16664
J. Phys. Chem. C 2007, 111, 16664-16669
Variation and Influence of the Local Structure around Ti in NaAlH4 Doped with a Ti-Based Precursor Aline Le´ on,*,† Jo1 rg Rothe,‡ and Maximilian Fichtner† Institut fu¨r Nanotechnologie, and Institut fu¨r Nukleare Entsorgung, Forschungszentrum Karlsruhe GmbH, P.O. Box 3640, 76021 Karlsruhe, Germany ReceiVed: April 30, 2007; In Final Form: July 27, 2007
The kinetics and hydrogen storage capacity of Ti-doped sodium alanate prepared by ball milling of an AlxTi(1-x) nanocomposite as Ti-containing starting material with NaH remain comparably stable upon cycling under hydrogen. The reaction rate of this material is comparable to that of TiCl3-doped samples. Extended X-ray absorption fine structure (EXAFS) analysis revealed that forming an AlxTi(1-x) nanocomposite by the reduction of TiCl4 by Al and subsequent annealing stabilizes the short-range order around Ti. The presence of stable Al-Ti phases at the beginning of the reaction prevents the subsequent formation of a Ti-Al cluster upon cycling under hydrogen, which is found in the case of doping with TiCl3. When the atomic-scale behavior of this new material is compared with that of the TiCl3- or Ti colloid-doped sodium alanates, it is found that the chemical state of Ti as well as its local structure seem to be relevant for the stability of the storage capacity and desorption/absorption reaction rate upon cycling under hydrogen.
1. Introduction The development of a hydrogen storage material which fulfils all the requirements for mobile applications is a challenge. So far, the most advanced system found with respect to thermodynamics, kinetics, and storage capacity has been sodium alanate, NaAlH4, doped with a Ti- or Ce-based precursor by ball milling.1-5 However, the behavior of this material is not fully optimized and the synthesis of the material may be costly. This is partly due to a lack of knowledge concerning the mechanism underlying the reversible decomposition reaction. The understanding of the mode of action of the dopant might allow for an optimization of the catalyst and, in turn, increase the material efficiency. The hydrogen storage material is usually obtained by ball milling pure NaAlH4 with a concentration of 2-5 mol % of Ti on the basis of either TiCl3 or Ti colloid; the best kinetics properties have been obtained by selecting these two precursors and this doping procedure. By now, it seems that titanium chloride based systems have the same performance than the Ti colloid-doped sodium alanate after more than 10-15 cycles. In all cases, the decomposition of the nanocomposite takes place at moderate working pressure and temperature according to the following two-step mechanism:
3NaAlH4 / Na3AlH6 + 2Al + 3H2
(1)
Na3AlH6 / 3NaH + Al + 1.5H2
(2)
The role of titanium or other dopants in this reversible decomposition reaction is still a matter of research. X-ray absorption spectroscopy of these materials revealed some important aspects of the Ti-based precursors concerning their short-range order.6-10 The investigation of the chemical state * Corresponding author. E-mail:
[email protected]. † Institut fu ¨ r Nanotechnologie. ‡ Institut fu ¨ r Nukleare Entsorgung.
