J. Phys. Chem. C 2010, 114, 13153
Comment on “Structural and Electronic Properties of the Hydrogenated ZrCr2 Laves Phases” H. Kohlmann* UniVersita¨t des Saarlandes, Fachrichtung 8.1 - Anorganische Festko¨rperchemie, Am Markt, Zeile 3, 66125 Saarbru¨cken, Germany ReceiVed: May 7, 2010; ReVised Manuscript ReceiVed: May 27, 2010 Recently, interesting structural and electronic properties of Laves phase hydrides ZrCr2H0.5 were predicted by means of theoretical calculations.1 The findings, for example, the atomlike nature of hydrogen in these hydrides, the site preference in the crystal structures, and the H-H repulsion, are important and mostly in accordance with experimental results. The comparison between theory and experiment, however, is almost completely lacking in that publication, making it difficult for readers to judge the impact of the study and its relevance to real materials. This Comment’s aim is to fill this gap and to add some remarks on the results. The investigation starts with treating the hydrogen-free intermetallics. Calculated lattice parameters of hexagonal ZrCr2 are given as a ) 509 pm and c/a ) 1.598, that is, c ) 813 pm (Table 1 in ref 1), in reasonable agreement with literature values (a ) 510.6(1) pm,2 509 pm,3 509.23 pm;4 c ) 827.5(2) pm,2 827 pm,3 828.81 pm4). These lattice parameters were apparently used to calculate the DOS and COOP of the corresponding hydrides (Figures 2-5 in ref 1), neglecting the considerable volume effect of hydrogen intercalation (a ) 515 pm, c ) 842 pm in hexagonal ZrCr2H0.53). The resulting volume error of 6% in the calculations gives rise to an overestimation of H-H repulsion effects predicted in ref 1, which are expected to trigger an order-disorder phase transition. This, however, is only known for the deuteride with a much higher deuterium content, hexagonal ZrCr2D3.8.5 Such low temperature ordering of hydrogen atoms, manifesting itself as a structural phase transition, is also common in cubic Laves phase hydrides, including cubic ZrCr2H0.2 and cubic ZrCr2H0.45.3 Investigation of the cubic deuteride with similar composition R-ZrCr2D0.66 revealed a phase transition with deuterium ordering to a monoclinic structure driven by D-D repulsion.6 Calculations done in ref 1, presumably assuming a temperature of 0 K, are based on a cubic instead of on the experimentally verified monoclinic model6 and are performed with the same volume of the unit cell as in the hydrogen-free compound. According to the authors, the difference between the two is very small,1 which is, however, not in good agreement with the experiment.3,6 Despite these simplifications, the calculations yield some interesting results, for example, the preference of hydrogen for tetrahedral voids surrounded by two zirconium and two chromium atoms, [Zr2Cr2] (Tables 7 and 8 in ref 1). This is in agreement with experimental structural data on all hydrides ZrCr2Hx investigated so far,3-7 thus backing the theoretical calculations. * Corresponding author. E-mail:
[email protected].
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The authors also tackle the problem of the chemical nature within the hydride ZrCr2H0.5 and find it to be atom-like rather than ionic.1 It should be emphasized here that in many such hydrides hydrogen is found to be atom-like with a slight tendency to carry a negative charge on the order of one or two tenths in many interstitial type hydrides, such as Laves phase hydrides,8,9 hydrides of palladium rich intermetallics,10 and transition-metal hydrides.11 The authors made a very interesting point on the importance of theoretical calculations for the understanding of possible reaction pathways of the hydrogenation reactions including hydrogen diffusion.1 Whereas this is strongly agreed upon, again a comparison with experimental data is missing. By temperaturedependent NMR measurements, two diffusion processes could be found in hexagonal ZrCr2H0.5 and cubic ZrCr2H0.45: a fast localized hydrogen motion between [Zr2Cr2] sites, thus forming a hexagon of diffusion pathways, and a slower hopping from one such hexagon to a neighboring one.12 The vast knowledge on diffusion processes in Laves phase hydrides13 does not back the speculation given in ref 1 that the formation of H2 plays a role in hydrogen diffusion. All in all, the authors of ref 1 have extracted useful information from their calculations on ZrCr2 hydrides, by and large confirming what has been previously known for other Laves phase hydrides: (1) Hydrogen does not form H2 molecules but is rather atom-like. (2) Hydrogen prefers [Zr2Cr2] voids over the others. (3) There is a strong H-H repulsion in the case of occupation of neighboring tetrahedral sites. I hope that the additional information given in this Comment, especially about the available literature on experimental results, will help the interested reader to better judge the subject of structural and electronic properties of these ZrCr2 hydrides and the achievements of both the theoretical and the experimental studies. References and Notes (1) van Midden, H. J. P.; Prodan, A.; Zupani, E.; Zitko, R.; Makridis, S. S.; Stubos, A. K. J. Phys. Chem. C 2010, 114, 4221–4227. (2) Kohlmann, H.; Fauth, F.; Yvon, K. J. Alloys Compd. 1999, 285, 204–211. (3) Skripov, A. V.; Natter, H.; Hempelmann, R. Solid State Commun. 2001, 120, 265–268. (4) Dorogova, M.; Hirata, T.; Filipek, S. M.; Bala, H. J. Phys.: Condens. Matter 2002, 14, 11151–11156. (5) Kohlmann, H.; Yvon, K. J. Alloys Compd. 2000, 309, 123–126. (6) Kohlmann, H.; Fauth, F.; Fischer, P.; Skripov, A. V.; Yvon, K. J. Alloys Compd. 2001, 327, L4–L9. (7) Irodova, A. V.; Suard, E. J. Alloys Compd. 2000, 299, 32–28. (8) Hong, S.; Fu, C. L. Phys. ReV. B: Condens. Matter Mater. Phys. 2002, 66, 094109. (9) Huang, R. Z.; Wang, Y. M.; Wang, J. Y.; Zhou, Y. C. Acta Mater. 2004, 52, 3499–3506. (10) Kohlmann, H.; Kurtzemann, N.; Weihrich, R.; Hansen, T. Z. Anorg. Allg. Chem. 2009, 635, 2399–2405. (11) Smithson, H.; Marianetti, C. A.; Morgan, D.; Van der Ven, A.; Predite, A.; Ceder, G. Phys. ReV. B: Condens. Matter Mater. Phys. 2002, 66, 144107. (12) Skripov, A. V.; Pionke, M.; Randl, O.; Hempelmann, R. J. Phys.: Condens. Matter 1999, 11, 1498–1502. (13) Hempelmann, R.; Skripov, A. Hydrogen Motion in Metals. In Hydrogen Transfer Reactions; Hynes, J. T., Klinman, J. P., Limbach, H. H., Schowen, R. L., Eds.;. Wiley-VCH: Weinheim, Germany, 2007; pp 787-829.
JP104156G
10.1021/jp104156g 2010 American Chemical Society Published on Web 07/07/2010
