Stability of the Ag42In42Ca16 icosahedral quasicrystal and its 2/1 cubic approximant

The simple cubic Ag42In45Ca13 (Pa (3) over bar, a=2.496 mu), which is the 2/1 crystalline approximant of the Ag42In42Ca16 icosahedral quasicrystal, has been found as the dominant phase in the ternary Ag3In3Ca13 alloy. Its close structural relation to the Ag42In42Ca16, icosahedral quasicrystal was shown by means of SAED (selected area electron diffraction). The decomposition of the cubic Ag42In45Ca13 after annealing or exposure in air was studied using EDS (energy dispersive X-ray spectroscopy), EELS (electron energy loss spectroscopy) and powder X-ray diffraction. Thin foils of the Ag3In3Ca alloy are easily oxidized in air and the product contains Ca, 0 and some C. In block samples of the Ag3In3Ca alloy exposed to air for 14 d, the cubic Ag42In45Ca13 gradually decomposes, while the amount of the binary AgIn2 and In4Ag9 phases increases. On the other hand, the Ag42In42Ca16 icosahedral quasicrystal is stable both chemically and structurally under the same conditions.