Solid/solid phase transition prediction

2000/12/01 Igartua Aldamiz, Josu M. Iturria: Elhuyar aldizkaria

• Fisika

Asking anyone on the street what phase transition (F-T) is, he will answer that he does not know. However, anyone knows that water heating, under certain conditions, will cause evaporation. Or in winter, at night (usually), for example, that the water on the roads freezes. If you remember that, people before you, of course! will say, refers to the transformation of liquid to steam or liquid to solid. Yes. But these are not the only transformations that occur in nature, by themselves or by human causes, that is, between the "states of matter" (solid, liquid or gas). In addition, although these are important, they are more interesting.

This is the case for solid/solid transformation types (see Figure 1). Solid/solid F-T are very important in technology, as changes in the internal structure can lead to technologically usable physical phenomena. Therefore, it is important to analyze them and under what conditions they will appear. This requires an experimental analysis. And, without experimentally analyzing, under what conditions will predictions appear if possible? It would save a lot of work (money). This is one of the objectives of the project we are working on: We seek conditions to predict certain F-T.

Several methods have been proposed for F-T prediction. For example, the study conducted by the Abrahams-Kurtz-Jamieson trio in 1968 revealed the relationship between the displacements of atoms applicable to certain substances and F-T. The materials analyzed were ferroelectric, and applying this relationship have been found in recent years a hundred new ferroelectric substances. The method consisted of pseudosimetry (see Figure 2). This is the method we are using. However, our work is systematic: we use the database information of atomic structures, and we search pseudosimetry using the theory of spatial groups. However, the discovery of pseudosimetry does not necessarily imply the experimental search for F-T, but the method takes into account only the geometric conditions and not the physical conditions. We talk about the candidates. Once the candidates are appointed, an experimental analysis of the substances will be carried out in order to detect the appearance or not of the possible F-T announced.

So far we have worked with three space groups: P212121, Pnma and I4/m. In total we have looked for pseudosimetry in 2,000 substances and have proposed a hundred new candidates.

Right now we are experimentally studying the third group. We speak of a single family of compounds, the A2BB'O6 family, the double psedoperovskite called elpasolite (where A=Ba, Sr, B=Co, Ni, Fe, Zn, Cu, Mn, Mg, Ca, Cd, W=Mo, Te, W). A total of 51 compounds, among which 10 have been synthesized. The DSC technique (differential sweep calorimetry) has been used for F-T observation. X-ray measurements have been performed for the analysis of the different phases.