I Koval'chuck, A Riabov, R Denys - Synthesis of hydrides of к-phases and their crystal structure - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ
Серія хім. 2005. Bun. 46. С. 90-95
VISNYK LVIV UNIV. Ser. Khim. 2005. No 46. P. 90-95
УДК 548.736.4; 546.3-19'11
SYNTHESIS OF HYDRIDES OF к-PHASES AND THEIR CRYSTAL STRUCTURE
І. Koval'chuck, A. Riabov, R. Denys, І. Zavaliy
Physico-Mechanical Institute of NAS of Ukraine, Naukova Str. 5, 79601 Lviv, Ukraine
The saturated hydrides of the Zr- and Hf-based к-phases were synthesized and the dependence of their H-capacity on oxygen and other inserted atoms content has been shown. Lattice parameters of parent compounds and their hydrides have been determined from X-ray powder diffraction data. The crystal structure of the Zr9Mo4NiO parent compound as well as of the Zr9Mo4NiOH18 saturated hydride has been determined and discussed in comparison with that for the Zr9V4SH23 deuteride. The redistribution of O-atoms has been found for the Zr9Mo4NiOx compounds during the hydrogenation.
Key words: Zirconium compounds, Hafnium compound, Nickel compounds, Metal hydrides, Crystal structure, powder X-ray diffraction analysis.
We have studied intensively hydride formation in oxygen-stabilised intermetallic compounds (IMC) based on Zr(Ti, Hf) with structure types of Ti2Ni and Re3B, efficient hydrogen absorbers [1, 2]. A new object of our studies is a large group of high-temperature IMC with Hf9Mo4B-type structure (so called к-phases), which are formed in systems of d-metals in the presence of p-elements (B, P, S, etc.). It is composed of a metal atom sublat-tice of the Mn3Al10-type, where the triangular-prismatic and/or octahedral interstices are filled with p-elements. In other cases trigonal prisms can be filled by Ni atom. In such Ni-containing к-phases octahedral interstices can accommodate light interstitials like oxygen . In the Zr-based systems к-phases can dissolve sulphur up to A9B4S4 stoichiometry (Zr9V4S4, Zr9Nb4S4, etc.) . This means that the number of inserted non-metallic atoms can vary in a wide range. The occupation of both octahedral and trigonal-prismatic interstices by the p-element leads to different hydrogen absorption-desorption properties and to the different hydrogenation capacity. In this work we present new experimental results on the formation of hydrides of Zr9Mo4NiOx (x=0-3), Hf9Mo4Ge к-phases as well as results of crystal structure determination of the corresponding hydrides by X-ray powder diffraction and compare these results with our previous results concerning hydrogenation of к-Zr^S [5,6].
A number of к-phases, namely Zr9Mo4NiOx (x=0-3), Hf9Mo4Ge, and their hydrides have been synthesized in this work. Intermetallic compounds were prepared by arc melting of constituent elements followed by high temperature annealing in evacuated quartz ampoules (1200°C, 5 h) and quenching into iced water. Hydrides have been synthesized at room temperature under 0.1-0.12 MPa hydrogen pressure. Reached hydro-genation capacities were determined volumetrically. All parent compounds and hydrides were characterised by XRD (DRON-3.0, Cu-Ka). The obtained XRD data were refined with the use of FullProf program .
© Koval'chuk I., Riabov A., Denys R. et al., 2005
SYNTHESIS OF HYDRIDES OF к-PHASES ...
The hydrogenation of the Zr9Mo4NiOx (x=0-3) and Hf9Mo4Ge compounds has not been studied so far, however, they could be interesting as hydrogen storage materials since they contain substantial amounts of hydride forming metal (Zr or Hf). Crystallography parameters of parent Zr9Mo4NiOx compounds and their hy-drides are provided in Table 1.
Crystallography parameters of studied к-phases and their hydrides
As can be seen from the table, hydrogenation capacity of all studied in this work compounds is lower than that of the phase, for which the capacity (1.77 Н/М)
was even higher than that of other earlier studied Zr-based intermetallic compounds (Zr2Fe-1.66, Zr3V3O06-1.6, ZrV2-1.63). In the row of Zr9Mo4NiOx alloys, which are simultaneously stabilized by metallic (nickel) and non-metallic (oxygen) atoms, the rise in the amount of inserted oxygen leads to contraction of the unit cell from 566.3 A3 for Zr9Mo4Ni to 556.6 A3 for Zr9Mo4NiO3. As could be expected accounting blocking of H site by insertion of oxygen atoms, Zr9Mo4NiO3 and Zr9Mo4NiO2 compounds reveal lowest hy-drogenation capacity and smallest unit cell expansion. However, hydrogenation capacity of Zr9Mo4NiO appears to be higher than that of the Zr9Mo4Ni compound, although unit cell increment per hydrogen atoms is larger in the latter case. Rietveld plots and results of Riet-veld refinement of XRD data for the Zr9Mo4NiO and its hydride are provided below.
Structural data of Zr9Mo4NiO refined from XRD data (P63/mmc, a = 8.7219(8) A, c = 8.5571(9) A, Rwp = 0.069, x2 = 2.78)
Koval'chuk I., Riabov A., Denys R. et al.
Fig. 1. The observed (+), calculated (—) and differential (bottom) X-ray diffraction patterns of compound Zr9Mo4NiO
Fig. 2. The observed (+), calculated (—) and differential (bottom) X-ray diffraction patterns of hydride Zr9Mo4NiOH18
Structural data of Zr9Mo4NiOH18 refined from XRD data (P63/mmc, a = 9.0588(4) A, c = 9.2592(1) A, Rwp = 0.049, x2 = 3.89)
As can be seen from the refinement results, hydrogenation of these alloy leads to the partial redistribution of oxygen atoms between two Zr6 octahedra (6(g) and 4(f) sites). In the parent alloy all O atoms are located in the 6g octahedra, whereas in hydride O atoms evenly occupy both types of octahedra. Such redistribution is similar to that observed in the case of the Zr3V3O06D96, Zr3V3(B,O)xDy and Zr3NiOxDy deuterides (comparatively to the corresponding parent compounds) [1,2,8].
