PHYSICAL REVIEW B VOLUME 51, NUMBER 17 Hydrogenation of titanium-based quasicrystals 1 MAY 1995-I A. M. Viano, R. M. Stroud, P. C. Gibbons, A. F. McDowell, M. S. Conradi, and K. .F. Kelton Department of Physics, Washington University, St. Louis, Missouri 63130 (Received 12 January 1995) Measurements of hydrogen loading and unloading in a quasicrystal are reported. The icosahedral phase (i phase) in a Ti-Zr-Ni alloy demonstrates hydrogen absorption from the gas phase at a temperature of 260 'C and a pressure of 40 atmospheres. The incorporation of hydrogen causes the quasilattice to expand, with an increase in quasilattice constant of almost 7%, from a;=5. 18 to 5. 52 k Similar abilities for storing hydrogen are found in high-order crystalline approximant and amorphous phases of like composition. Of the phases examined, the i phase absorbs the most hydrogen, giving a hydrogen atom to metal atom ratio ([H]/ [M]) of 1. 6. Initial studies indicate that hydrogen desorption is hindered by phase transitions to stable hydride phases. Hydrogen loading at lower temperatures with higher H2 pressures favors higher [H]/[M] ratios for the i phase and the crystal approximant, and also minimizes the growth of more stable crystalline hydride phases. Because of their ability to reversibly absorb significant quantities of hydrogen, crystalline and amorphous transition metals and their alloys have been studied extensively for potential use in hydrogen storage applications. Crystalline Ti and Zr, for example, are capable of storing up to 66 atomic percent (at. %) of hydrogen, corresponding to a hydrogen atom to metal atom ratio ([H]/[M]) of 2. ' Amorphous Zr-Ni is capable of storing hydrogen with an [H]/[M] of 1. 6. Key factors for determining which materials can store hy- drogen include the chemical interactions between metal and hydrogen atoms and the number, type, and size of interstitial sites in the host metal. In most transition metal alloys, the hydrogen prefers to sit in tetrahedrally coordinated sites, making the polytetrahedral Laves phases particularly attractive. The icosahedral phase, the most common quasi- crystal phase showing the rotational symmetry of the icosa- hedral point group, is likely dominated by local tetrahedral order. Surprisingly, hydrogen absorption in i-phase materials has received almost no attention. This is probably because most quasicrystals are formed in aluminum alloys, which do not have favorable chemistry for hydrogen storage. Though less well studied, icosahedral quasicrystals are also found frequently in titanium-3d transition metal alloys, ' which have favorable chemistry. The dominance of polytetrahedral order and the favorable hydrogen-metal chemistry make these materials potentially useful for hydrogen storage appli- cations, motivating the present study. Early studies by us and others have demonstrated that Ti-3d transition metal-Si icosahedral phases can absorb hy- drogen from the gas phase and by electrolytic methods. The amount of hydrogen loaded in these cases, however, was not determined. Further, the inAuence of surface conditions, the effect of alloy composition, the kinetics of sorption and de- sorption, and the unloading and cycling characteristics for these materials are as yet unknown. Here, we report the first measurements of hydrogen loading in Ti-Zr-Ni icosahedrally ordered alloys. Constituting the most ordered class of Ti- based quasicrystals, the phases formed in these alloys are known to be extremely sensitive to Si composition. By vary- ing the Si over a range of only 4 at. %, it is possible to go continuously from the i phase to high-order cubic approxi- mants to an icosahedrally ordered metallic glass. As we show here, all of these phases absorb significant amounts of hydrogen (up to [H]/[M] = 1. 6 in the i phase). In addition to pointing to possible applications, these results open up new windows of investigation into quasicrystal structures. Planned NMR and neutron diffraction studies on hydroge- nated and deuterated samples will provide information about the local atomic structure and can probe the dynamics of hydrogen diffusion through a quasilattice. Alloy ingots prepared by arc melting were crushed and melt-spun to form rapidly quenched ribbons as described elsewhere. The alloy composition was adjusted to give the i phase, crystal approximant, or metallic glass on quenching. Three alloys were investigated; Ti45Zr38Ni&7, Ti53Z127Ni2p, and Ti53Zr27Ni2p(+Sl). Tile rapidly querlclled TigsZr3sNll7 alloy contained only the icosahedral phase. The amorphous T153ZI27N12p alloy contained between 2 and 3 at. % Si, result- ing from contamination from the fused silica quenching tube. Si has been found to enhance amorphous phase formation in these alloys. The Ti53ZI27Nizp alloy made with no measur- able Si contamination contained a phase mixture of high- order Fibonacci rational approximants, including the 3/2 and the 5/3 cubic approximants. Samples were investigated with x-ray powder diffraction (XRD) and transmission electron microscopy (1'I'M). TEM investigations using a JEOL 2000FX equipped with a Noran energy dispersive spectrometer (EDS) were made to obtain local cornpositional and phase microstructural information. TEM specimens of the as-quenched ribbon were prepared by ion milling, using a liquid nitrogen cooled sample stage to minimize milling-induced artifacts. Samples of the hydroge- nated materials were ground into a fine powder for TEM and XRD investigations. Hydrogenation of the ribbon was carried out in one of two Sieverts'-type systems. Initially, a low-pressure (less than one atmosphere) system was used. In this case, the hydroge- nation temperatures were determined simply by heating the sample until surface barriers were overcome and hydrogen absorption occurred, as indicated by a decreasing gas pres- 0163-1829/95/51(17)/12026(4)/$06. 00 12 026 1995 The American Physical Society