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

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.

💡 Research Summary

This paper investigates the polymorphism of the Fe₁₋ₓCoₓSe₂ system, focusing on the ability to control the formation of the orthorhombic marcasite (o‑Fe₁₋ᵧCoᵧSe₂) and cubic pyrite (c‑Fe_zCo₁₋zSe₂) phases through synthesis temperature and composition. The authors first prepared composition‑gradient thin‑film libraries by magnetron co‑sputtering Fe and Co onto oxidized silicon substrates, achieving a continuous Fe‑Co gradient across a ~200 nm thick film. The libraries were then sealed with high‑purity selenium in evacuated quartz tubes and subjected to ex‑situ selenization at three distinct temperatures: 250 °C, 350 °C, and 430 °C, each for 12–60 h with a heating rate of 100 °C h⁻¹.

Energy‑dispersive X‑ray spectroscopy (EDS) confirmed a linear Co concentration gradient (x_avg ranging from ~0.10 to ~0.80) and a near‑stoichiometric (Fe+Co):Se ratio of 1:2. X‑ray diffraction (XRD) performed at 4 mm intervals revealed a clear temperature‑dependent phase evolution. At 430 °C, samples with x_avg < 0.56 crystallized exclusively in the orthorhombic marcasite structure; above this threshold, pyrite reflections emerged and grew in intensity with increasing Co content. A similar trend was observed at 350 °C, but the pyrite onset shifted to x_avg ≈ 0.62. In stark contrast, the lowest temperature (250 °C) yielded almost pure marcasite across the entire composition range, with only a minor pyrite fraction (≈25 %) even for the pure CoSe₂ end member. Longer selenization times (up to 60 h) reduced residual metallic Fe and Co peaks that appeared at shorter dwell times.

Rietveld refinements provided lattice parameters for both phases. All lattice constants expanded with increasing Co content, following Vegard‑like behavior. At high temperatures the orthorhombic lattice parameters saturated near x_avg ≈ 0.56 (430 °C) or 0.62 (350 °C), whereas at 250 °C no saturation was observed, indicating a more continuous compositional tuning of the marcasite cell. The cubic lattice parameter a_c remained roughly constant for 0.75 ≤ x_avg ≤ 1 and decreased slightly at lower Co fractions. By assuming only the two phases are present, the authors deconvoluted the average composition into the individual phase compositions y (for o‑Fe₁₋ᵧCoᵧSe₂) and z (for c‑Fe_zCo₁₋zSe₂). The derived y values plateau around 0.6 as x_avg increases, confirming that the orthorhombic phase can accommodate substantial Co substitution before the pyrite phase becomes competitive.

Density functional theory (DFT) calculations using the PBE functional and norm‑conserving pseudopotentials were performed for both crystal structures across the full compositional range. The orthorhombic phase is predicted to be the ground state for all compositions, with an energy advantage of only 18 meV per atom for pure CoSe₂, rising to 29 meV per atom for FeSe₂. This small energy gap corresponds to a thermal energy of ~209 K, explaining the observed temperature‑driven phase competition. The calculations also show that the energy difference is sensitive to volume, and the barrier for the structural transition (involving Se‑bond reorientation) is about 67 meV per atom. The computed lattice parameters match the experimental values within 0.8 %, and the electronic density‑of‑states analysis yields an indirect band gap of ~0.3 eV for orthorhombic FeSe₂ and 0.9–1.1 eV for cubic CoSe₂, consistent with earlier experimental reports.

The combined experimental‑theoretical study leads to three central conclusions: (1) the orthorhombic marcasite structure is the thermodynamic equilibrium phase for the entire Fe₁₋ₓCoₓSe₂ solid solution; (2) low‑temperature synthesis (250 °C) stabilizes the marcasite phase even for the Co‑rich end member, providing a practical route to synthesize pure CoSe₂ in its predicted ground‑state structure; (3) the energy landscape between the two polymorphs is extremely flat, making the phase distribution highly sensitive to synthesis temperature, composition, and processing time. These insights open pathways for tailoring the functional properties (catalytic activity, thermoelectric performance, optoelectronic behavior) of Fe‑Co‑Se materials by deliberate control of their crystal structure.