Giant pressure-induced volume collapse in the pyrite mineral MnS ₂

Abstract

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spinstate transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S = 5 2 mineral hauerite (MnS 2 ) undergoes an unprecedented (ΔV ∼ 22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S = Magnetism plays an important role in determining the properties of the minerals that make up Earth's crust and mantle (1, 2). This is due to the ubiquity of transition metals (notably Fe) with unpaired electrons. Under geological pressures, these may undergo abrupt volume collapses due to spin-state transitions. These occur when the crystal field splitting Δ, competes with Hund's rule and electronic Coulomb repulsion to determine the size of the magnetic moment. High pressure can thus make a high-spin (e.g., S = 5 2 ) state unstable with respect to a low-spin state (1-5), because Δ increases as the metal-ligand bond distances decrease. For simple materials, volume collapses of ΔV ' 5 % are regarded as notable (3-9). Understanding the mechanisms (5) of such magnetically driven transitions, and predicting the high-pressure structures, is ultimately of great importance for modeling the mantle in particular.The simple pyrite-structured mineral hauerite (10) (MnS 2 ) was reported nearly 30 years ago to undergo a pressure-induced transition (11, 12). The pyrite structure (13) (Fig. 1A) is cubic, with molecular disulfide S 2− 2 groups, which octahedrally coordinate an fcc lattice of Mn 2+ sites. Under ambient conditions, a high-spin t 3 2g e 2 g S = 5 2 moment is stabilized by Hund's rule coupling, and long-range magnetic order results (10). Early work on MnS 2 , which used laboratory energy-dispersive diffraction (11), detected a structural change at 11 GPa. These results were interpreted as a spin-state transition to a t 5 2g e 0 g S = 1 2 state in the marcasite structure. This was supported by later calculations (14). However, when the experiment was repeated using synchrotron radiation (12), an unidentified disordered phase was observed. Together with the large estimated volume collapse (∼15%), this little-remarked-upon finding was one of the motivations for the present work.We used the same natural sample as previous investigations (11, 12), and confirmed its purity by diffraction and X-ray fluorescence measurements (15) (Fig. S1). Pressure was applied using gas-loaded diamond anvil cells. Up to 11.7 GPa, X-ray diffra...

Spin-driven symmetry breaking in the frustratedfccpyrite MnS2

“…Due to their scarcity, we were unable to identify these trace impurities. Together with the XRF results reported previously [9], this indicates that our sample is of remarkable purity and stoichiometry for a natural mineral.…”

“…While little has been reported at ambient pressure, with no detected anomalies in e.g. thermal expansion [5], we recently discovered [9] spin-lattice coupling at low temperatures has also been previously reported. In 1996, Chattopadhyay and Liss performed pioneering hard x-ray triple axis experiments on MnS 2 .…”

“…Note that these are the same samples used for earlier single crystal neutron and high pressure diffraction studies [5,[7][8][9]. We used a small part of a large crystal of octahedral habit.…”

“…As reported previously, the elemental composition of our sample was checked using X-ray fluorescence measurements (see the supplementary info in Ref. [9]). …”

Spin waves in the fcc lattice antiferromagnet: competing interactions, frustration, and instabilities in the Hubbard model

“…17 Giant pressure-induced volume collapse accompanied by high spin (S = 5/2) to low spin (S = 1/2) transition involving interplay between crystal field splitting and Hund's rule coupling has been observed in this pyrite mineral. 18 The measured Néel temperature of MnTe 2 has been found to show unusually large pressure dependence of 12K/GPa, giving rise to large violation of Bloch's rule. 19 Based on IR reflection measurements at room temperature, MnTe 2 appears to undergo pressure-induced semiconductor-metal transition in the pressure range of 8-25 GPa.…”