of Ti in Ti-doped sodium alanate on the basis of TiCl3 and Ti colloid suggested that the presence of metallic Ti favors the formation of small bimetallic entities between Ti and Al, which consumes a significant Al fraction that is then missing for the alanate reconstruction. X-ray absorption measurements showed that, after several cycles under hydrogen, the Ti-Al intermetallic consists of small clusters composed of Ti surrounded by about 10 Al atoms at 2.80 ( 0.02 Å and a small Ti contribution (about 1 atom) at 3.88 ( 0.02 Å, no matter what the nature of the Ti-based precursor was. The resulting local structure around Ti is not the same as in the TiAl3 bulk phase (with a local structure around Ti consisting of four Al at 2.72 Å, eight Al at 2.88 Å, and four Ti at 3.85 Å). The formation of this nanoscale Ti-Al alloy has been correlated to the decrease of the hydrogen storage capacity and desorption/absorption rates. These fundamental studies indicate that in order to gain stability in the kinetics and the reversible storage capacity, the formation of these bimetallic entities should be prevented. To increase the efficiency of the material, the titanium should therefore be bound to an element preventing the reduction to the metallic state and, thus, inhibiting its alloying with Al. A possible solution is to use one of the dehydrogenated compounds, either NaH or Al, as starting material to synthesize a new titanium-containing phase. Chemically and thermodynamically, the most favorable route is the development of an Al-Ti phase at the surface of Al particles.11 This pretreated Al is then mixed with an equimolar amount of NaH and ball-milled to form a functional nanocomposite which resembles the desorbed state of Ti-doped NaAlH4. A subsequent rehydrogenation reaction at 100 °C under 100 bar of hydrogen for a period of 20 h yields the Ti-doped sodium alanate.12 X-ray diffraction (XRD) analysis of the material as synthesized indicates the presence of Al and TiAl3 phases. However, this long-range ordered structure relaxes into a more disordered state upon ball milling and subsequent cycling, which will be shown below. The kinetics and evolution of the short-range order structure of samples obtained by this synthesis route shall be investigated
10.1021/jp073298p CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007
Local Structure around Ti in Ti-Doped NaAlH4
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TABLE 1: Samples Description labels
sample description
(TiCl3) (Ti) (TiAl3) (Al0.98Ti0.02) (NaH + Al0.98Ti0.02) bm (NaH + Al0.98Ti0.02) a5a
TiCl3 as received (99.999%, Sigma Aldrich) Ti metallic foil Ti/Al alloy as received (99.5%, Alfa Aesar) Ti-based nanocomposite as synthesized NaH mixed with Al0.98Ti0.02 and ball-milled for 2 h NaH mixed with Al0.98Ti0.02, ball-milled for 2 h, then cycled five times under hydrogen and stopped after the fifth absorption NaAlH4 + 2 mol % Ti on the basis of TiCl3 (NaH + Al + TiCl3) with a concentration of 2 mol % Ti after one absorption at T ) 100 °C under a pressure of pH2 ) 100 bar (NaH + Al0.98Ti0.02) with a concentration of 2 mol % Ti after one absorption at T ) 100 °C under a pressure of pH2 ) 100 bar
(sample A) (sample B) (sample C)
using volumetric measurements and X-ray absorption spectroscopy. The findings will be compared with the results obtained for sodium alanate doped by ball milling with 5 mol % Ti on the basis of TiCl3 or Ti13‚6THF. 2. Experimental Section Chemical operations were performed on the bench under purified N2 atmosphere using Schlenk tube techniques. Materials handling and sample preparation were done in a glovebox filled with argon and equipped with a recirculation system, where the oxygen and water concentrations were kept below 1 ppm. The AlxTi(1-x) nanocomposite was synthesized by adding 3.79 g of liquid TiCl4 (>99% purity, Merck) to a suspension of 27 g of Al powder (325 mesh, purity 99.8%, Alfa Aesar) in 200 ml of dry THF (THF ) tetrahydrofuran, analytic grade, Merck) and stirred for 24 h under a nitrogen atmosphere. In this process a partial reduction of the Ti into the trivalent state occurred:
xAl + (1 - x)TiCl4 f (x - x/3)Al + (x/3)AlCl3‚(THF)x + (1 - x)TiCl3‚(THF)x The Ti complex formed was identified by single-crystal X-ray diffractometry. The solvent was then drawn off, and the material was heattreated in an argon stream at a temperature around 500 °C for 90 min. Thus, the chlorine from the precursor was driven out as Cl2 gas which was trapped and destroyed in a sodium hydroxide solution. The final product after annealing was described as an AlxTi(1-x) nanocomposite with low chlorine content. The chlorine content was determined using the Mohr method (titration with AgNO3 solution). The chlorine content varied from 1 to 2 wt % in the different batches. The average value from eight measurements was 1.5 wt %. The storage material, Ti-doped NaAlH4, was obtained by the hydrogenation reaction of an NaH-Al0.98Ti0.02 nanocomposite at 100 °C under a hydrogen pressure of 100 bar for 24 h. The NaH-Al0.98Ti0.02 nanocomposite was prepared by ball milling a mixture of 888 mg of NaH (95% purity, Sigma Aldrich), 1034 mg of Al0.98Ti0.02, and 20 mg of Al in a silicon nitride vial containing balls made of the same material (Table 1). Milling was performed under argon atmosphere in a Fritsch P6 planetary mixer/mill at a rotation speed of 600 rpm. The ball to powder weight ratio was about 20:1. Absorption and desorption of hydrogen were carried out in a modified Sieverts apparatus. A more detailed description of the apparatus, the reactor, and the measurement procedure can be found elsewhere.13,14 Desorption was performed at 150 °C under a residual hydrogen pressure of 0.5 bar and absorption at 100 °C under a hydrogen pressure of 100 bar. X-ray absorption fine structure spectroscopy (XAFS) of the samples was performed at the Ti K edge (4966 eV). Both the
