Peer review of: Tuning the hysteresis of a metal-insulator transition via lattice compatibility (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The authors studied WxV1-xO2 strips grown on Si and c-Al2O3 substrates, and observed that the 
hysteresis of MIT curves can be tuned by tungsten doping. By applying the non-linear theory to WxV1-
xO2, the authors explained the reason of hysteresis variation, and provided a possible way to 
minimize the hysteresis and optimize the phase transformation performance. The results are simple 
yet elegant and clean. A few questions need to be addressed: 

1. The authors prepared lateral graded doped WxV1-xO2 thin films, then patterned strips with 
different x. How were x values of these strips determined? BTW, the text said "...patterned into 
multiple strips along the composition gradient direction...", while it should be corrected as 
"...patterned into multiple strips perpendicular to the composition gradient direction...". 

2. The width of the strips is 0.2mm, which is not very small. Can the lateral x difference be neglected 
at this scale? The resultant error bars in x in their Fig.4 and Fig.5 should then be equal to the 
difference between x of each data points, i.e., +/- 0.25%. Please add the error bars. 

3. The WxV1-xO2 strips are grown on Si/ c-Al2O3, what is the role of the substrates? If the 
measurements are performed on single-crystalline, free standing WxV1-xO2, will they give the same 
result? If not, need a thorough discussion on and outlook of using substrate for this purpose. This is 
what is useful for readers. 

4. Which sample the data in Table. S1 corresponds to, WxV1-xO2/Si or WxV1-xO2/c-Al2O3? 



Reviewer #2 (Remarks to the Author): 

The idea of relating lattice compatibility tensors, specifically the middle eigenvalue and co-factor 
conditions across Martensitic transitions, to obtain a strain-free interface between the martensite and 
austenite, and to thereby suppress hysteresis has been demonstrated elegantly in previous work by 
these authors in the case of two shape memory alloy systems. These ideas of lattice compatibility are 
sought to be extended here to electron correlated oxides. While the article demonstrates 
phenomenological validation of the core ideas of the model to some extent, claims of universality of 
the this approach need must be tempered by the relatively modest suppression of the hysteresis (a 4K 
hysteresis remains even under the best conditions, which is orders of magnitude higher than 
demonstrated for the shape memory alloys or even previously achieve with VO2), the almost complete 
degradation of the magnitude of the metal—insulator transition for the optimal samples, absence of 
any demonstration beyond a single cycle, and the extremely slow scan rates utilized in this work. 
Below, I’ve provided a pointwise list of issues that ought to be addressed before I am able to 
recommend this work for publication in Nature Communications: 
(1) The entire premise of this work, as laid out in Figure S1, is that there are several monoclinic 
variants corresponding to the low-temperature polymorph and a rutile phase corresponding to the 
high temperature phase of VO2. What this work seems to neglect is the idea, now quite well 
established, of several energetically proximate polymorphs in VO2, e.g., the M2 and triclinic phases 
that are known to be stabilized under conditions of strain and are nucleated upon appraoching the 
phase transition. The idea of a “triple point” first established in work by Cobden also makes VO2 a 
rather distinctive system that is quite different both in terms of its structural phase diagram and strain 
coupling as compared to the shape memory alloys with which the authors make their comparisons. I 
provide here a partial list of articles: (a) Strelcov, E.; Ievlev, A.; Belianinov, A.; Tselev, A.; Kolmakov, 
A.; Kalinin, S. V., Local coexistence of VO2 phases revealed by deep data analysis. Scientific Reports 
2016, 6, 29216; (b) Braham, E. J.; Sellers, D.; Emmons, E.; Villarreal, R.; Asayesh-Ardakani, H.; 
Fleer, N. A.; Farley, K. E.; Shahbazian-Yassar, R.; Arròyave, R.; Shamberger, P. J.; Banerjee, S., 
Modulating the Hysteresis of an Electronic Transition: Launching Alternative Transformation Pathways 
in the Metal–Insulator Transition of Vanadium(IV) Oxide. Chemistry of Materials 2018, 30 (1), 214-
224; (c) Shi, Y.; Xue, F.; Chen, L.-Q., Ginzburg-Landau theory of metal-insulator transition in VO2: 
The electronic degrees of freedom. EPL (Europhysics Letters) 2017, 120 (4), 46003; (d) Tselev, A.; 
Luk’yanchuk, I. A.; Ivanov, I. N.; Budai, J. D.; Tischler, J. Z.; Strelcov, E.; Kolmakov, A.; Kalinin, S. 
V., Symmetry Relationship and Strain-Induced Transitions between Insulating M1 and M2 and Metallic 
R phases of Vanadium Dioxide. Nano Letters 2010, 10 (11), 4409-4416. 

