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I'm from a CS background and just trying to understand the real meaning behind multi phase materials. Following is the experiment which drove me towards this question.

With the help of DFT and machine learning, we identified that the material LaBiO3 could display interesting dielectric properties. Hence, a team here mixed the oxides in stoichiometric ratio in a ball mill and observed the XRD patterns of the formed composite powder.

The XRD data suggest that LaBiO3 is unstable and form Bi1.4La0.6O3 R-3m phase and Bi2La4O9 R3m phases.

So my question is, is this a composite that has nothing to do with LaBiO3 and they are just two different materials? Or could this be categorized as a metastable phase of LaBiO3?

Could anyone explain this with the definitions?

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  • $\begingroup$ La2O3 and Bi2O3 have different crystal structures. This suggests fairly strongly that a (BiLa)O3 phase will not be stable. $\endgroup$
    – Jon Custer
    Jul 10, 2019 at 18:02

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The definition of a substance that is "multi-phase" is that there is more than one distinct compound in it and the compounds form distinct regions in the substance with different properties.

Examples make this clearer. Salt dissolved in water forms a uniform mixture: a salt solution. There is more than one compound there but they are perfectly mixed.

Fatty organic compounds are not soluble in water but can form small dispersed fatty particles. We call the result milk (I'm simplifying a little as there are other things in there as well). The result has more than one phase: a microscope would reveal very small, dispersed particles of fat floating in water. Milk looks like a single uniform substance, but isn't when you look more closely.

Fibreglass is, essentially, a bunch of small fibres made of glass embedded in a matrix of some polymer. There are two distinct phases with different compositions in the substance.

The characteristic of multi-phase compounds is that different regions of the substance contain different compounds (these regions can be very small as in some metal alloys). In your case the reaction seems to produce two distinct compounds which may be finely mixed together. The XRD pattern shows two different crystal structures. This may result because this specific reaction does not create the compound you wanted under these conditions. Which could be because the compound you want is not stable or could be because the reaction conditions allow more than one product to form. Either way the reaction is giving a mixture not a single compound.

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  • $\begingroup$ Thanks @matt_black for the reply, it does simplify the term "multi-phase". So just to make sure I'm getting this right, salt solution you mention is single phase, result milk is multi-phase but looks like a uniform substance (clarification asked below) and fibreglass is multi-phase with visible signs to the outside? If so, does this result milk show uniform physical properties as well? like uniform density, viscosity and all? If it does we can clearly differentiate these from fibreglass kind of compositions. $\endgroup$
    – Achintha Ihalage
    Jul 11, 2019 at 17:13
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    $\begingroup$ @avin "Uniform" depends on what scale you measure on. Milk looks uniform in bulk, but not when you look closely enough. Then you see droplets of oil which have different density and viscosity to the water than surrounds them. $\endgroup$
    – matt_black
    Jul 11, 2019 at 22:35
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This is actually a super important concept in chemistry. Without belaboring it. There is a difference between having a mixture of different substances (e.g., carbon and oxygen, graphite and $\ce{O2}$ molecules) and having a new compound, the carbon and oxygen reacted to make $\ce{CO2}$.

In solid state compounds, this can sometimes be confusing, because there is no big change evident (usually) in terms of a gas being generated or the like. So, for instance (true story), I found $\ce{ZnSnO3}$ reported as a "compound" in the powder diffraction database. But when I looked at the spectrum--and I was intimately familiar with this phase space--I saw that the spectrum of "$\ce{ZnSnO3}$" exactly matched a mixture of $\ce{ZnO}$ and $\ce{Zn2SnO4}$ (known compounds). An email to the JCPDF database got the spurious $\ce{ZnSnO3}$ deleted.

The reason I am telling this story is it is directly analogous to what your guys are going through. And it is not common, but also not vanishingly rare, in solid state chemistry to get spurious new compound reports based purely on powder x-rays (even when just looking at that would tell you, you have a mixture). This is especially the case when people are "trying" to get a certain new compound. Lesson is....watch out for mixtures.

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