Evidence for anomalous pressure dependence of the spontaneous strain in PbTiO3

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Abstract
The a and c lattice parameters of tetragonal PbTiO3 have been measured by X-ray diffraction techniques, using a diamond-anvil cell, up to a pressure of 6.35(5) GPa at room temperature. The a lattice parameter is found to show a small expansion with pressure up to approximately 1.7 GPa, beyond which da/dP then becomes negative (as is more usual). The compressibility is highly anisotropic: between atmospheric pressure and 6.35 GPa the c parameter is compressed more than ten times as much as the a parameter. The spontaneous strain, z12/=(c/a-1)12/, exhibits significantly anomalous behaviour: the magnitude of d(z12/)/dP decreases with pressure, from -3.4(2)*10-2 GPa-1 at low pressures to about -1.3*10-2 GPa-1 at 6.0 GPa. There is some evidence that this can be interpreted as a discontinuity in the behavior of PbTiO3 at approximately 1.7 GPa, close to the pressure where da/dP passes through zero.