Stoichiometry of tetragonal and hexagonal Fe_xSe: phase relations

Precise compositional analyses at spatial resolution <1 μm, combined with structure determination using bulk (i.e. powder XRD) and individual grain (i.e. EBSD) techniques, show that both β-Fe_xSe and δ-Fe_xSe form as solids in a two-phase field above and below the apparent peritectic temperature of 457 °C. Microstructures show that β-Fe_xSe and δ-Fe_xSe form together via exsolution when cooled from this two-phase field; evident when annealing time and temperature are optimised. Using a facile one-pot method with elemental Fe:Se reactant ratios ranging from 0.95 to 1.14, β-Fe_xSe occurs as the predominant phase in association with hexagonal δ-Fe_xSe in the temperature range 330 °C < T_max < 750 °C, where T_max is the sintering temperature. Maximum yield of β-Fe_xSe occurs after sintering at T_max > 690 °C with annealing at 420 °C for ∼24 h. We define a modified phase diagram that includes this two phase field with β-Fe_xSe and δ-Fe_xSe for 1.02 < x < 0.90 and a second two phase field with β-Fe_xSe and α-Fe for 1.08 < x < 1.02. This revised phase diagram for Fe:Se ∼ 1.0 suggests that the peretectic transition nominally identified at x = 1.04 is not evident. (paper)