Formation and structure of 1/1 cubic approximants in the Cd-Mg-Ce system: The identification of a new structural variant

The stability region of the 1/1 cubic crystalline approximant to icosahedral quasicrystal in the Cd-Mg-Ce system was investigated in detail. The partial isothermal section of the ternary phase diagram at 673 K was determined in the range from 0 to 18 at% Ce, where it was shown that the approximant spans a wide compositional range of Cd57-86Mg0-29Ce14-15. Mg addition was found to shift the stability region to lower temperatures. Besides, the approximant was found to be in equilibrium not with the Cd-Mg solid solution but with (Cd,Mg)17Ce2 and (Cd,Mg)11Ce compounds as opposed to the previous work [J. Alloys Compd. 822 (2020) 153541]. Based on the diffraction data (both X-ray and electron), to our surprise, a new variant of 1/1 cubic approximant was identified within the compositional range. The prototype Cd37Ce6 structure (space group Pn3) which dominates near the Cd-rich end was found to be partly taken over by the Cd25Eu4-type structure (space group Fd3) with a doubled unit cell in the higher-Mg part of the compositional range. The atomic structure with both Pn3 and Fd3 symmetries were determined by means of single-crystal X-ray diffraction measurement. Their refined atomic structures were compared and the preferential sites for Mg substitution are revealed.(c) 2023 Elsevier B.V. All rights reserved.