X-ray absorption near-edge structure (XANES) and the extended X-ray fine structure (EXAFS) were investigated. The measurements were carried out at the ANKA-INE beamline,15 Forschungszentrum Karlsruhe, Germany. The Ti K edge XANES spectrum of a Ti metal foil was measured for energy calibration in the transmission mode. Due to the low concentration of Ti in the doped sodium alanate samples, these spectra were taken in the fluorescence mode at room temperature, collecting Ti KR radiation (∼4510 eV) by a solid-state detector (five-element Canberra LEGe). Special sample holders were used in order to prepare, transfer, and measure the samples without exposing them to air. Up to nine scans were averaged to improve the signal-to-noise ratio. A detailed description of the background removal, normalization procedure, and EXAFS data analysis can be found elsewhere.6 Powder X-ray diffraction patterns were obtained with a Philips X’PERT diffractometer (Cu KR radiation, 2 kW, with X’Celerator RTMS detector, automatic divergence slit). The powder was measured on a Si single crystal and sealed in the glovebox by an airtight hood made of Kapton foil, the foil being mounted out of the focus of the spectrometer. The patterns were recorded at 295 K with a step size of 0.02° in a 20-80° 2θ range using a counting time of 120s per step. 3. Results and Discussion 3.1. Kinetics. Figure 1 displays the first (Figure 1a) and the second (Figure 1b) decomposition kinetics of Ti-doped NaAlH4 synthesized from the Al0.98Ti0.02 nanocomposite. For comparison, the decomposition kinetics of sodium alanate doped with 2 mol % Ti on the basis of TiCl3 directly by ball milling (sample A) and indirectly by the rehydrogenation reaction of the mixture (NaH + Al + TiCl3) (sample B) are presented. After the first desorption, the storage capacity is 4.9, 4.2, and 3.9 wt % H2 for samples (NaAlH4 + TiCl3), (NaH + Al + TiCl3), and (NaH + Al0.98Ti0.02), respectively. The sample (NaH + Al0.98Ti0.02) after one absorption (sample C) exhibits the slowest kinetics compared to samples A and B, although the reaction rate is faster at the beginning. After the second desorption, however, the storage capacity is around 4 wt % H2 in all cases. The kinetics of the first step (eq 1) is faster for sample C, whereas the reaction rate in the second step (eq 2) is almost the same for the three samples. As shown in Figure 2, cycling of sample C under hydrogen indicates that the kinetics improves with increasing number of cycles, which is not the case with the other materials. Moreover, in contrast to TiCl3-doped NaAlH4 (where the storage capacity decreases from 4.8 to 3.7 wt % within the first five cycles12), the hydrogen storage capacity remains stable at around 4 wt % of H2 over the number of cycles investigated. These results indicate that the initial state of the Ti-based precursor may be relevant to a stable storage capacity and kinetics of sodium alanate upon cycling under hydrogen. Further
16666 J. Phys. Chem. C, Vol. 111, No. 44, 2007
Figure 1. First (a) and second (b) decomposition kinetics of samples A, B, and C.
investigations of sample C were carried out to determine the evolution of the chemical state of Ti as well as the local structure around Ti. 3.2. Chemical State and Local Structure. Figure 3 displays the Ti K edge XANES spectra of the nanocomposite Al0.98Ti0.02 as prepared and of the sample (NaH + Al0.98Ti0.02) ballmilled for 2 h and subjected to subsequent five times of cycling under hydrogen and stopped in the absorbed state. For comparison, the XANES spectrum of TiAl3 is added. As can be seen, the valence state of Ti in the nanocomposite does not change during ball milling or cycling under hydrogen. The first inflection point in the XANES spectrum remains unchanged at 4967 eV, suggesting that Ti is never fully reduced to the zerovalent state in this material, in contrast to sodium alanate doped with Ti on the basis of TiCl3 or Ti colloid. Moreover, a comparison with the TiAl3 spectrum indicates that the electronic structure in the Al0.98Ti0.02 nanocomposite differs significantly from the one of bulk TiAl3. When compared with the corresponding EXAFS χ(k) functions, Figure 4, it is found that the local structure around Ti in the (NaH + Al0.98Ti0.02)-doped alanate ball-milled for 2 h and around Ti in the doped alanate after five absorption cycles are also similar. Therefore, it can be assumed that the local structure around Ti in this material persists during the hydrogenation/ dehydrogenation cycles. Comparison of the EXAFS to that obtained from bulk TiAl3, Figure 5, indicates that a significant formation of this crystalline phase can be ruled out. Moreover, the local structure of Ti in the new material is also different
Le´on et al.
