Abstract:Room-temperature Raman scattering and x-ray diffraction measurements together with first-principles calculations were employed to investigate the behavior of disordered Pb(In 1=2 Nb 1=2 ÞO 3 (PIN) under pressure up to 50 GPa. Raman spectra show broad bands but a peak near the 380 cm À1 increases its intensity with pressure. The linewidth of the band at 550 cm À1 also increases with pressure, while two of the Raman peaks merge above 6 GPa. Above 16 GPa, we observe additional splitting of the band at 50 cm À1 . … Show more
The pressure-induced structural changes in the perovskite-type (ABO$$_3$$
3
) ferroelectric solid solution (1-x)Na$$_{0.5}$$
0.5
Bi$$_{0.5}$$
0.5
TiO$$_3$$
3
-xBaTiO$$_3$$
3
(NBT-xBT) at the morphotropic phase boundary (MPB) ($$x_{\text {MPB}}=0.048$$
x
MPB
=
0.048
) have been analyzed up to 12.3 GPa by single-crystal x-ray diffraction with synchrotron radiation. A pressure-induced phase transition takes place between 4.4 and 5.0 GPa, where the pseudocubic low-pressure phase transforms into an orthorhombic high-pressure phase with space group Pnma. The high-pressure phase is comprised of mixed BO$$_6$$
6
tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti$$^{4+}$$
4
+
cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT-$$x_\mathrm{{MPB}}$$
x
MPB
BT structure possesses anti-phase BO$$_6$$
6
tilts with a relatively large correlation length and the same type of polar distortions as those present in pure NBT, but with strongly violated correlation length due to Ba$$^{2+}$$
2
+
-induced local elastic-stress fields. For $$x_{\text {MPB}}$$
x
MPB
the effect of Ba on the mesoscopic-scale structure is compensated by a mild external pressure of only 0.7 GPa, resulting in structural features resembling those of pure NBT at ambient conditions.
The effects of hydrostatic pressure at ambient temperature on the structural and dielectric properties of PbFe0.5Nb0.5O3 (PFN) were investigated using second harmonic generation (SHG) and powder x-ray diffraction measurements to 31 GPa. The results demonstrate that PFN undergoes a pressure-induced structural transition from the R3m ferroelectric to the R3¯m paraelectric phase. SHG measurements showed a continuous decrease in the signal with pressure and complete disappearance at 7.1 GPa. Effective nonlinear optical coefficients were determined from the SHG data, and their pressure behavior was used to infer the nature of the transition. The loss of the SHG signal is accompanied by drastic changes in line widths of Bragg reflections, but no discontinuous change in volume was observed. The pressure-volume data were fit to various equations of state, and a bulk modulus K0 = 136 (±2) GPa, bulk modulus pressure derivative K0′ = 4.0 (±0.2), and initial volume V0 = 64.5 (±0.1) Å3 were obtained.
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