An in situ neutron diffraction study of the thermal disproportionation of the Zr 2FeD 5 system

Abstract

The Zr 2FeD 5 system has been annealed to 680 °C under ultra high vacuum, and studied in situ by neutron diffraction. The system disproportionates through three distinct regions in temperature. Initially, the tetragonal Zr 2FeD 5 ( P4/ ncc) is retained up to 330 °C, while steadily depleted of D. From 330 °C to 530 °C, a complex multi-phase disproportionation occurs, with the production of cubic ZrD 2, tetragonal ZrD 2, tetragonal Zr 2FeD 5 ( I4/ mcm), and growth of the intermetallic ZrFe 2. At the beginning of the 330–530 °C period, the total atom count from quantitative phase analysis (QPA) indicates the formation of amorphous (a-) Zr 56Fe 44. By 530 °C, QPA and peak breadth analysis indicate that ca. 2/3rd of the sample is consumed as very small nanocrystals (<150 Å coherence length) of strained ZrD 2. From 530 °C to 680 °C, the cubic ZrD 2 is almost entirely consumed and depleted of D to form the final mixture of the intermetallic phases Zr 3Fe and ZrFe 2. QPA of the final intermetallic mixture yields a Zr:Fe ratio greater than that observed in either the arc melted alloy or the initial Zr 2FeD 5 deuteride, indicating that a ca. Zr 71Fe 29 amorphous component was present in the initial arc melted alloy. According to the total atom count by QPA, crystallisation of the Fe richer amorphous Zr 56Fe 44 phase formed at 330 °C begins at ca. 530 °C, and later by 680 °C, all amorphous phases have completely crystallised to yield a 70.77:26.75:2.47 mol.% mixture of Zr 3Fe:ZrFe 2:ZrD 2− x .