Synthetic control over marcasite-pyrite polymorph formation in the Fe1-xCoxSe2 series

TL;DR

Fe$_{1-x}$Co$_x$Se$_2$ exhibits near-degenerate marcasite and pyrite polymorphs; the paper develops a combinatorial thin-film selenization platform to controllably synthesize the series across composition and temperature. Combining ex-situ selenization at $T_ ext{synthesis}=250$–$430$ °C with XRD and EDS, the authors show marcasite CoSe$_2$ can be the majority phase at low temperature, while cubic CoSe$_2$ emerges at higher temperatures and higher Co content; DFT calculations reveal the two CoSe$_2$ polymorphs are energetically close with an orthorhombic ground state, and Fe substitution stabilizes the orthorhombic phase, explaining the observed phase behavior. Overall, the study demonstrates that marcasite is the equilibrium phase of Fe$_{1-x}$Co$_x$Se$_2$ under low-temperature synthesis across the full composition range, enabling synthetic control of polymorph formation.

Abstract

Transition-metal dichalcogenides of the pyrite-marcasite family are model systems of crystal chemistry. A few of these show polymorphism. The theoretical ground state of CoSe2 is marcasite, but the material is typically synthesized in the pyrite structure. Polymorphism has been observed in nanoparticles and synthetic control of the polymorphs of CoSe2 has not been achieved. We have synthesized material libraries of the Fe1-xCoxSe2 series by combining combinatorial deposition and ex-situ selenization. The approach allows to efficiently explore substitution ranges and crystal structures that form for different synthesis conditions. We find that higher levels of Co content x within the marcasite structure are possible when synthesizing at low temperatures. At a synthesis temperature of only 250° C, we have successfully synthesized marcasite CoSe2 as the majority phase. Density functional theory simulations reveal that the two isomorphs of CoSe2 are extremely close in energy and that the orthorhombic phase is the energetic ground state. Our experimental and theoretical data show that the marcasite structure is the equilibrium phase of Fe1-xCoxSe2 in the entire composition range.

Figures (7)

• Figure 1: Synthesis of Fe$_{1-x}$Co$_x$Se$_2$ thin-film materials libraries (a) Variation of the relative Co content $x_\mathrm{avg.}$ along the materials library as determined by EDS. (b) Sketch of the materials library enclosed in a quartz tube for selenization, not to scale. (c) Temperature profile of the selenization process. (d,e) SEM images of a materials library before and after the selenization process, respectively.
• Figure 2: XRD patterns for Fe$_{1-x}$Co$_x$Se$_2$ thin-film materials library measured in 4 mm steps. Data for films of pure Fe and Co are added. The data are stacked according to $x_\textnormal{avg.}$ for clarity, with the scale indicated on the right axis. (a),(b) XRD patterns for the films synthesized at 430° C. (c),(d) XRD patterns for the film synthesized at 350° C. (e),(f) XRD patterns for the films synthesized at 250° C. Green ticks and lines represent the Bragg peak positions of the orthorhombic marcasite o-FeSe$_2$ phase. The blue ticks and lines represent the Bragg peak positions for the cubic pyrite c-CoSe$_2$ phase. Orange stars show the Bragg peak positions of elemental Se. In panel (e), the closed diamond indicates the (101) peak position of elemental Fe and the open diamond indicates the position of (111) Bragg peak of elemental Co.
• Figure 3: Structural parameters of o-Fe$_{1-x}$Co$_x$Se$_2$ and c-Fe$_{1-x}$Co$_x$Se$_2$ as a function of $x_\textnormal{avg.}$ for different synthesis temperatures. Experimental and DFT orthorhombic $a_{\textnormal{o}}$ lattice parameter (a), orthorhombic $b_{\textnormal{o}}$ lattice parameter and cubic $a_{\textnormal{c}}$ lattice parameter (b), orthorhombic $c_{\textnormal{o}}$ lattice parameter (c), as well as orthorhombic and cubic unit-cell volumes, $V_{\textnormal{o}}$ and $V_{\textnormal{c}}$ (d). Note that the cubic unit cell contains twice as many formula units as the orthorhombic unit cell.
• Figure 4: Phase fraction as function of the experimentally determined average Co content $x_\textnormal{avg.}$ for films synthesized at different temperatures. The green area represents the orthorhombic phase fraction and blue area represents the cubic phase fraction. The beige area represents the unreacted fraction of Fe and Co. All films were selenized for 12 hours (open symbols) except for the pure iron and cobalt at 250° C (closed grey symbols), which were selenized for 60 hours.
• Figure 5: Estimated cobalt content $y$ in the orthorhombic marcasite phase, and cubic pyrite phase, $z$, as a function of $x_\mathrm{avg.}$ for the film selenized at 430° C, where the most accurate structural data were obtained.