Reversible bi-metastable phase switching in the VO2 system - Dr. Wenge Yang
Shanghai July 18, 2016 — By using the in situ synchrotron techniques at HPCAT and a series of self-designed experiments, an international joint team co-led by Dr. Wenge Yang from HPSTAR, realized a controllable phase switching between pressure-induced amorphization and thermal-driven recrytallization in VO2(B) nanosheets. They claimed that it was the ﬁrst ever example of a structural memory effect observed in a strongly correlated material. The story is published in theJuly 18th edition of Nature Communications (doi:10.1038/ncomms12214).
VO2(B) is a metalstable phase among at least five or six VO2 phases, adopting an anisotropic layered structure at ambient conditions. Its well-embedded layers and hierarchical V-O bonding makes it a good candidate for seeking exceptional structural evolutions. Nanocrystallized VO2(B) is another advantage for such study because of the many oxygen vacancies and stresses on the surface in accelerating the collapse of the original structure.
In the experiment, the researchers firstly demonstrated the pressure-induced amorphization of VO2(B) nanosheets around 20 GPa at room temperature. They used multiple measurements including X-ray diffraction, Raman, and IR spectra to reveal the changes of long-range and short-range orders including the coordination changes of VOx polyhedral during the amorphization process.
What surprised the researchers is that, when considering the pressure-induced amorphous VO2(B) (PIA-VO2(B)) as a new metastable phase similar as that of VO2(B) crystalline phase, they are able to realize controllable switching between VO2(B) and PIA-VO2(B).
The method to achieve the recrystallization is simple heating at about 200 °C for ~ 5 mins. They revealed the role of VOx polyhedral during the reversible phase transformations and compared with that of the PO4 units in AlPO4, which was known as a unusual "memory glass".
The authors provided a brief phase diagram to illustrate the phase relationship and pointed out the kinetic hindrance of the phase transformation from PIA-VO2(B) to thermodynamically stable VO2 phases. They also performed theoretical calculations to provide a clear phase diagram of VO2 system under compression.
“You may think that recrystallization of an amorphous material to its virgin structure by annealing is a common phenomenon. I can't agree with you anymore but only if the returning phase is thermal-stable”, said Yonggang, the lead author of this paper. “The switching between two thermally-metastable phases was rarely observed and dependent highly on the temperature parameter”, he explained.
So, that's the first key point of their work: they believe that the key factor enabling their observation is the temperature, which is kinetically low enough for sufﬁcient atomic mobility and thus avoid the transformation to other more stable phases such as VO2(M).
“Another key point is that we also give some evidences of the participation of spin, orbital and magnetic interactions in the transformations", Wang added. "20 GPa is not too high. We made big PIA-VO2(B) sample by using the large-volume press apparatus in Dr. Yingwei Fei's Lab at Geophysical lab. So, we can measure the magnetic susceptibility of PIA-VO2(B) and link the results with the structural and electric resistance measurements".
“Our findings highlight the importance of pressure and temperature in exploration of thermodynamically hindered phase transformations”, said Dr. Yang, one of the principal investigators and a staff scientist of HPSTAR.
Caption:Density-energy scheme of the VO2 system and the reversible structure switching, courtesy of Yonggang Wang.