# Sintering of Yttrium barium copper oxide

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?

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.