Figure 2. (a) Decomposition kinetics of sample C upon cycling under hydrogen; (dx) is labeled for the xth decomposition reaction (x varies from 1 to 5). (b) Decomposition kinetics of NaAlH4 doped with 2 mol % TiCl3 upon cycling under hydrogen; (dx) is labeled for the xth decomposition reaction (x varies from 1 to 4).
from that in TiCl3- or Ti cluster-doped sodium alanate after eight absorption cycles. The EXAFS analysis carried out for Al0.98Ti0.02 as prepared, the ball-milled sample, and the sample after five times of cycling under hydrogen gives similar results. We present in the following only the representative result from the ball-milled sample. Figure 6a displays the FT magnitude of the k3-weighted EXAFS χ(k) function. The corresponding Fourier-filtered data and the k-space fit are shown in Figure 6b. The FT magnitude exhibits three resonances; the best fit is obtained with 9-10 Ti at 2.98 Å, 6 Al at 4.13 Å, and 12 Ti at 5.25 Å (cf., Table 2). The FT is indicative of the presence of well-ordered entities. It should be noted that herein the first shell is composed of Ti instead of Al as observed with TiAl3-, TiCl3-, or Ti clusterdoped sodium alanate. Moreover, this well-ordered structure involving Al and Ti atoms is already present after ball milling and does not form slowly as in the alanate activation process by adding TiCl3 or Ti13‚6THFsafter a couple of hydrogenation cycles. 3.3. Evolution on the Long- and Short-Range Order. Figure 7 displays the XRD pattern of the samples in different stages of the reaction. In the Al0.98Ti0.02 nanocomposite as synthesized diffraction peaks from Al and TiAl3 phases are present. After ball milling and after the fifth desorption, only Al and NaH phases are visible. The long-range order initially associated with at least a fraction of Ti disappears during ball milling. It is neither present in the absorbed nor in the desorbed
Local Structure around Ti in Ti-Doped NaAlH4
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16667
Figure 5. EXAFS spectra χ(k) of samples (NaH + Al0.98Ti0.02) after five desorption/absorption cycles. For comparison, EXAFS spectra of Ti, TiAl3, and sodium alanate doped with 5 mol % Ti on the basis of TiCl3 after eight absorption cycles are presented.
Figure 3. (a). Normalized Ti K edge XANES spectra of sample (NaH + Al0.98Ti0.02) ball-milled for 2 h and after five desorption/absorption cycles. For comparison, the spectra of the Al0.98Ti0.02 nanocomposite as synthesized as well as of TiAl3 are presented. The dashed line at 4966 eV corresponds to the first inflection point in the Ti K edge XANES spectrum of the pure metal. (b). Normalized Ti K edge XANES spectra of TiCl3-doped NaAlH4 samples at different stages upon cycling under hydrogen.
Figure 6. (a) FT magnitude and imaginary part of the k3-weighted EXAFS χ(k) function of (NaH + Al0.98Ti0.02) after ball milling and (b) corresponding Fourier-filtered data in k-space.
