Hydriding and dehydriding behavior of lanthanum silicides

Hydriding and dehydriding behavior of lanthanum silicides

Solid State Ionics 141–142 Ž2001. 599–602 www.elsevier.comrlocaterssi Hydriding and dehydriding behavior of lanthanum silicides K. Bohmhammel, E. Hen...

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Solid State Ionics 141–142 Ž2001. 599–602 www.elsevier.comrlocaterssi

Hydriding and dehydriding behavior of lanthanum silicides K. Bohmhammel, E. Henneberg ) Department of Physical Chemistry, Freiberg UniÕersity of Mining and Technology, Leipziger Strasse 29, D-09596 Freiberg, Germany

Abstract The reactivity of hydrogen towards the lanthanum silicides, La 5 Si 3 , LaSi and LaSi 2 , has been studied by gravimetric and differential thermoanalytical methods. Synthesized silicides and hydrogenated phases have been characterized by X-ray diffraction. The hydrogen interaction is attributed to the lanthanum content of the silicide phase. Whereas LaSi 2 does not react with hydrogen over a wide pressure–temperature range, La 5 Si 3 takes up hydrogen even at 1008C. Hydrogenation of La 5 Si 3 leads from a metastable hydride with about one hydrogen atom per lanthanum atom to the decomposition into lanthanum hydride, LaH 2qx , and another phase without X-ray pattern. On the contrary, hydrogen release is not complete at temperatures up to 9008C. LaSi reacts reversibly with hydrogen under formation of a ternary hydride phase. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Lanthanum; Silicon; Hydrides

1. Introduction The synthesis of silicides is ordinarily realized by high temperature reactions of a metal with silicon. In this way, thermodynamically stable phases are formed. Alternatively, addition of halogen andror hydrogen to the binary metal–silicon system offers new ways of silicide-synthesis at lower temperatures w1x. In respective ternary or quaternary systems, the behavior of hydrogen is not well-understood. Strong metal hydride formers Že.g. rare-earth elements. are expected to enhance the hydrogen affinity of their silicides considerably. Assumed reactions during the hydrogenation of an intermetallic compound are the formation of a ternary

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Corresponding author. Tel.: q49-3731-392538; fax: q493731-393588. E-mail address: [email protected] ŽE. Henneberg..

hydride, the decomposition into another Žstable. intermetallic phase and a metal hydride, the formation of a free metal and a metal hydride, or both components would form metal hydrides. In analogy to the system, La–Mg–H, where a reversible reaction of the intermetallic phase with hydrogen leads to the hydrides of Mg and La w2x, thermodynamic approach predicts a reversible reaction according to LaSi y q xr2 H 2 l La H x q y Si.

Ž 1.

This reaction may provide the synthesis of the intermetallic compound on the one side, and the formation of nanocrystalline substances on the other side at moderate temperatures in each case. The present work is an attempt to study the forming conditions of stable and metastable phases during introduction of hydrogen to the system lanthanum– silicon. Whereas the binary systems of rare-earth metals and hydrogen are well investigated there are only a few examinations about the system rare-earth

0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 7 4 - 3

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K. Bohmhammel, E. Hennebergr Solid State Ionics 141–142 (2001) 599–602

metal–silicon–hydrogen. Bruer et al. w3x investigated the hydrogenation of rare-earth alloys with silicon, germanium and tin. Observing that Ln 5 Si 3 ŽLn s Y, La, Nd, Ho. shows a hydrogen absorption behavior similar to its parent metal.

