Phase Transitions in Antiferroelectric PbHfO3

Phase transitions in ceramic PbHf${\mathrm{O}}_3$ have been studied by dielectric and structural measurements. The dielectric constant is about 90 at room temperature, and its temperature dependence shows a small anomaly at 163°C and a pronounced peak of 540 at 215°C. The $P-E$ relation, however, is almost linear, showing no ferroelectric hysteresis loops within this temperature range. At room temperature PbHf${\mathrm{O}}_3$ has a tetragonal lattice of the perovskite type with $a=4.136\mathrm{}\mathrm{A}$ and $\frac{c}{a}=0.991\mathrm{}$, and a powder x-ray photograph shows some superstructure lines which have essentially the same character as those of PbZr${\mathrm{O}}_3$. This shows that PbHf${\mathrm{O}}_3$ is an antiferroelectric of the PbZr${\mathrm{O}}_3$ type below 163°C. The crystal structure between 163°C and 215°C is also based on a tetragonal lattice; but the axial ratio $\frac{c}{a}$ is much closer to unity (0.997), and the observed superstructure lines are different from those of the lowest phase. Thus the intermediate phase is another antiferroelectric phase, with a different type of dipole arrangement from that of the lowest phase. At 215°C, it becomes paraelectric, accompanied by a change to a cubic structure.