Abilio Almeida

The role of the structural distortions on the metal to insulator transition (MIT) of NdNiO3 is still a controversial topic. NdNiO3 ceramic, 260 nm and 110 nm thin films deposited onto (001)-oriented LaAlO3 substrates were studied to unravel the effect of structural symmetry breaking on the electronic phase transition. Resistivity measurements evidence the coexistence of metallic and insulating states below the TMI, while magnetization measurements confirm the expected TMI = TN for all samples. The structural symmetry was investigated across the MIT, through Raman scattering spectroscopy, with the orthorhombic Pnma phase being found at high temperatures, but the emergence of the monoclinic P21/n phase only occurring well below TMI, which is an unexpected experimental result since the structural transition has been understood as a necessary trigger for the MIT. These experimental results, which are noticeably independent of the sample type/quality, imply a temperature difference between the structural and electronic/magnetic ordering, thus contrasting with current theoretical models.

Alkaline niobate and tantalate perovskites have attracted attention as polar dielectrics for electronics and telecommunications. Here, we studied the polar behaviour, lattice dynamics, and local structure in conventionally processed K_0.985Mn_0.015TaO_3±δ ceramics using a combination of variable-temperature dielectric and Raman spectroscopies, and X-ray absorption fine structure (XAFS) measurements, respectively. Mn doping induces a low-frequency dielectric relaxation in KTaO₃ (KT), which follows the Arrhenius law with an activation energy U ≈ 105 meV and the characteristic relaxation time τ₀ ≈ 4.6 × 10⁻¹⁴ s. Our XAFS results support preferential Mn occupancy of the cuboctahedral sites as Mn²⁺, with these cations strongly off-centred in the oversized oxygen cages. Such disordered Mn displacements generate electric dipoles, which are proposed as the source of the observed dielectric relaxation. We show that in Mn-doped ceramics, the low-frequency polar TO1 mode softens on cooling and, at low temperatures, exhibits a higher frequency than in undoped KT. This mode displays no detectable splitting, which contrasts with Li-doped KT that also contains off-centred Li⁺ species on the cuboctahedral sites. Therefore, we conclude that the coupling between the Mn displacements and the lattice is weaker than in the Li case, and Mn-doped KT therefore exhibits a dielectric relaxation but no ferroelectric transition.

YMnO₃ thin films were prepared based on a chemical solution deposition route. Their structure was studied as a function of the annealing temperature using X-ray diffraction, Raman spectroscopy, and electron backscatter diffraction. For annealing temperatures above 850°C the films, though crystallizing predominantly in a P6₃cm hexagonal structure, exhibit a thin Pnma orthorhombic phase close to the film-substrate interface. It is shown that the residual orthorhombic structure is associated with a substrate induced strain state, characterized by an expansion in the in-plane a direction, and a compression in the out of plain c direction.

Oxygen deficient KTaO₃ thin films were grown by RF magnetron sputtering on Si/SiO₂/Ti/Pt substrates. Room temperature X-ray diffraction shows that they are under a compressive strain of 2.3%. Leakage current results reveal the presence of a conductive mechanism, following Poole-Frenkel formalism. The existence of a polar response below T_pol ∼ 367°C was ascertained by dielectric, polarization, and depolarization current measurements. A Cole-Cole dipolar relaxation was evidenced which is associated with oxygen vacancies induced dipoles. After annealing the films in an oxygen atmosphere above T_pol, the aforementioned polar response is suppressed, due to significant oxygen vacancies reduction.