Abstract
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-FexSe and δ-FexSe form as solids in a two-phase field above and below the apparent peritectic temperature of 457 °C. Microstructures show that β-FexSe and δ-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, β-FexSe occurs as the predominant phase in association with hexagonal δ-FexSe in the temperature range 330 °C < Tmax < 750 °C, where Tmax is the sintering temperature. Maximum yield of β-FexSe occurs after sintering at Tmax > 690 °C with annealing at 420 °C for ∼24 h. We define a modified phase diagram that includes this two phase field with β-FexSe and δ-FexSe for 1.02 < x < 0.90 and a second two phase field with β-FexSe 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.
Highlights
Transition metal chalcogenides have received significant interest in recent years due to their extensive range of applications in thermoelectrics, solar cells, sensors, magnetic devices, batteries, photocatalysts, semiconductors and superconductors [1]
We present a selection of experimental data to demonstrate the range of conditions for which high yields of tetragonal β-FexSe may form
While we have not explored this region of the phase diagram in detail, we suggest that the extent of this β-FexSe and α-Fe two phase field extends to lower temperatures [9]
Summary
Transition metal chalcogenides have received significant interest in recent years due to their extensive range of applications in thermoelectrics, solar cells, sensors, magnetic devices, batteries, photocatalysts, semiconductors and superconductors [1]. The discovery of superconductivity in iron based compounds [2, 3] has led to intense scrutiny of chalcogenides including of the basic structural units that make up new families of superconductors through judicious substitution of other elements [4]. Iron selenides are found in several stable crystalline forms: tetragonal β-FexSe, hexagonal δFexSe, orthorhombic FeSe2, hexagonal Fe7Se8 and monoclinic Fe3Se4 [8, 9]. Among these phases, the tetragonal βFexSe with PbO structure (space group P4/nmm) transitions to a superconductor (SC) with orthorhombic space group Cmma at ∼8 K [2]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
