Guirong Liu

Solid solution Pb1-xSrxTiO3 and its thermal expansion

Abstract The solid solution limit of Pb 1− x Sr x TiO 3 was determined in the composition range of 0≤ x ≤1.0 at room temperature (RT). The phases were isolated and indexed in a tetragonal system with x x ≥0.5. The cell parameters of Pb 1− x Sr x TiO 3 continuously, but nonlinearly, change with solubility x . The intrinsic thermal expansions of the solid solution compounds Pb 1− x Sr x TiO 3 ( x =0, 0.15, 0.20, 0.50, 0.90, 1.0) were obtained in the temperature range from RT to 1173 K with high-temperature X-ray powder diffraction. Negative thermal expansion coefficients of Pb 1− x Sr x TiO 3 ( x =0, 0.15, 0.20) were found below the Curie points. The thermal expansions of these titanate ceramics were highly correlated with the solubility in the solid solution Pb 1− x Sr x TiO 3 .

Structure and enhancement of negative thermal expansion in the PbTiO3–CdTiO3 system

The structures of Pb1−xCdxTiO3 (x=0.03 and 0.06) were investigated by the x-ray Rietveld method at room temperature. It is surprising to find that the spontaneous polarization displacements of Pb∕Cd and Ti atoms according to the oxygen polyhedron decrease, although the c∕a ratio increases with doping-cadmium content. Cadmium substitution plays a unique role of enhancement of the negative thermal expansion in PbTiO3. The average bulk thermal expansion coefficient decreases from a¯=−1.99×10−5∕°C for pure PbTiO3 to a¯=−2.40×10−5∕°C for Pb0.94Cd0.06TiO3. The negative thermal expansion of PbTiO3 might be a consequence of hybridization between Pb and O atoms.The structures of Pb1−xCdxTiO3 (x=0.03 and 0.06) were investigated by the x-ray Rietveld method at room temperature. It is surprising to find that the spontaneous polarization displacements of Pb∕Cd and Ti atoms according to the oxygen polyhedron decrease, although the c∕a ratio increases with doping-cadmium content. Cadmium substitution plays a unique role of enhancement of the negative thermal expansion in PbTiO3. The average bulk thermal expansion coefficient decreases from a¯=−1.99×10−5∕°C for pure PbTiO3 to a¯=−2.40×10−5∕°C for Pb0.94Cd0.06TiO3. The negative thermal expansion of PbTiO3 might be a consequence of hybridization between Pb and O atoms.

Structural investigations on ferroelectric Pb 1−3/2 x La x TiO 3 using the x-ray Rietveld method

The structures of the defect perovskite Pb1-3/2xLaxTiO3 prepared by the solid-state method were investigated in the solubility range of 0.05 less than or equal to x less than or equal to 0.30 with 0.05 increment by x-ray Rietveld method. In the system Pb1-3/2xLaxTiO3, the atom displacements of Ti (delta(Ti)) and Pb/La (delta(Pb/La)) along the spontaneous polarization (P-s) direction (c axis) decrease nonlinearly with increasing La content, while the value of delta(Ti)/delta(Pb/La) decreases linearly. The shape of oxygen octahedron of compounds Pb1-3/2xLaxTiO3 is independent of the La content. The calculated value of P-s decreases linearly in the solubility range of 0.05 less than or equal to x less than or equal to 0.30. In the refinement process, the hkl dependence of diffraction line broadening was also taken into account. The anisotropy of microstrain-like broadening observed in Pb1-3/2xLaxTiO3 might be ascribed to occurrence of compositional inhomogeneity between the crystallites.

BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Abstract Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3-xBiFeO3 (x=0∼0.07) were synthesized by the solid-state reaction. Differential scanning calorimetry (DSC) measurements revealed that an increase in the amount of BiFeO3 dopant resulted in a decrease in the orthorhombic-tetragonal and tetragonal-cubic phase transition temperature of the material. One percent BiFeO3 additive suppressed grain growth, which not only benefits the sintering of ceramics but also enhances the piezoelectric and ferroelectric properties, where d33=145pC/N, kp=0.31, Qm=80, Pr=11.3 μC cm−2 and Ec=16.5 kV cm−1. As xBF>0.01, both piezoelectric and ferroelectric properties decreased rapidly with an increasing amount of dopant.

Neutron diffraction studies of structure and increasing splitting of LO-TO phonons in Pb1−xCdxTiO3

Pb1−xCdxTiO3 (0.0⩽x⩽0.08) compounds in the tetragonal phase were prepared by the solid state reaction. Neutron powder diffraction study revealed that Pb1−xCdxTiO3 exhibits an unusual structural property. Although the tetragonality (c∕a) is enhanced by Cd substitution for Pb, the cation displacement of spontaneous polarization unexpectedly decreases, which is associated with the reduced Curie temperature. The increasing of the LO-TO splitting in Pb1−xCdxTiO3 shows an anomalous behavior in comparison with other previously studied PbTiO3-based systems. It was interestingly found that a similarity exists in the behavior of the solubility dependence of the LO-TO splitting and the negative thermal expansion (NTE) in the PbTiO3-based systems, which suggests that Pb–O hybridization plays an important role in NTE mechanism of PbTiO3.

Molten salt synthesis and phase evolution of Ba(Cd1/3Nb2/3)O3

Abstract Ba(Cd1/3Nb2/3)O3 powder with an octahedral shape stacked by small particles was successfully synthesized by the molten salt synthesis method at a low temperature of 850 °C, which was much lower than the columbite method, two-step solid-state reaction process. Thermogravimetry and differential thermal analysis curves combined with X-ray diffraction analysis determined the phase evolution of Ba(Cd1/3Nb2/3)O3 powder at relative high temperature. Furthermore, Raman results showed that the Ba(Cd1/3Nb2/3)O3 powder had a trigonal order structure. It was indicated that the “template formation mechanism” and crystal habit growth which obeyed the model of anion coordination polyhedral, play an important role in the formation and growth processes. The resulting ceramics exhibited favorable dielectric properties.

Crystal structure of microwave dielectric ceramics Ba[(Mg 1− x Cd x ) 0.33 Nb 0.67 ]O 3

A series of complex perovskite solid solutions of Ba[(Mg1-xCdx)(0.33)Nb-0.67]O-3 have been synthesized by the columbite method. Detailed Rietveld refinement of their X-ray diffraction data show that Ba[(Mg1-xCdx)0(.33)Nb(0.67)]O-3 has an order trigonal structure. The ordering degree as determined by the B-site occupancies increases with the partial substitution of Cd for Mg.