An in situ neutron diffraction study of the thermal disproportionation of the Zr2FeD5 system

Abstract

The Zr2FeD5 system has been annealed to 680 àunder ultra high vacuum, and studied in situ by neutron diffraction. The system disproportionates through three distinct regions in temperature. Initially, the tetragonal Zr2FeD5 (P4/ncc) is retained up to 330 ì while steadily depleted of D. From 330 àto 530 ì a complex multi-phase disproportionation occurs, with the production of cubic ZrD2, tetragonal ZrD2, tetragonal Zr2FeD5 (I4/mcm), and growth of the intermetallic ZrFe2. At the beginning of the 330-530 àperiod, the total atom count from quantitative phase analysis (QPA) indicates the formation of amorphous (a-) Zr56Fe44. By 530 ì QPA and peak breadth analysis indicate that ca. 2/3rd of the sample is consumed as very small nanocrystals (<150 Šcoherence length) of strained ZrD2. From 530 àto 680 ì the cubic ZrD2 is almost entirely consumed and depleted of D to form the final mixture of the intermetallic phases Zr3Fe and ZrFe2. QPA of the final intermetallic mixture yields a Zr:Fe ratio greater than that observed in either the arc melted alloy or the initial Zr2FeD5 deuteride, indicating that a ca. Zr71Fe29 amorphous component was present in the initial arc melted alloy. According to the total atom count by QPA, crystallisation of the Fe richer amorphous Zr56Fe44 phase formed at 330 àbegins at ca. 530 ì and later by 680 ì all amorphous phases have completely crystallised to yield a 70.77:26.75:2.47 mol.% mixture of Zr3Fe:ZrFe2:ZrD2-x.