Abstract:
Zirconia ZrO$_2$ is one of the most important modern materials, but its use faces the problem of complex polymorphism. It is generally accepted that, under normal conditions, it exists in the monoclinic form $P2_1/c$ (No 14), which undergoes an extended martensitic transformation into the tetragonal form $P4_2/nmc$ (No 137) when heated above 1000$^\circ$C. However, the group-theoretic analysis forbids a direct transition between these space groups and requires at least two intermediate forms. The experiment carried out in this work using structure- sensitive methods (dilatometry and electrical conductivity) indeed revealed the existence of singular points on the temperature dependences of expansion and conductivity in the temperature range of 350 $\pm$ 20 and 730 $\pm$ 20$^\circ$C. This agrees with the literature data, which demonstrate singular points on the temperature dependences of the ZrO$_2$ monoclinic lattice parameters. According to the dilatometry data, these transitions are phase transitions of the second order, which correlates with the obtained Raman data (measurements at temperatures up to 900$^\circ$C in the range 60–900 cm$^{-1}$), showing that the local structure changes slightly, therefore, the transitions occur within the framework of the monoclinic syngony.
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