Synthetic control over polymorph formation in the d-band semiconductor system FeS$_2$

TL;DR

This study demonstrates controlled hydrothermal synthesis of FeS₂ polymorphs, marcasite and pyrite, enabling detailed physical property measurements and highlighting marcasite's potential as a semiconductor material comparable to pyrite.

Contribution

It introduces a systematic synthesis method for phase-pure FeS₂ polymorphs and provides comprehensive physical property data for marcasite, previously difficult to obtain.

Findings

High-quality marcasite crystals were successfully synthesized.

Marcasite has a direct optical bandgap of 0.76 eV and a band gap of 0.73 eV from spectroscopy.

Marcasite exhibits temperature-independent diamagnetism and a carrier concentration of 4.14×10^{18} cm^{-3}.

Abstract

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS$_2$. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS$_2$. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high-quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H$_2$S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 $\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite -- that is now synthetically available in a straightforward fashion -- is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.