Pressure-Driven Cooperative Spin-Crossover, Large-Volume Collapse, and Semiconductor-to-Metal Transition in Manganese(II) Honeycomb Lattices

Abstract

Spin-crossover (SCO) is generally regarded as a spectacular molecular magnetism in 3d-3d metal complexes and holds great promise for various applications such as memory, displays, and sensors. In particular, SCO materials can be multifunctional when a classical light- or temperature-induced SCO occurs along with other cooperative structural and/or electrical transport alterations. However, such a cooperative SCO has rarely been observed in condensed matter under hydrostatic pressure (an alternative external stimulus to light or temperature), probably due to the lack of synergy between metal neighbors under compression. Here, we report the observation of a pressure-driven, cooperative SCO in the two-dimensional (2D) honeycomb antiferromagnets MnPS and MnPSe at room temperature. Applying pressure to this confined 2D system leads to a dramatic magnetic moment collapse of Mn (d) from S = 5/2 to S = 1/2. Significantly, a number of collective phenomena were observed along with the SCO, including a large lattice collapse (∼20% in volume), the formation of metallic bonding, and a semiconductor-to-metal transition. Experimental evidence shows that all of these events occur in the honeycomb lattice, indicating a strongly cooperative mechanism that facilitates the occurrence of the abrupt pressure-driven SCO. We believe that the observation of this cooperative pressure-driven SCO in a 2D system can provide a rare model for theoretical investigations and lead to the discovery of more pressure-responsive multifunctional materials.

Mott Metal-Insulator Transitions in Pressurized Layered Trichalcogenides

“…Note that the previous experimental study suggested the formation of Mn zigzag chains in the high-P phase [22], in contrast to our DFT+U and eDMFT results.…”

“…1(a) in the main text, the value of critical pressure predicted by DFT+U calculations is twice bigger than the one reported in Ref. 22 (63 vs. 30 GPa). For a better understanding of this discrepancy, we perform calculations with interpolating structures between the honeycomb high-spin and the dimerized low-spin structures as shown in Fig.…”

“…1(b)). We note that the theoretical critical pressure of 63 GPa is somewhat overestimated compared to the experimentally reported value of ∼ 30 GPa [22]. However, we show in the SI that within eDMFT, spinodal lines extend down to a much lower pressure of 40 GPa with a much reduced energy barrier be-tween the metallic and insulating solutions compared to the DFT+U results.…”

“…Structural relaxations for all compounds were performed in the presence of the DFT+U eff (4 eV) on-site Coulomb interaction and a Néel-type antiferromagnetic order [26], which gives reasonable agreements of lattice parameters and gap sizes with experimentally observed values [22,29]. It should be mentioned that, without incorporating magnetism and U eff to open the gap, the volume is severely underestimated for both compounds, especially ∼ 20% smaller in MnPS 3 .…”

Emergent superconductivity in an iron-based honeycomb lattice initiated by pressure-driven spin-crossover

“…In the pursuit of abrupt pressure-driven SCOs in TM chalcogenides, we became aware of the essential role of the TM-sublattice dimensions in the cooperativeness of SCO 26 , 27 . We have discovered abrupt pressure-driven phase transitions accompanied by a high-to-low spin-state transition of Mn 2+ in the transition from three-dimensional (3D) NaCl-type Mn X to two-dimensional (2D) MnP X 3 ( X = S, Se) with an Mn 2+ honeycomb lattice.…”