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The dance of crystal structures


The word “crystal” is a technical term; iron and steel, for example, are crystals whereas glass is not. In fact, "crystal" means materials of a crystalline structure.

Just like any other kind of material, crystals can change their structure. For example, if the temperature rises sufficiently, it passes from a solid to a liquid state. But other, not so noticeable, structural changes also take place, such as those that occur in the solid state, itself. These changes are known as solid-to-solid phase transitions and are induced by changes in either temperature or pressure. Moreover, the electrical and magnetic properties of the crystals are affected during these transitions and are, thereby, of great interest for technology.

At the Leioa (Bizkaia) campus of the University of the Basque Country (EHU), a research team has been analysing solid-to-solid transitions of crystals. They selected a group of crystals known as double perovskites for this purpose. Prior to the analysis a certain amount of preparation work is required in the lab: the perovskites have to be synthesised.

Synthesis of crystals in the laboratory

The synthesis of crystals in the laboratory is not a simple process. First, the component powders of the crystal have to be mixed and compacted and then they have to be stabilised in the kiln. For example, to mention one method of synthesis, in order to obtain the Sr2NiWO6 perovskite, SrCO3, NiO and WO3 powders have to be blended. The result of the blend will also be a powder - perovskite powder.

But the resulting perovskite is not always the desired one and this is why it is necessary to characterise the compound obtained once it has been synthesised. That is, in this example, it has to be confirmed that it is, in fact Sr2NiWO6 perovskite and not another one, or, at least the same perovskite but with a few impurities.

Finally, once the desired perovskite has been obtained, various techniques are applied: X-ray diffraction, neutron diffraction, synchrotron radiation, Raman spectroscopy, etc. With all these, information about the crystal structure is obtained - location of the elements, their vibration frequency and a number of other properties. In order to carry this out, moreover, researchers have to travel to France and the United States, given that, in the University of the Basque Country, there is no synchotron; neither can Raman spectroscopy be carried out.

Solid structure is adaptable

So, the solid structure of the crystal is adaptable. But how? For example, when pressure increases, the structure of the crystal compacts so crystal atoms are nearer to each other. This may produce an increase in the vibration frequency of these atoms. At some point, the stability of the structure will break and change the location of components, thus, a solid-to-solid phase transition will have taken place.

Must be remenbered that these transitions are often accompanied by changes in the electrical and magnetic properties of the crystals: for example, the conductivity of the crystal can change when it reaches a certain temperature. The work at Leioa is targeting those values of temperature and pressure accompanied by transitions in the perovskites.

Garazi Andonegi | alfa
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