2. Experimental details The used raw materials were lanthanum lumps and silicon powder, both with purities better than 99.9% ŽAlfa Aesar.. The lanthanum lumps were cut into small pieces in a glove box. The silicides, LaSi 2 , La 5 Si 3 and LaSi w4x, were prepared by arc melting under an argon atmosphere in a water-cooled copper tray. For the preparation of the silicides from a mixture of lanthanum hydride ŽLaH x with 2 F x F 3. and silicon, pressed pellets were heated in a quartz tube under high vacuum Ž p - 10y7 bar.. The used lanthanum hydride was synthesized in an autoclave at 1008C and 30-bar hydrogen over 24 h. For both preparation methods, the lanthanum silicide preparation was followed by annealing of tantalum foilcovered samples in sealed quartz ampoules under argon atmosphere at temperatures between 7008C and 9008C. The samples were crushed and ground in a glove box. To investigate the reactivity of the silicides towards hydrogen, we applied a high-pressure symmetric microbalance Žmodel Sartorius S3DP. at temperatures between 258C and 5508C and hydrogen pressures up to 30 bar. The thermogravimetric experiments were accomplished by differential thermoanalysis ŽDTA, Setaram. under hydrogen or argon atmosphere and by differential scanning calorimetry Žhigh pressure DSC, Setaram.. The prepared compounds and hydridingrdehydriding products were analyzed by X-ray powder diffraction ŽXRD. using a Siemens D5000. Because all used substances show a high affinity against oxygen and moisture, air contact was avoided consequently.

tion were detected in a linear gravimetric temperature scan and a heating rate of 5 Krmin. The onset temperatures of these steps show strong pressure dependence. Fig. 1 shows the weight increase during hydrogenation expressed by the atomic ratio of HrLa and the appropriate differential gravimetric curve at a pressure of 13 bar. The first uptake of about one hydrogen atom per lanthanum atom is the largest. The X-ray powder diffraction analysis of the quenched intermediate product shows that the hydride phase retained the structure of their parent silicide, but with anisotropic distortion of the lattice. After the second step at about 3008C, when the HrLa-ratio approaches 1.4, the XRD-measurement shows almost no signals. With increasing temperature, a hydrogen uptake up to an HrLa-ratio of 1.6 was obtained, and the appropriate XRD-pattern shows widened reflexes of lanthanum hydride. No other phase could be detected by X-ray diffraction. In agreement with the pattern of the gravimetric temperature scan, the DTA-curve of La 5 Si 3 under hydrogen flow shows three exothermic processes owing to the lower pressure of 1 bar hydrogen at higher temperatures Žsee Fig. 4.. Isotherm gravimetric absorption curves at a constant pressure Žshown in Fig. 2., at 3008C, 3508C and 4008C, indicate the existence of two kinetically determined reaction steps. At lower temperatures, a first, very fast process with a final HrLa-ratio of 1

3. Results 3.1. ReactiÕity of hydrogen towards La5 Si 3 Hydrogen strongly reacts with La 5 Si 3 at a temperature of 1008C. Three steps of hydrogen absorp-

Fig. 1. Gravimetric linear temperatures scan of La 5 Si 3 at 13-bar hydrogen.

K. Bohmhammel, E. Hennebergr Solid State Ionics 141–142 (2001) 599–602

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Fig. 2. Isothermal hydrogenation of La 5 Si 3 at a pressure of 3 bar in a microbalance, HrLa atomic ratio vs. reaction time plot.

Fig. 3. X-ray powder diffraction patterns of La 5 Si 3 , hydrogenated at various temperatures.

can be obtained, followed by a slow process. The reaction rate of this second step has a strong temperature dependence and cannot be separated from the first step above 3508C. A kinetic interpretation of these curves is not possible because this reaction is heat-controlled. Isotherm measurements at 1008C and 2008C show only the first, very fast reaction step. The second step is very slow, and results in a steady increase of weight over a few days. From Fig. 3, it can be seen that with increasing temperature of hydrogenation, the intensity of the X-ray lines of the lanthanum hydride increase. The pattern at 3008C shows traces of the silicide. The degree of the amorphisation during the hydrogenation decreases with increasing temperature and does not depend primarily on the hydrogen content. Desorption measurements were performed under argon flow by differential thermoanalysis ŽFig. 4.. It was established that only a minimal hydrogen release occurred around 3008C. This effect is clearly related to desorption of H from LaH 2q x Ž x F 1.. A few endothermic effects were detected at temperatures between 6508C and 8508C. Complete desorption could not be reached at temperatures up to 9008C. Attempts to prepare La 5 Si 3 by heating a mixture of LaH x and Si Žsee Eq. Ž1.., require very high temperatures over 9008C and a high vacuum.

