Phase transitions in monoclinic ZrO-=SUB=-2-=/SUB=-

Abstract

Zirconia ZrO2 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 (N 14), which undergoes an extended martensitic transformation into the tetragonal form P4_2/nmc (N 137) when heated above 1000oC. 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±20 and 730±20oC. This agrees with the literature data, which demonstrate singular points on the temperature dependences of the ZrO2 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 900oC 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. Keywords: ZrO2, monoclinic phases, phase transitions, dilatometry, electrical conductivity, Raman spectroscopy.