We have observed smaller volume in nanoparticles of half-doped manganites LCMO (La0.5Ca0.5MnO3) and PCMO(Pr0.5Ca0.5MnO3). We explained this by the effective surface tension of the nanoparticles. This effect is similar to the pressure effects. Is it a general tendency or have you observed contrary results?
Hi Tapan, what I have seen in my experiments in the contrary tendency: the smaller the nanoparticles the bigger the unit cell. At least, this is what clearly happens with apatite. apparently, at least in this case, is that there are more defects if the nanoparticles are very small, so the atoms requiere slightly more space. As long as the particles size increases, the structure relaxes, making the unit cell smaller. This could be an explanation.
I don't know if this is a general tendency, but I can tell you without any doubt that I clearly saw that in those compounds.
So my idea of surface tension reducing the volume may not be the whole story then. I guess it contributes only partly. The defect may be the other contribution.
Can one model these two contribution and estimate what the total contribution should be?
Well, I must say that in the case of the particles I studied, there was like a coating by molecules sorrounding the nanoparticles, so this could reduce the surface effect in this case. So myabe this is different in your case.
Anyway I would expect bigger cells for smaller nanoparticles due to the defects inside the unit cell (being only a few cells in the nanoparticle)
If your sample is Ca5(PO4)3(OH)0.33F0.33Cl0.33 then I guess it is difficult to compare nano volume with that of the bulk. How good can you control the OH, Fl and Cl stoichiometry during synthesis?
For semiconductor nanoparticles unit cell in NC is usually smaller than in bulk.
That is in agreement with our results on half-doped manganites and also MnO;
I wonder whether one can make some theory of the volume difference between the bulk and nano particles. Since nonoparticle volume can be either smaller or larger it appears to me that there exists two contribution to it and depending on the substance one wins over the other. Is the idea of surface tension in nanoparticles causing volume contraction justified?
One more possible contribution is a transition to the quasicrystalline order at the size reduce. e.g. lack of translational symmetry at small dimensions changes "unit cell" concept completely. This situation can be observed as a lot of defects, cause difference between crystal and quasicrystal can be interpreted as defects by definition, and this is correct. Also, experimental detection of quasicrystalline transition is a five-fold symmetry observation in a diffraction pattern (or other non-crystal symmetry: n>=7).
Has anyone reported such crystalline to quasicrystalline transition with size reduction? I shall be much interested. I mean of course whether anyone has seen electron diffraction pattern having 5-fold or 10-fold symmetry or other symmetries incompatible with translational symmetry?
I cannot show good review publication right now (if it exists), but can share some occasional references. Generally, on post-soviet states I've seen a lot of such reports: both experimental and theoretical (even, complete symmetry classification of nanoparticles structure opposed to bulk).
Regarding experimental one, see for instance [1] - the investigation of fcc vs. icosahedron structure of Ni nanoparticles.
I personally have been worked a lot with symmetry-related theories (including magnetic symmetry and magnetoelectric materials), so if you wish to investigate all this deeply, please, contact me, I can contact my Ukraine and Russian colleagues, which reported such kind of results, to collect complete review.
[1] http://link.springer.com/article/10.1134/1.1688426
The paper you refer gives only Monte Carlo simulations. I shall be pleased to see some electron diffraction picture of such clusters.
In my opinion you should have a higher distortion of the unit cell in nanoparticles of half-doped manganites LCMO (La0.5Ca0.5MnO3) and PCMO(Pr0.5Ca0.5MnO3), that can contribute for a reduction of volume.
I'd suggest looking for a pattern. I'd imagine the unit cell dimensions (unit volume) scaling with the surface area to volume ratio. So try making your nanoparticles with different diameters (sizes), as close to the same conditions for each. Then plot S/V ratio to unit volume. If you see a pattern (linear perhaps), then the two can be related. And this provides a reasonable argument.
Yes that is a very good idea. In fact we have done that for half-doped manganites and we found a systematic reduction of the cell volume with the decrease of particle size. Unfortunately we had data for bulk and nanoparticles of only three different sizes. Similar results have also been reported for nanocrystalline ferroelectric semiconductor GeTe.
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