X-ray diffraction study of the icosahedral AlCuFe quasicrystal at megabar pressures
Introduction
The first natural occurring quasicrystal (QC), icosahedrite (ideally Al63Cu24Fe13), was discovered from the Khatyrka meteorite in 2009 [1], which opens the new field of quasi-crystallography on the Earth and planetary sciences. The discovery of the natural QC fundamentally alters the concept of the conventional QCs because all quasicrystalline materials had been obtained only under highly controlled laboratory conditions. Very recently, the second natural QC, decagonal quasicrystal (ideally Al71Ni24Fe5), was found in the same meteorite [2]. The fact that these natural QCs were contacted with stishovite provides strong evidence for ultra-high pressure formation [1], [2]. Bindi and coworkers therefore suggested that the Khatyrka meteorite was formed in early solar system about 4.5 Gya and experienced at least 5 GPa and 1200 °C [3], [4]. However, presence of metallic Al and Cu with the natural QCs requiring a highly reducing environment is still puzzling. The exotic assemblage makes it extremely complex to understand how the natural QCs were formed. A recent experimental study investigated the thermodynamic stability of icosahedral (i)-AlCuFe QC and showed that the i-AlCuFe QC remains its structure up to 5 GPa and 1673 K [5]. To date, a few experimental studies on the high-pressure behavior of i-AlCuFe QC at ambient temperature have been published [6], [7], [8]. Previous in situ XRD study at ambient temperature revealed that i-AlCuFe QC can persist up to a pressure of 35 GPa [7]. Since QCs are not close packed structures, it can be assumed that phase transitions occur at high pressure [9]. The last two decades, however, almost all studies have been conducted below 40 GPa [10]. Hence, the upper pressure stability limit of i-AlCuFe QC has not been determined. An appreciation of temperature and pressure at which the i-AlCuFe QC is stably maintained can give critically important information to understand the thermodynamic process of natural QC formation. In this paper, we report high-pressure XRD experiment on the i-AlCuFe QC up to 104 GPa. These results suggest the occurrence of a phase transition from i-AlCuFe QC to its higher pressure phase.
Section snippets
Experimental methods
The i-AlCuFe QC was synthesized in our laboratory following previous study [11]. Commercially available Al (Nirako Co., Ltd., purity≥99.8%), Cu (Wako Pure Chemical Co., Ltd., purity≥99.8%), and Fe (Wako Pure Chemical Co., Ltd., purity≥95%) used as starting materials were mixed using an agate mortar with an atomic ratio of Al:Cu:Fe=65:20:15. The mixed powders were sealed under vacuum in a quartz vessel and placed in a furnace. The sample was then heated up to 1173 K and kept for 6 h. After cooling
Results and discussion
Fig. 1 shows the XRD pattern of the synthesized i-AlCuFe QC in the study. The XRD profile is completely consistent with that of i-AlCuFe QC previously reported [11]. We detected additional peaks due to a small amount of AlFe3 which was observed in the previous study as well [5]. The AlFe3 commonly occurs as an accessory compound during the procedure for synthesis of i-AlCuFe QC. The presence of AlFe3 can be neglected taking into consideration the previous experimental result of high-pressure
Acknowledgments
We greatly appreciate an anonymous reviewer for providing constructive comments and suggestions. The authors would like to thank N. Chino for kindly providing technical help with the EPMA analyses. The synchrotron XRD measurements were performed at the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2014B1136).
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