Figure 4. EXAFS spectra χ(k) of samples corresponding to those of Figure 3.
state during the first cycles under hydrogen. Diffraction peaks from Al, NaAlH4, and Na3AlH6 phases can be identified in the
pattern taken after the fifth absorption. The absence of TiAl3 in these different stages of the reaction suggests that the initial long-range ordered Ti structure, disappearing after ball milling, is not responsible for the activation of the hydrogenation process in the Ti-doped alanate. The short-range order around Ti in the Al0.98Ti0.02 nanocomposite is reflected by XAFS spectroscopy. EXAFS analysis
16668 J. Phys. Chem. C, Vol. 111, No. 44, 2007
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TABLE 2: Data Range and Metric Parameters Obtained by EXAFS Least-Square Fita sample labels
fit range R-∆ (Å)
shell
R (Å) (0.02 Å
N (1 3.5b 7.1b 3.5b
σ2 (Å2)
∆E (eV) 6.6c 6.6c 1.3
R-factor
TiAl3
1.23-3.71
Al Al Ti
2.71 2.86 3.83
TiCl3 (a8a) [8]
2.02-3.71
Al Ti
2.80 3.85
10.7 0.8
0.0057 0.0002
5.9 -2.2
0.009
(NaH + Al0.98Ti0.02) bm
2.21-5.25
Ti Al Ti
2.98 4.13 5.25
9.8 6.1 12.4
0.0063 0.0031 0.0070
2.9 2.0 3.4
0.046
0.0037 0.0063 0.0100
0.003
a S02 fixed at 0.6. b Denotes that fit parameters have been correlated according to the known crystal structure. c Denotes that fit parameters are forced to be equal for both shells.
Figure 7. XRD pattern of sample (NaH + Al0.98Ti0.02) ball-milled for 2 h (bm), after five desorption cycles (a5d), and subsequent absorption (a5a). For comparison, the spectrum of the Al0.98Ti0.02 nanocomposite as synthesized is presented.
reveals a pronounced short-range order with three neighbor shells visible at radial distances up to 5 Å. The data indicates that the local structure around Ti does not evolve upon ball milling or with subsequent cycling under hydrogen (cf., Figures 3 and 4). However, in case of sample AlxTi(1-x) as synthesized there is a discrepancy between the XRD pattern, giving evidence for the presence of Ti in a crystalline TiAl3 phase (which is no longer detectable after ball milling), and the EXAFS spectrum, pointing to a structure prevailing unchanged during all stages of the reaction. XRD indicates that the major part of Ti in AlxTi(1-x) nanocomposite is in a state lacking any long-range ordering only after ball milling and subsequent cycling under hydrogen. Therefore, in the following we will discuss the discrepancy between the XRD and the EXAFS data of the AlxTi(1-x) nanocomposite as synthesized. XRD and XANES deliver complementary results, allowing us to get the overall picture of crystalline (i.e., long-range ordered) and amorphous (i.e., invisible by XRD) phases possibly coexisting in the material. One has to keep in mind that a EXAFS spectrum is always a weighted superposition of the spectra of all individual species present in the material according to their relative abundance, regardless of the chemical state or physical structure. If the crystalline Ti phase would dominate the nanocomposite as synthesized, one would expect both the XANES and the EXAFS of AlxTi(1-x) to be similar to those obtained for the TiAl3 bulk phase. Admixtures of >5-10% Ti in a crystalline TiAl3 phase would be easily discernible from a simple fingerprint analysis of the EXAFS data. However, Figures 3 and 4 clearly indicate that this is neither the case for the XANES, reflecting a different oxidation state and electronic
(band) structure of the Ti sites in the nanocomposite compared to the bulk alloy, nor for the EXAFS, pointing to a different arrangement of the first-neighbor shells. Hence, EXAFS generally indicates a phase different from bulk TiAl3 dominating throughout the whole reaction pathway. Moreover, comparison of the intensity of the first shoulder around 4970 eV in the XANES of the nanocomposites in Figure 3a indicates the presence of very small Ti entities as forming in TiCl3-doped sodium alanate upon ball-milling (cf., ref 6). As previously observed, the particles in the new phase may be again too small to give any Bragg reflections, hence being invisible in XRD analysis. Additional XPS measurements performed on these samples indicate that there is only a small contribution from metallic Ti, while the major part is in a slightly oxidized state. According to the different steps involved in the chemical reaction during the synthesis (see the Experimental Section), one might expect that residues from the organic compounds are still associated with the AlxTi(1-x) (as in Ti13‚ 6THF6). However, this coordination seems not to result in a highly organized structure as in the Ti13 cluster, preventing the oxygen shell from being unequivocally discernible in the EXAFS signal (cf., the weak signal in the region around 2 Å (R-∆) preceding the Ti peak in Figure 6a). 