Stoichiometry Of Tetragonal And Hexagonal Fexse: Phase Relations

Precise compositional analyses at spatial resolution mu m, combined with structure determination using bulk (i.e. powder XRD) and individual grain (i.e. EBSD) techniques, show that both beta-FexSe and delta-FexSe form as solids in a two-phase field above and below the apparent peritectic temperature of 457 degrees C. Microstructures show that beta-FexSe and delta-FexSe 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, beta-FexSe occurs as the predominant phase in association with hexagonal delta-FexSe in the temperature range 330 degrees C < T-max < 750 degrees C, where T-max is the sintering temperature. Maximum yield of beta-FexSe occurs after sintering at T-max > 690 degrees C with annealing at 420 degrees C for similar to 24 h. We define a modified phase diagram that includes this two phase field with beta-FexSe and delta-FexSe for 1.02 < x < 0.90 and a second two phase field with beta-FexSe and alpha-Fe for 1.08 < x < 1.02. This revised phase diagram for Fe:Se similar to 1.0 suggests that the peretectic transition nominally identified at x = 1.04 is not evident.