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I will do an experiment on superconductors, specifically $\text{YBa}_2\text{Cu}_3\text{O}_7$, and I am curios about some steps in the preparation. The starting materials are $\text{Y}_2\text{O}_3$, $\text{BaCO}_3$ and $\text{CuO}$.

First, all three starting materials will be crushed in a mortar. After that, the powder will be heated in the furnace for a few hours and cooled down. What exactly happens here? Does heating the powder give us a better crystal structure of $\text{YBa}_2\text{Cu}_3\text{O}_7$ which we already got through crushing the materials in the mortar? Or does heating the materials give us $\text{YBa}_2\text{Cu}_3\text{O}_7$?

Second, after it is cooled down it will be crushed and formed into pellets. The pellets are then going to be heated in an environment which purely consists of oxygen. Heating it in oxygen apparently turns our $\text{YBa}_2\text{Cu}_3\text{O}_7$ into $\text{YBa}_2\text{Cu}_3\text{O}_{7-x}$. Why does our material lose oxygen when heated in an oxygen environment?

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For the first question: Crushing the precursors in a mortar does not create YBCO. It only creates tiny particles of each precursor. Heating to high temperatures allows the tiny particles to chemically react into YBCO. Tiny particles are much more reactive that large chunks as the surface to volume ratio is much higher. So you are exposing more of each particle composition to the reaction. Chances are that once you crush the precursors, heat, cool down slowly (to give time to the reaction), and check for the composition (x-rays diffraction for instance), you will find out that all is not reacted. You will have to get this new mixture and start over as many times until you get a single phase.

For the second question: YBCO is stable with any amount from 6 to 7 atoms of oxygen per unit formula. YBCO$_6$ is insulating. From the top of my mind, superconductivity appears around 6.3-6.4 atoms of oxygen per unit cell. The highest (optimal) $T_c$ (~91K) has something close to 6.9 atoms of oxygen per unit cell. So, once you synthesize YBCO, you have to carefully treat it in a controlled oxygen atmosphere to get the proper amount of oxygen inside. Note that there are recipes to get to any amount of oxygen in the YBCO$_6$ - YBCO$_7$ range.

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