Fragment of the structure of the Zr9Mo4NiOx compounds in shown on the Figure. In the IMC compound O atoms are situated in the B octahedra only, whereas in the hydride they occupy the C octahedra as well.
Fig. 3. Fragment of the structure of oxygen-doped Zr9Mo4NiO к-phase (Hf9Mo4B-type). Chains of tetrahedra made of Mo atoms, as well as polyhedra - Ni-centred Zr16 trigonal prisms, O2-centred Zr14Zr22 (B) and O1-centred Zr13Zr23 octahedra, are shown
In our previous work  we have analysed the possible structure of hydrogen sublat-tice in the Zr9V4SH23 deuteride, which revealed that the assumption of insertion of hydrogen into tetrahedral sites with Zr and V in surrounding allow, after accounting the limitation applied by the "2A rule", positioning of only 18 hydrogen atoms per formula unit.
Further analysis, with accounting structural similarity Zr9V4S compound to the Zr containing compounds with Ti2Ni type of structure  (the presence of a spatial framework created by Zr(Hf)6 octahedra, we assumed that extra positions for accommodation of hydrogen could be triangular faces between neighbouring Zr6 octahedra, which gave surplus of 6 H/f.u., thus giving well agreement with experimentally observed hydrogenation capacity. Later this assumption had been perfectly confirmed by neutron diffraction studies of the Zr9V4SD23 deuteride . However, in the case of Zr9Mo4NiO, filling of octahedral 6g and 4f sites by O atoms can block hydrogen positioning in the triangular faces, thus perhaps limiting the maximum capacity by the value of 18 H/f.u., which coincides with the volu-metrically obtained one. Further structural studies with the use of neutron diffraction are needed to verify these suggestions.
Hydrides of the Zr9Mo4NiO0-3 and Hf9Mo4Ge, synthesised for the first time, reveal lower hydrogenation capacity than the Zr9V4S к-phase, studied before. For the studied in this work hydrogenation of the K-Zr9Mo4NiO0-3 intermetallics small amounts inserted oxygen (up to 1 O/f.u.) leads to the 20% rise in the hydrogenation capacity compared to oxygen-less compound with sharp drop in capacity with futher increase of O. Comparison of capacities of these compounds with the structural data for the Zr9V4SD23 allowed to suggest that the presence of O atoms in octahedral interstices blocks filling of Zr3 triangles by hydrogen atoms, thus decreasing maximum capacity by 25%. Neutron diffraction studies of these new hydrides are in progress.
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2. Zavaliy I.Yu., Cerny R., Koval'chuck I.V., Saldan I.V. Hydrogenation of oxygen-stabilized Zr3NiOx phase with filled Re3B-type of structure // J. Alloys Comp. 2003. Vol. 360. P. 173-182.
3. Mackay R., Franzen H.F. New Zirconium Kappa Phases // Z. Anorg. Allg. Chem.
1992. Vol. 616. P. 154-156.
4. Marking G.A., Young V.G., Franzen H.F. New group V к-phases // J. Alloys Comp. 1996. Vol. 241. P. 98-111.
5. Zavaliy I.Yu., Riabov A.B., Cerny R., Denys R.V., Koval'chuck I.V. The crystal struture analysis for prediction of H-sublattice in new K-Zr9V4SH235 hydride // Proc. VIII Int. Conf. on Crystal Chemistry of Intermetallic Compounds, Lviv, Ukraine, September 25-28, 2002. P. 161.
I 6. Zavaliy I.Yu., Koval'chuck I.V., Cerny R., Riabov A.B. Hydrogenation an crystal struc--* ture of k-Zr9V4SD23 // J. Alloys Comp. in press. 7. Rodrisuez-Carvaial J. (2002). Program FullProf.2k, Version 2.20, Laboratoire Leon
Brillouin (CEA-CNRS), France. Yartys V.A., Riabov A.B., Hauback B.C. Neutron diffraction studies of Zr-containing intermetallic hydrides. Cubic Zr3V3B0.24O0.36D8.0 and Zr3V3B0.40O0.60D6.4 with filled /-type structures // J. Alloys Comp. 2001. Vol. 317-318. P. 92-97.
СИНТЕЗ ГІДРИДІВ к-ФАЗ ТА ЇХ КРИСТАЛІЧНА СТРУКТУРА
I. Ковальчук, О. Рябов, Р. Денис, І. Завалій
Фізико-механічний інститут НАН України, вул. Наукова, 5, 79601 Львів, Україна
Синтезовано насичені гідриди к-фаз на основі Цирконію та Гафнію, показано залежність воденьсорбційної ємності цього типу сполук від кількості кисню та інших легких атомів втілення. За даними порошкової рентгенівської дифракції визначено періоди Ґратки вихідних фаз та їх гідридів. Проведено уточнення кристалічної структури для вихідної сполуки Zr9Mo4NiOx та її насиченого гідриду Zr9Mo4NiOH18, обговорено одержані результати порівняно до структури дейтериду Zr9V4SD23. Виявлено перерозподіл атомів Оксигену в сполуках Zr9Mo4NiOx при гідруванні.
Ключові слова: сполуки Цирконію, сполуки Гафнію, сполуки Ніколу, металогідриди, кристалічна структура, рентгеноструктурний аналіз.
Стаття надійшла до редколегії 28.10.2004 Прийнята до друку 04.02.2005