Romano, C.; Paris, E.; Poe, B. T.; Giuli, G.; Dingwell, D. B.; Mottana, A. Effect of aluminum on Ti-coordination in silicate glasses: A XANES study. Am. Mineral. 2000, 85, 108−117. (47) Bender, S.; Franke, R.; Hartmann, E.; Lansmann, M.; Hormes, J. X-ray absorption and photoemission electron spectroscopic investigation of crystalline and amorphous barium silicates. J. NonCryst. Solids 2002, 298, 99−108. (48) Gaultois, W. M.; Grosvenor, A. P. XANES and XPS investigations of the local structure and final-state effects in amorphous metal silicates: (ZrO2) x(TiO2) y(SiO2)1−x−y. Phys. Chem. Chem. Phys. 2012, 14, 205−217. (49) Westre, T. E.; Kennepohl, P.; DeWitt, J. G.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes. J. Am. Chem. Soc. 1997, 119, 6297−6314. (50) De Groot, F. High-Resolution X-ray Emission and X-ray Absorption Spectroscopy. Chem. Rev. 2001, 101, 1779−1808. (51) Grosvenor, A. P.; Ramezanipour, F.; Derakhshan, S.; Maunders, C.; Botton, G. A.; Greedan, J. E. Effects of bond character on the electronic structure of brownmillerite-phase oxides, Ca2B′xFe2−xO5 (B′ = Al, Ga): an X-ray absorption and electron energy loss spectroscopic study. J. Mater. Chem. 2009, 19, 9213−9220. (52) Walker, J. D. S.; Grosvenor, A. P. The Effect of Synthetic Method and Annealing Temperature on Metal Site Preference in Al1−xGaxFeO3. J. Solid State Chem. 2013, 197, 147−153. (53) Van Bokhoven, J. A.; Nabi, T.; Sambe, H.; Ramaker, D. E.; Koningsberger, D. C. Interpretation of Al K and L2,3 − edges of aluminum oxides: differences between tetrahedral and octahedral explained by different local symmetries. J. Phys.: Condens. Matter 2001, 13, 10247−10260. (54) Weigel, C.; Calas, G.; Cormier, L.; Galoisy, L.; Henderson, G. S. High-resolution Al L2,3-edge X-ray absorption near edge structure spectra of Al-containing crystals and glasses: Coordination number and bonding information from edge components. J. Phys.: Condens. Matter 2008, 20, 135219. (55) Momma, K.; Izumi, F. J.; Newville, M. VESTA: a threedimensional visualization system for electronic and structural analysis. J. Appl. Crystallogr. 2008, 41, 653−658.
I
DOI: 10.1021/acs.inorgchem.8b02464 Inorg. Chem. XXXX, XXX, XXX−XXX
