Pressure-driven cooperative spin and lattice collapses in Mn(II) Chalcogenides - Dr. Wenge Yang

AUGUST 2, 2016

Recently, giant pressure-driven volume collapse (> 20%), a rarely reported phenomenon in condensed matter, was observed in MnS2 and MnS. The intriguing behavior was considered to be associated with the pressure-driven high-spin to low-spin transition of Mn(II), but lacking experimental evidences and in-depth understandings. An international team — UNLV, HPSynC and HPSTAR of scientists co-led by Dr. Wenge Yang dug into this phenomenon and found that the giant volume collapse was coupled with the spin state transition of Mn(II) and a semiconductor-to-metal transition. This work is published online in Angew. Chem Int. Ed., (doi:10.1002/anie.201605410).

Pressure-induced structural phase transition has been a common phenomenon in the field of condensed matter of physics. During such a phase transition, most of the materials will suffer a volume change of no more than 5%. Very recently, giant lattice collapses have been reported in MnS2 and MnS by Kimber, et al. (PNAS, 2014, 111, 5106) and Xiao, et al. (JACS, 2015, 137, 10297), respectively.

“A large volume collapse is always associated with dramatic changes in electronic configuration of transition metals, either spin, charge or orbital, or two or more of them. More characterizations besides the XRD data can help to verify the underlying relationship”, said Dr. Yonggang Wang, the lead author of the paper. "And what I was wondering was that whether the pressure-driven lattice collapse was universal in all manganese chalcogenides or not ".

So they chose two stable materials, MnS and MnSe for the experiments. Firstly, structure studies were performed by in situ XRD measurements, where volume collapses of more than 20% were observed in both materials. The materials were found to transform from the low-pressure rocksalt (NaCl)-type to the high-pressure MnP-type structure. Mn-Mn intermetallic bonding was found during the transition, which is considered to be incidentally happened during the giant lattice collapse in manganese chalcogenides. Then the X-ray emission spectroscopy (XES) results revealed that a high-spin (S = 5/2) to low-spin (S = 1/2) state transition occurred accompanied with the structural phase transition. Finally, high pressure transport measurements on the two samples showed that a semiconductor-to-metal transition was involved in the event.

“The giant lattice collapse, the high-to-low spin state transition, and the formation of Mn-Mn metallic bonds, which one is the reason? Which one is the result? The answer should be, pressure is the original driving force and the three phenomena are cooperative results”, Yonggang added.

“Now we know that pressure-driven lattice and moment collapses are universal in most manganese chalcogenides. The most important thing is that, such an interesting phenomenon is predictable. I can tell you whether or not a material will undergo the high-spin to low-spin transition under high pressure, just by a glance at the crystal structure. We picked out several other compounds and performed the high pressure measurements. The phenomenon was found in all of them”, said Wenge, a staff scientist of HPSTAR. “You will see the results soon.”