3.2. ReactiÕity of hydrogen towards LaSi A gravimetric linear temperature scan of LaSi at 12-bar hydrogen pressure is shown in Fig. 5. It

Fig. 4. Comparison of the hydrogen absorption and desorption DTA curves of La 5 Si 3 .

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K. Bohmhammel, E. Hennebergr Solid State Ionics 141–142 (2001) 599–602

LaSi 2 , the most silicon-rich lanthanum silicide, does not react with hydrogen over a wide pressure– temperature range. But the inverse reaction, the formation of LaSi 2 from lanthanum hydride and silicon, gives pure LaSi 2 with a high degree of crystallinity.

4. Discussion

Fig. 5. Gravimetric linear temperatures scan of LaSi at 12-bar hydrogen.

shows a small initial hydrogen uptake of around 2808C, and a large weight increase at 4258C with a final HrLa ratio of about 1, followed by a small shoulder at 5008C. The first small absorption effect of up to 0.15 HrLa cannot be observed in the DSC curve under similar conditions. This indicates a solubility of hydrogen in the lattice Žformation of an a-phase. before the hydride phase is formed. The XRD pattern of hydrogenated LaSi shows an expanded lattice structure of the parent, high temperature LaSi phase ŽhT-LaSi.. The DTA desorption curve under argon, up to 9008C, presents a broad endothermic effect at about 4758C, and it has the same peak area of the summarized absorption signals. This result leads to the conclusion that the hydrogenation of LaSi is a reversible process. LaSi was synthesized from a mixture of lanthanum hydride and silicon Žsee Eq. Ž1.. by heating in a dynamic high vacuum up to 8508C. The product obtained consists of about 50% high temperature LaSi ŽhT-LaSi. w4x and 50% low temperature LaSi ŽlT-LaSi. after annealing, as estimated from the X-ray intensities. The lT-LaSi is stable below 11008C and was recently presented by Mattausch et al. w5x. The X-ray powder diffraction pattern of the dehydrogenated phase does not show the lines of initial hT-LaSi. A change of the lattice structure from hT-LaSi to lT-LaSi, which is the stable phase under those low temperature conditions, is assumed and is the subject of current investigations.

The reactivity of lanthanum silicides towards hydrogen depends strongly on their lanthanum content. La 5 Si 3 , the most lanthanum-rich compound shows a very strong affinity to hydrogen, already at temperatures over 1008C. The hydrogenation leads from an intermediate metastable ternary hydride to a decomposition into lanthanum hydride and a siliconcontaining phase. This phase is assumed to be amorphous, because there is no indication of their existence in the X-ray powder diffraction pattern. The hydrogenation of the intervening LaSi is reversible. A stable ternary hydride is formed. On the other hand, the most silicon-rich compound, LaSi 2 , does not react with hydrogen under the investigated temperature–pressure conditions. The formation of La 5 Si 3 from lanthanum hydride and silicon requires very high temperatures. In the case of LaSi, the reaction of lanthanum hydride with silicon partially leads to the low temperature modification of LaSi, which is stable under these conditions. The formation of LaSi 2 proceeds without restriction. These results are transferable to the hydridingr dehydriding behavior of intermetallic phases in other systems.

References w1x J. Acker, PhD Thesis, University Freiberg, 1999. w2x B. Christ, PhD Thesis, University Freiberg, 1998. w3x B.A. Bruer, N.J. Clark, I.J. McColm, J. Less-Common Met. 110 Ž1985. 131–137. w4x T.B. Massalski, L. Bennett, L.H. Murray ŽEds.., 3rd edn., Binary Alloy Phase Diagram, vol. 3, ASM International, Metals Park, OH, 1986, 2424. w5x H. Mattausch, O. Oeckler, A. Simon, Z. Anorg. Allg. Chem. 625 Ž1999. 1151–1154.