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

Technical Summary: Synthetic Control over Marcasite-Pyrite Polymorph Formation in the Fe1−xCoxSe2 Series

Problem Statement
Transition-metal dichalcogenides of the pyrite-marcasite family serve as model systems for crystal chemistry, with specific interest in FeSe2 and CoSe2 for catalytic and optoelectronic applications. While FeSe2 naturally forms in the orthorhombic marcasite structure, bulk CoSe2 is typically synthesized in the cubic pyrite structure. This presents a discrepancy with Density Functional Theory (DFT) predictions, which suggest an orthorhombic marcasite ground state for CoSe2. Although polymorphism (the coexistence of both structures) has been observed in CoSe2 nanoparticles, achieving synthetic control over these polymorphs in bulk or thin-film forms has remained elusive. The specific challenge addressed is determining the conditions under which the thermodynamically predicted marcasite phase can be stabilized over the commonly observed pyrite phase across the entire Fe1−xCoxSe2 composition range.

Methodology
The authors employed a combinatorial materials science approach to synthesize and analyze thin-film libraries of the Fe1−xCoxSe2 series. The process involved a two-step synthesis:

1. Deposition: A thin-film library with a continuous Fe-Co gradient (approximately 200 nm thick) was deposited on thermally oxidized silicon substrates using magnetron co-sputtering. This created a composition gradient ranging from Fe-rich to Co-rich.
2. Ex-situ Selenization: The libraries were sealed in evacuated quartz tubes with high-purity selenium and annealed at three distinct temperatures: 250°C, 350°C, and 430°C. Selenization times varied between 12 and 60 hours to ensure complete reaction, particularly at lower temperatures.

Structural characterization was performed using X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) at 4 mm intervals along the gradient. Rietveld refinement was used to determine lattice parameters and phase fractions. Complementing the experiments, DFT simulations were conducted using the abinit package with PBE functionals to calculate energy-volume curves, electronic densities of states (DOS), and the relative stability of the cubic and orthorhombic phases.

Key Results

• Temperature-Dependent Polymorphism: The synthesis temperature was identified as the critical factor controlling polymorph selection. At higher temperatures (350°C and 430°C), the cubic pyrite phase becomes dominant for Co-rich compositions (x ≥ 0.62), while the orthorhombic marcasite phase dominates Fe-rich compositions.
• Stabilization of Marcasite CoSe2: At the lowest synthesis temperature of 250°C, the orthorhombic marcasite structure was successfully stabilized as the majority phase even for pure CoSe2 (x = 1), with only 25% of the cubic phase present. This demonstrates synthetic control over the polymorphs of CoSe2, a feat not previously achieved in bulk materials.
• Phase Coexistence and Composition: In the coexistence region (x ≈ 0.6–1.0) at higher temperatures, the orthorhombic phase was found to be Fe-enriched (y ≈ 0.65 at x_avg = 0.8), while the cubic phase was Co-enriched (z ≈ 0.87). This indicates that transition-metal atoms possess sufficient mobility to segregate into grains of different phases with distinct compositions.
• Theoretical Validation: DFT calculations confirmed that the orthorhombic marcasite phase is the energetic ground state for all compositions in the Fe1−xCoxSe2 series. However, the energy difference between the orthorhombic and cubic phases is extremely small for pure CoSe2 (18 meV/atom), corresponding to a thermal energy of ~209 K. This small energy gap explains the sensitivity of the structure to synthesis temperature. The energy difference increases with Fe substitution, reaching 29 meV/atom for FeSe2.
• Electronic Structure: The DOS analysis revealed that the valence band is dominated by Co and Fe 3d-states. In the orthorhombic phase, these states split into a characteristic double-peak structure due to the distortion of metal-Se octahedra.

Significance and Claims
The paper claims to have achieved synthetic control over the polymorphic phases of CoSe2 by utilizing low-temperature selenization. The authors conclude that the orthorhombic marcasite structure is the true equilibrium phase for Fe1−xCoxSe2 across the entire composition range, including pure CoSe2. The prevalence of the cubic pyrite structure in bulk CoSe2 synthesized at higher temperatures is attributed to kinetic inhibition; the energy barrier for the transition from cubic to orthorhombic (involving Se-bond reorientation) is approximately 67 meV/atom, preventing the system from reaching its thermodynamic ground state at elevated temperatures.

The study highlights that the material exists at the "brink of a crystallographic transition," where the two polymorphs are nearly degenerate in energy. By lowering the synthesis temperature, the authors successfully bypassed the kinetic barriers that typically favor the cubic phase, thereby stabilizing the theoretically predicted ground state. This work provides a method to reproducibly access the marcasite polymorph, which has been suggested to possess superior catalytic performance compared to its cubic counterpart.