Indeed, the stabilization of the M2 phase of VO2 upon W incorporation has been extensively 
documented and directly observed in studies of W-doped (or for that matter, undoped) VO2 
nanobeams. If lattice strain can drive a transition to entirely different polymorphs (M2 and T) and not 
just a twin variant (the ORNL group has shown using extensive symmetry analysis that the M2 
polymorph is not in fact a twin variant but a distinctive polymorph), the simple biphasic model of 
Figure S1 does not hold. Indeed, I suspect that if the authors were to undertake a detailed 
examination of the systems under consideration here, they would indeed find these polymorphs and it 
is the existence of these polymorphs that results in the substantial remnant hysteresis. 
(2) The X-ray diffraction data shown in the manuscript corresponds to a narrow range of 2-theta 
values and thus it is impossible to rule out the presence of M2 and triclinic phases within the films, 
which are especially expected to be observed when approaching the transition temperature. Given the 
relative low intensities of reflections, Raman microprobe analysis performed as a function of 
temperature might perhaps be the best way to examine whether the simplified view presented here of 
M1/T transformations, which ignores extensive work on M2 nucleation in VO2 thin films (particularly 
epitaxial films), is indeed credible. 
(3) I find it puzzling that the authors are reporting results of Rietveld refinements without actually 
showing their data and the quality of the fits. Based on the limited diffraction data that has been 
presented, I expect that large error bars are warranted for Figure 4. 
(4) The remnant hysteresis observed under the optimal conditions is >4K, which is substantially 
higher than results observed for the shape memory alloys and indeed this is a matter that deserve 
further comment. If the “happy accident” makes VO2 such a perfect material, why then cannot the 
hysteresis be reduced further. Indeed, the results here are entirely not a record even for VO2: for 
example, the following work has noted the almost complete elimination of hysteresis upon co-doping: 
Miyazaki, K.; Shibuya, K.; Suzuki, M.; Sakai, K.; Fujita, J.-I.; Sawa, A., Chromium–niobium co-doped 
vanadium dioxide films: Large temperature coefficient of resistance and practically no thermal 
hysteresis of the metal–insulator transition. AIP Advances 2016, 6 (5), 055012. I suspect the strain 
induced nucleation of other phases has a lot to do with the remnant hysteresis and ultimately 
represents a substantial limitation of this appraoch. 
(5) Two critical parameters need further elucidation: (a) what happens at faster scan rates and (b) 
during subsequent cycles? The hysteresis in W-doped VO2 is known to be scan rate dependent, 
reflective in some measure, the available nucleation sites. Further work has shown the kinetic 
asymmetry between the forward and reverse transitions derived from their distinctive origins (Fan, W.; 
Cao, J.; Seidel, J.; Gu, Y.; Yim, J. W.; Barrett, C.; Yu, K. M.; Ji, J.; Ramesh, R.; Chen, L. Q.; Wu, J. 
Large Kinetic Asymmetry in the Metal-Insulator Transition Nucleated at Localized and Extended 
Defects. Phys. Rev. B - Condens. Matter Mater. Phys. 2011, 83 (23), 235102-7.). Lattice strain 
derived from epitaxial matching strongly alters the formation of twin boundaries and as a result 
modifies the nucleation probabilities of the M2 and T polymorphs. I expect this explains the differences 
the authors observe between the films grown on Si and Al2O3. 
(6) In Figure 5b, there seems to be no formal metal—insulator transition. The transport data indicate 
a semiconductor-semiconductor transition. Can the authors explain the strong suppression of the 
magnitude of the transition and its abruptness (indicating a protracted phase coexistence regime) in 
these samples (which is not in fact observed for high-quality W-doped VO2 films and nanobeams). 
How repeatable are these measurements? Past calorimetry measurements reported by the authors 
have gone out to thousands of cycles but only singular sweeps are shown here. 
(7) In a similar context of the above point, many measurements of VO2 thin films and nanobeams 
indicate a distinctive step-like alteration of transport corresponding to switching of individual domains-
the extended and broad coexistence regimes observed here are likely due to a complex domain 
microstructure (can this be imaged?) and the stabilization of various polymorphs. If such texturation is 
indeed observed and given the substantial degradation of the magnitude of the phase transition, this 
would negate some of the intended benefits of hysteresis reduction. 
(8) Have the authors attempted to image domain evolution within these samples? Considerable work 
on phase contrast microscopy is now available and given the broad transitions, I expect that the 
diminished hysteresis has much to do with the facilitation of percolative mesoscale textures through 
this system. 
(9) As a semantic but nonetheless important point, VO2 is not a representative electron correlated 
material given the substantial role of phonons in underpinning the MIT (see for example, Budai, J.D., 
Hong, J., Manley, M.E., Specht, E.D., Li, C.W., Tischler, J.Z., Abernathy, D.L., Said, A.H., Leu, B.M., 
Boatner, L.A., McQueeney, R.J., and Delaire, O. (2014) Metallization of vanadium dioxide driven by 
large phonon entropy. Nature 515, 535–539). As such, extrapolating from VO2 to Mott systems 
wherein electron correlation effects are of paramount importance is entirely unjustified and not 
supported by any of the data presented here. 
(10) Can the authors provide example of representative WDS data and discuss the accuracy of this 
quantitation approach. I note that there are no horizontal error bars in Figure 4. 

