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Content archived on 2024-05-29

High-pressure phase transitions inpolyamorphic systems of semiconducting elements

Final Activity Report Summary - POLYAMORPHIC SYSTEMS (High-pressure phase transitions in polyamorphic systems of semiconducting elements)

The initial goal of our project was to prepare bulk samples of amorphous Ge and GexSi1-x alloys and study their polyamorphic transformation as a function of pressure. This would result in new metallic amorphous materials that might be recoverable to ambient conditions.

Firstly, we had to synthesise the amorphous Ge and GexSi1-x alloy samples. A procedure for preparing amorphous Si and Ge was recently described using a metathesis reaction between Zintl phases, like NaSi or NaGe, and an ammonium halogenide, such as NH4Cl or NH4Br. We developed and optimised the procedure in order to increase the yield of amorphous product and eliminate traces of crystalline material formed. Using this new method we could prepare hundred milligram quantities of amorphous Ge powder. The disordered (amorphous) structure of the final product was confirmed by X-ray diffraction and Raman spectroscopy.

The effect of pressure on the amorphous Ge was probed by in situ Raman spectroscopy. Upon increasing pressure the main Raman peak weakened and shifted abruptly to lower wavenumber above 11 to 12 GPa pressure (P) confirming the presence of a polyamorphic transition to a high-density form of a-Ge with a higher coordination number. During decompression the reverse transition took place at lower pressure, ranging between 6 and 7 GPa. The large hysteresis indicated that the polyamorphism resembled a first order phase transition.

Moreover, we attempted to use the synthesis method to prepare samples of amorphous GexSi1-x alloys to carry out similar pressurisation studies; however the syntheses were not successful and another method had to be devised. During our study we investigated the high pressure phase transformation behaviour of the Zintl phase NaSi, which appeared to undergo an amorphisation and polymerisation reaction at pressure greater than 12 GPa.