Cooperative spin-crossover under pressure - Dr. Wenge Yang
Shanghai Dec. 5th, 2016 — Spin crossover (SCO), an intriguing phenomenon that magnetic ions can switch between high-spin (HS) and low-spin (LS) states in response to light irradiation or temperature, is mostly observed as a spectacular molecular magnetism in 3d4-3d7 metal complexes. In particular, SCO materials can be multifunctional when a light- or temperature-induced SCO occurs along with structural and/or electrical alterations, and thus hold great promise for applications such as memory, display and sensor. Contrastively, pressure-induced SCO often occurs as a progressive process due to the gradual effect of pressure on the inter-atomic distance and bandgap. This greatly hinders harnessing pressure-induced SCO for potential applications in sensors or memory devices. A joint team of researchers from UNLV, HPSynC and HPSTAR reported their breakthrough in pursuing "cooperative" pressure-driven SCO in J. Am. Chem. Soc. (DOI: 10.1021/jacs.6b10225). An abrupt pressure-driven SCO accompanying with large lattice collapses and semiconductor-to-metal transitions was achieved in two-dimensional honeycomb lattices, MnPS3 and MnPSe3, for the first time. The work opens a new avenue for the exploration of pressure-responsive multifunctional materials.
According to the crystal-field theory, whether or not a transition metal ion embedded in the crystalline matrix will have a HS or LS electron configuration is determined by the magnitude of the crystal-field splitting energy (∆=10Dq) along with the Hund's intra-atomic exchange energy (J). Pressure, as an alternative external stimulus to light or thermal excitation, is an efficient tool for altering crystal field strength by shortening the metal-ligand bond-lengths, and thus has promise to drive SCO in strongly correlated systems. Previous researches on representatives including FeS, (Mg,Fe)O and (Mg,Fe)(Si,Al)O3 have provided in-depth understanding of the concurrent structural and electronic anomalies, and seismic-wave heterogeneity in the Earth's lower mantle.
“However, when revisit from the viewpoint of material science, one of the most challenge question is, can we make pressure-induced SCO potential as practical switching materials, just as those of light- or temperature-induced SCO materials?”, said a co-lead author on the research, Wenge Yang, also a staff scientist of HPSTAR. "To approach this goal, the pressure-driven SCO process should be sharp rather than gradual or progressive".
This work is a continue of their previous work on MnS and MnSe (Angew. Chem. Int. Ed., 2016, 55, 10350), where they predicted that the pressure-induced SCO should be a universal behavior in manganese chalcogenides. Now they switch to two-dimensional honeycomb compounds MnPS3 and MnPSe3. X-ray emission spectra shows an abrupt SCO (from HS (S=5/2) to LS (S=1/2)) in both materials around 25−30 GPa. Their following in-situ high-pressure structural and transport characterizations show that the sharp pressure-driven SCO accompanies large in-plane lattice collapse and a semiconductor-to-metal transition, indicating a strongly cooperative mechanism that facilitates the occurrence of the abrupt pressure-driven SCO.
“From three-dimensional to two-dimensional, from Geosciences to Material Chemistry, I got this inspiration from the light- or temperature-induced SCO in ambient metal-organic complexes, where an abrupt spin change indicates a remarkable "cooperative behavior" between neighboring metal centers.”, said by the project leader Dr. Wang from UNLV and HPSynC. “MnPS3 and MnPSe3 are ideal low-dimensional confined systems, better than those mineral candidates to realize cooperative pressure-driven SCO. The magnetic spins located on the confined Mn honeycomb lattice are expected to accommodate neighboring spins more collectively than in other 3D crystalline materials”.
"Although the cooperative pressure-driven SCO is already a big-step ahead towards the rational design of multifunctional pressure-responsive materials, there is still a long way to go." said Yonggang. "What should a pressure-driven SCO be in a one-dimensional system? Is it possible to realize a metal-to-insulator transition through pressure-driven SCO, just as what happens in transparent sodium under ultra high pressure?"
Caption: IAD and spin values as a function of pressure, obtained relative to the ambient spectra of MnPS3 and MnPSe3; Proposed high-pressure structure for MnPS3 and MnPSe3