4. Conclusion Structural investigations and cycling experiments indicate that the stability of the local structure around Ti is relevant to the stability of the reaction rate and hydrogen storage capacity. In the case of TiCl3- or Ti colloid-doped samples, an evolution occurs upon cycling under hydrogen in the valence state as well as in the local structure of Ti. The formation of a Ti-Al cluster which immobilizes a fraction of Al is correlated with the decrease of the hydrogen storage capacity and the reaction rate. On the basis of these structural studies, a Ti-doped sodium alanate has been synthesized by the hydrogenation of (NaH + Al0.98Ti0.02) nanocomposite. The kinetics and hydrogen storage capacity of this material do not change significantly upon five cycles under hydrogen, compared to the Ti colloid- or the TiCl3doped sodium alanate. Analysis of the local environment of this precursor in different stages of the reaction indicates that no evolution of the valence state of Ti and the local environment around Ti occurs. With a first shell of Ti instead of Al atoms around Ti, the microstructure of this nanoscale entity is different compared to the one which was found in samples produced by the standard method, i.e., adding TiCl3 or other Ti compounds and ball-milling with the sodium alanate. Hence, the stability of the short-range order upon cycling under hydrogen is obviously correlated with the stability of the kinetics and hydrogen storage capacity. Another advantage of this new material is that the synthesis of an AlxTi(1-x) nano-
Local Structure around Ti in Ti-Doped NaAlH4 composite with a low chlorine content allows us to reduce the amount of storage-inactive material like NaCl in the TiCl3-doped sodium alanate and that the kinetics of this material is comparable to that of TiCl3-doped samples. Acknowledgment. Financial support of the work by EU-IP “StorHy” (contract no. 502667) and the Helmholtz initiative “FuncHy” is gratefully acknowledged. We appreciate beamtime allotment by ANKA. The authors cordially thank Christoph Frommen for providing the AlxTi(1-x) nanocomposite and Stefan Wetterauer for his assistance in the laboratory. References and Notes (1) Bogdanovic´, B.; Brand, R.; Marjanovic´, A.; Schwickardi, M.; To¨lle, J. J. Alloys Compd. 2000, 302, 36. (2) Sandrock, G.; Gross, K. J. Alloys Compd. 2002, 339, 299. (3) Schu¨th, F.; Bogdanovic´, B.; Felderhoff, M. Chem. Commun. 2004, 20, 2249. (4) Fichtner, M. AdV. Eng. Mater. 2005, 6, 443.
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16669 (5) Bogdanovic´, B.; Felderhoff, M.; Pommerin, A.; Schu¨th, F.; Spielkamp, N. AdV. Mater. 2006, 18 (9), 1198. (6) Le´on, A.; Kircher, O.; Rothe, J.; Fichtner, M. J. Phys. Chem. B 2004, 108, 16372. (7) Graetz, J.; Ignatov, A.; Tyson, T.; Reilly, J.; Johnson, J. Appl. Phys. Lett. 2004, 85, 500. (8) Felderhoff, M.; Klementiev, K.; Gru¨nert, G.; Spliethoff, B.; Tesche, B.; Bellosta von Colbe, J. M.; Bogdanovi, B.; Ha¨rtel, M.; Pommerin, A.; Schu¨th, F.; Weidenthaler, C. Phys. Chem. Chem. Phys. 2004, 6, 4369. (9) Graetz, J.; Ignatov, A.; Tyson, T.; Reilly, J.; Johnson, J. Mater. Res. Soc. Symp. Proc. 2005, 837. (10) Le´on, A.; Kircher, O.; Fichtner, M.; Rothe, J.; Schild, D. J. Phys. Chem. B 2006, 110, 1192. (11) Fichtner, M.; Frommen, C. German Patent DE 10 2005 037 772 B 3. (12) Le´on, A.; Frommen, C.; Rothe, J.; Schild, D.; Fichtner, M. Proceedings World Hydrogen Energy Conference, Lyon, France, 2006. (13) Fichtner, M.; Fuhr, O.; Kircher, O.; Rothe, J. Nanotechnology 2003, 14, 778. (14) Kircher, O.; Fichtner, M. J. Appl. Phys. 2004, 95, 7748. (15) Rothe, J.; Denecke, M. A.; Dardenne, K.; Fangha¨nel, Th. Radiochim. Acta 2006, 94, 691.
