Preparation and single-crystal structure of a new high-pressure modification of baal₂si₂

Abstract

A ternary element mixture of Ba, Al, and Si in a molar ratio of 1 : 2 : 2 was arc-melted and treated under a high-pressure and high-temperature condition of 5 GPa at 1200 °C. X-ray structural analysis was performed on the single crystal obtained by this treatment. The crystal was found to be a new high-pressure modification of BaAl₂Si₂, and isotypic with layer structured ThCr₂Si₂ in the space group I4/mmm. The crystal obtained by the simple arc-melting also had the same structure (high-pressure phase). The low-pressure phase α-BaAl₂Si₂ was prepared by annealing the arc-melted sample at 1200–1000 °C. The single crystals of the α-phase were also obtained, which crystallized in the space group Cmcm. This structure was closely related to the structure of α-BaAl₂Ge₂ (space group Pnma). It is interesting to note that BaAl₂Si₂ has a pressure induced polymorph, whereas BaAl₂Ge₂ has a temperature dependent dimorphism.

Keywords:

• Silicide
• High pressure
• Synthesis
• Clathrate
• BaAl₂Si₂
• Phase transition

Acknowledgements

This study has been supported by a Grant-in-Aid for Scientific Research (A) (no. 16205027) and the COE Research (no. 13E2002) of the Ministry of Education, Culture, Sports, Science and Technology of Japan, and Electric Technology Research Foundation of Chugoku.