Conclusions

The major accomplishment of this work was the study on the influence of perovskite structure by substitution, size and strain effects on characteristics of ferroics, namely, Er³⁺-doped Pb0.8La0.2TiO3, Er³⁺-Yb³⁺ codoped ABO3 perovskites with different degrees of tetragonality (including PbTiO3, BaTiO3, and SrTiO3), BaTiO3 nanoparticles, and PbTiO3-CoFe2O4 multiferroics synthesized by chemical solution methods. All the results and discussions are summarized as follow:

Due to the substitution effect, the Ba1-xSrxTiO3 (BST) system exhibits structural ordering transition points at x~0.4-0.5 and x~0.75 and Pb1-xSrxTiO3 (PST) exhibits one structural ordering transition at x~0.5. Our investigations indicate that the lattice ratio c/a is around 1.002 for the undoped Pb0.8La0.2TiO3 (PLT) polycrystalline films, and slightly increases and then declines to around 1.001 at 7 mol% Er³⁺-dopant, and then increases to 1.007 for the samples with 9 and 15 mol% in sol-gel-derived Er³⁺-doped PLT polycrystalline films. However, our investigations also found that the maximum emission was observed at 7 mol% but no emission for Er³⁺ dopant larger than 9 mol%, which possesses the higher tetragonality than for the 7 mol% one.

From the results of Raman spectra, the peak attributed to A1(TO3)+E(TO3) phonon

modes keeps almost unchanged as Er³⁺ concentration increases from 0 mol% to 7 mol%, but vanishes for samples with 9 and 15 mol% Er³⁺ concentrations. The disappearance of Raman modes due to weak disorder effect (resulting from a displacement of Ti ion is caused by decreasing the crystal diameter to result in the formation of a centrosymmetric structure with increasing Er³⁺ concentration. Finally, our investigation show destruction to a displacement of Ti in the short-range structure by observing the disappearance of fluorescence emission and Raman signals when the Er³⁺ concentration exceeds 7 mol% in sol-gel-derived Er³⁺-doped PLT polycrystalline films.

For another study on the substitution effect, the mechanism of upconversion (UC) emission in Er³⁺-Yb³⁺ co-doped ABO3 perovskites was discussed. As increasing Yb³⁺ co-doped concentrations in 6 mol% Er³⁺ doped PbTiO3, BaTiO3, and SrTiO3

polycrystalline powder samples synthesized by sol-gel method, we have observed the room-temperature green UC emission at 550 nm being quenched by the simultaneously enhanced red UC emission at 660 nm under the 980-nm laser excitation. For codoping Yb³⁺ ions up to 6 mol% in PbTiO3 and only Er³⁺ doped PbTiO3 samples, which still possess relatively large tetragonality, the green UC emission is still much stronger than red one. In these cases, both the UC emissions are dominated by the two-photon process. But, as further increasing the Yb³⁺ ion

concentration, the crystal structures tend to become cubic phase with enhancing red UC emission and almost diminishing in green emission. Since the pure BaTiO3

crystal exhibits weaker tetragonality than PbTiO3, the stronger red emission and weaker green one were expected at the lower codoped Yb³⁺ concentration in BaTiO3

system than in PbTiO3 one. The observed quench of green radiation accompanied with enhancement of red radiation should be due to the efficient energy back-transfer (EBT) process by raising Yb³⁺ concentration. The efficient EBT process requires not only Yb³⁺ concentration but also level match of ⁴S3/2→⁴I13/2 in Er³⁺ and

2F7/2→²F5/2 in Yb³⁺ under assistance of Boltzmann distributed population within the manifold of ⁴S3/2 or/and ⁴I13/2 (Er³⁺) state affected by the crystal field with different symmetries. As a result, declining tetragonality results in the centrosymmetric crystal field for high Yb³⁺-ion concentration to achieve the level match required for the EBT process that may be difficult to be fulfilled with asymmetric crystal field in the tetragonal phase.

Additionally, we report, in contrast to the substitution effect, the size effect on structure for BaTiO3 nanoparticles synthesized by the glycothermal method. We applied a single tetragonal-phase model to refine the crystal structure and the coupled-phonon model to analyze the coupled A1(TO) modes upon particle size decreasing from 140 nm to 30 nm. Our investigations indicate that the weak strength

of coupling between A1(TO1) and A1(TO2) leads to a change from a spectral dip at 180 cm^-1 for A1(TO1) phonon to a peak at the same position and found that the approach of uncoupled2 and 3 with less tetragonality due to decreasing the particle size results in slightly increasing in 23. The larger coupling strength repels these two modes farther so that the less reducing in spectral separation. According to the results of decomposition, we also observed the decreasing LO-TO splitting with the decline of tetragonality and expansion of the unit-cell volume. And the change of unit-cell volume is the dominant mechanism for the tendency of the LO-TO splitting in BaTiO3

system without complication of ion replacement.

Finally, regarding the strain effect, the characteristics of three different geometric forms of multiferroics fabricated by the simple solution method has been investigated using optical microscopy (OM), field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), superconducting quantum interference device (SQUID), and micro-Raman spectroscopy. We report on the stress dependence of the behavior of interfacial phonon and the magnetic properties in three multiferroics consisting of the different geometric shapes of ferromagnetic CoFe2O4 (CFO) embedded in ferroelectrics PbTiO3 (PTO). Their energy of interfacial phonon and ferromagnetic properties depend on stress due to not only the lattice misfit but also the degree of chemical bonding at the interface between CFO and PTO matrices. The

disk-3 type structure, the self-assembled CFO disks embedded in PTO matrix, illustrates the strong elastic interactions between the two phases. The larger nonsymmetrical coercivity Hc and the absence of saturation magnetization Ms of CFO matches with the larger Raman shift of A1(TO2) and A1(TO3) modes of PTO found in the disk-3 type than the other types of the CFO and PTO multilayered structure and the CFO particles embedded in PTO matrix.