ISSN 1063-7834, Physics of the Solid State, 2006, Vol. 48, No. 1, pp. 30–36. © Pleiades Publishing, Inc., 2006. Original Russian Text © S.N. Nemnonov, V.M. Cherkashenko, V.N. Kozhanov, A.V. Skripov, É. Z. Kurmaev, E. P. Romanov, 2006, published in Fizika Tverdogo Tela, 2006, Vol. 48, No. 1, pp. 30–35. 30 † 1. INTRODUCTION It is known that Hf and Ti form only disordered solid solutions with a hexagonal close-packed lattice. Recently, the problem of order–disorder phase transi- tions in interstitial systems has aroused considerable interest. The possibility of these transitions occurring follows directly from the fact that the number of inter- stices in interstitial solid solutions, as a rule, exceeds the number of intercalated atoms. It is reasonable to expect that intercalated atoms are randomly distributed over interstitial positions at high temperatures and can “crystallize” in the crystal field of host atoms with decreasing temperature. It is characteristic that this crystallization, which is accompanied by an ordered arrangement of intercalated atoms in interstitial posi- tions, should be related to single-type interactions, namely, interactions between intercalated atoms, whereas the type of occupied interstices (octahedron, tetrahedron) is governed by the interactions with host atoms. The absorption of hydrogen in alloys with hydrogen-induced ordering of the host lattice is of par- ticular interest [1–3]. The mechanism of hydrogen- induced ordering in different alloys is still not clearly understood. In our recent work [4], we revealed a new hydrogen-stabilized phase with an ordered lattice in the HfTi 2 –H system. In [4], the structure and phase compo- sition of the samples formed upon interaction of the HfTi 2 alloy with hydrogen were investigated using x- ray diffraction. Among the HfTi 2 H x phases at different hydrogen contents, we identified the cubic phase with an ordered host lattice of the C15 type (the Laves phase). A similar hydrogen-stabilized phase of the C15 † Deceased. type was previously revealed in the ZrTi 2 –H(D) system [1, 5]. Previous investigations of similar systems have been concerned for the most part with phase transitions, phase compositions, structural parameters, and the influence of the hydrogen concentration on the order parameter of the lattice [1, 4, 5]. There are only a few works in which the positions occupied by hydrogen are determined using x-ray diffraction analysis. First and foremost, the reason for this is that the majority of inter- stitial solid solutions are systems composed of atoms whose masses differ significantly. In this situation, determination of the location of easier atoms by x-ray diffraction analysis is complicated because the scatter- ing power of these atoms is less than that of the host atoms. The electron energy spectra of typical Laves phases and their hydrides have been studied using elec- tronic spectroscopy and x-ray spectral analysis. How- ever, the features observed in the spectra of hydrides, as a rule, have not been analyzed with due regard for the type of crystallographic positions occupied by hydro- gen, even though this is one of the most important char- acteristics of hydrides [6–9]. Most likely, only neutron diffraction methods can provide direct information on the location of interstitial atoms. In the HfTi 2 H(D) x , ZrTi 2 H(D) x , and ZrV 2 H(D) x phases stabilized by hydrogen, the occupancy changes with variations in the temperature and, possibly, in the hydrogen content [1, 4, 6, 10–12]. In a previous study [13], we obtained sufficiently reliable data on the loca- tion of hydrogen atoms when analyzing the x-ray emis- sion spectra of Ti 3 SbH x hydrides. In our present work, we developed the method proposed in [13] and attempted to determine not only the location of hydro- gen atoms but also the hydrogen occupancies of differ- X-ray Spectroscopic Study of the Electronic Structure and Location of Hydrogen Atoms in HfTi 2 H x Hydrides S. N. Nemnonov, V. M. Cherkashenko, V. N. Kozhanov † , A. V. Skripov, É. Z. Kurmaev, and E. P. Romanov Institute of Metal Physics, Ural Division, Russian Academy of Sciences, ul. S. Kovalevskoœ 18, Yekaterinburg, 620219 Russia e-mail: nemnonov@ifmlrs.uran.ru Received December 7, 2004; in final form, April 25, 2005 Abstract—The Ti Kβ 5 spectra and the electronic structure of HfTi 2 H x hydrides (x = 0, 0.5, 3.0, 4.0, 4.5, 5.95) are investigated. The location of hydrogen atoms in the structure of the HfTi 2 H x hydrides is determined, and the hydrogen occupancies of the positions e and g are calculated. The inference is made that the hydrogen atoms occupy only the e positions at a low hydrogen content up to x = 3.0 (this is confirmed by the experimental crys- tallographic data) and the positions e and g at a higher hydrogen content. PACS numbers: 61.10.Nz, 61.66.Fn DOI: 10.1134/S1063783406010070 METALS AND SUPERCONDUCTORS