Publicação
Diffraction and microscopy studies of novel functional complex oxides obtained via high pressure synthesis
| Resumo: | The demand for alternatives to lead-based materials will increase in the future, along with multiferroic materials and improved manufacturing techniques. This PhD work focuses on the broad investigation of the structure, magnetic and electromechanical properties of novel oxide materials. Many potentially promising Bi-containing complex perovskite oxides are hard to process via conventional methods. High-pressure synthesis is one of the few approaches to produce such compounds. After synthesis these materials can remain at ambient pressure and temperature in a metastable state. Based on the revision of the Bi-based systems already studied, new systems were considered and investigated in detail. In the BiCrO₃–BiScO₃ system, a substitution of 10 at% Scᶟ⁺ was considered. The monoclinic C2/c structure was preserved, with a small increase in volume of the crystal cell. The phase transitions observed in BiCrO₃ were still present in BiCr₀.₉Sc₀.₁O₃, although at higher temperatures and a wider transition range, which were also observed in the complex dielectric permittivity. The substitution also severely disrupted the magnetic ordering, retaining the antiferromagnetic order but with a lower Néel temperature (85 K) as compared to 110 K in undoped BiCrO₃. The solid solutions between the rhombohedral (R3c) BiFeO₃ and the tetragonal (P4mm) BiZn₀.₅Ti₀.₅O₃ was studied in the whole substitutional range. Up to 25 at% [Zn₀.₅Ti₀.₅] the R3c structure endured, with gradual increase in volume of the unit cell. Between 30 and 90 at% [Zn₀.₅Ti₀.₅] the R3c and P4mm phases coexist. In the same work, the magnetic properties and stability temperatures were compared for the various isovalent substitutions of the BiFeO₃-based perovskites. The BiMg₀.₅Ti₀.₅O₃–BiZn₀.₅Ti₀.₅O₃ system was studied in the full compositional range and a morphotropic phase boundary (MPB) was identified at 75 at% [Zn₀.₅Ti₀.₅]. A ~5 % jump in volume occurs at a crossover from the orthorhombic (Pnnm) to the tetragonal (P4mm) phase. It was shown that annealing above 573 K results in irreversible changes in the phase diagram of this system. Namely, for compositions with [Zn₀.₅Ti₀.₅] content between 20 and 60 at%, the initial orthorhombic phase transforms into a ferroelectric rhombohedral (R3c) phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower [Zn₀.₅Ti₀.₅] content of 60 at. %. The phase diagram of the annealed BiMg₀.₅Ti₀.₅O₃ BiZn₀.₅Ti₀.₅O₃ ceramics is formally analogous to that of lead zirconate titanate. This makes this lead-free system promising for the development of environmentally friendly piezoelectric ceramics. |
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| Autores principais: | Cardoso, João Pedro Veleirinho |
| Assunto: | Perovskite High-pressure synthesis Phase transitions Conversion polymorphism In situ studies |
| Ano: | 2023 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Aveiro |
| Idioma: | inglês |
| Origem: | RIA - Repositório Institucional da Universidade de Aveiro |
| Resumo: | The demand for alternatives to lead-based materials will increase in the future, along with multiferroic materials and improved manufacturing techniques. This PhD work focuses on the broad investigation of the structure, magnetic and electromechanical properties of novel oxide materials. Many potentially promising Bi-containing complex perovskite oxides are hard to process via conventional methods. High-pressure synthesis is one of the few approaches to produce such compounds. After synthesis these materials can remain at ambient pressure and temperature in a metastable state. Based on the revision of the Bi-based systems already studied, new systems were considered and investigated in detail. In the BiCrO₃–BiScO₃ system, a substitution of 10 at% Scᶟ⁺ was considered. The monoclinic C2/c structure was preserved, with a small increase in volume of the crystal cell. The phase transitions observed in BiCrO₃ were still present in BiCr₀.₉Sc₀.₁O₃, although at higher temperatures and a wider transition range, which were also observed in the complex dielectric permittivity. The substitution also severely disrupted the magnetic ordering, retaining the antiferromagnetic order but with a lower Néel temperature (85 K) as compared to 110 K in undoped BiCrO₃. The solid solutions between the rhombohedral (R3c) BiFeO₃ and the tetragonal (P4mm) BiZn₀.₅Ti₀.₅O₃ was studied in the whole substitutional range. Up to 25 at% [Zn₀.₅Ti₀.₅] the R3c structure endured, with gradual increase in volume of the unit cell. Between 30 and 90 at% [Zn₀.₅Ti₀.₅] the R3c and P4mm phases coexist. In the same work, the magnetic properties and stability temperatures were compared for the various isovalent substitutions of the BiFeO₃-based perovskites. The BiMg₀.₅Ti₀.₅O₃–BiZn₀.₅Ti₀.₅O₃ system was studied in the full compositional range and a morphotropic phase boundary (MPB) was identified at 75 at% [Zn₀.₅Ti₀.₅]. A ~5 % jump in volume occurs at a crossover from the orthorhombic (Pnnm) to the tetragonal (P4mm) phase. It was shown that annealing above 573 K results in irreversible changes in the phase diagram of this system. Namely, for compositions with [Zn₀.₅Ti₀.₅] content between 20 and 60 at%, the initial orthorhombic phase transforms into a ferroelectric rhombohedral (R3c) phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower [Zn₀.₅Ti₀.₅] content of 60 at. %. The phase diagram of the annealed BiMg₀.₅Ti₀.₅O₃ BiZn₀.₅Ti₀.₅O₃ ceramics is formally analogous to that of lead zirconate titanate. This makes this lead-free system promising for the development of environmentally friendly piezoelectric ceramics. |
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