Reviewer #3 (Remarks to the Author): 

The authors applied the non-linear theory of martensite to a strongly correlated oxide system V(1-
x)WxO2, showing that the hysteresis of the MIT is minimized when the cofactor conditions are 
satisfied. The minimized hysteresis may feature high reversibility of the material, which is desired in 
applications. 

This work provides a systematic way to reduce the hysteresis of the MIT in W-doped VO2 and 
improves our understanding of the hysteresis of the MIT. It will be of interest to the MIT research 
community. Below are my comments on this work. 

1. The local structure around the W dopant is a tetragonal-like structure [Scientific Reports 2, 466 
(2012)], which may serve as nucleation sites at phase transitions. This will have an effect on the 
hysteresis: more nucleation sites are likely to lead to smaller hysteresis. Compared to the lattice 
compatibility between phases, what is the role of this effect in regulating the hysteresis? 

2. Intuitively, the more deviation from 1 for bM1/aT, the larger lattice incompatibility between phases, 
and thus the larger hysteresis. This is however not the case according to Fig. 4 and Fig. 5. Can the 
authors discuss this point? Maybe this is related to the first comment, that is, besides the lattice 
compatibility aspect, there is another aspect (nucleation sites) that can influence the hysteresis. 

3. Figure 5(c) shows that the hysteresis of the polycrystalline V(1-x)WxO2/Si is larger than that of the 
epitaxial V(1-x)WxO2/Al2O3. But in general better crystals should exhibit larger hysteresis due to 
fewer nucleation sites. Can the authors give a more detailed explanation for the measured result?