Papers by Author: Francesco Cordero

Abstract: Anelastic and dielectric relaxation measurements have been carried out on poled and unpoled samples of the ferroelectric perovskite (Na_0.5Bi_0.5)_1−xBa_xTiO₃ (NBT-xBT), with composition in the range between pure NBT and the morphotropic phase boundary, 0 ≤ x ≤ 0.08. The complex elastic compliance spectra contain clear indications of both the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the phase diagram, which is difficult to obtain by diffraction techniques due to the very low distortions in both the tetragonal and rhombohedral phases and to the structural disorder in the Na/Bi sublattice. An extensive study is made for concentrations in the region of the morphotropic boundary (x ∼ 0.06) in order to find possible signatures of monoclinic phase, as for the case of PbZr_1-xTi_xO₃ (PZT). The main features in the anelastic curves are compared with those in the dielectric spectra and are tentatively related to different modes of octahedral rotations and polar cation shifts.

Abstract: Anelastic and dielectric spectroscopy measurements are presented, which, together with previous measurements [1], clarify some controversial aspects of the phase diagram of PbZr_1xTi_xO₃ close to the border with the antiferroelectric (AFE) phase, and at the morphotropic phase boundary (MPB). No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, supporting recent structural studies according to which the two phases coexist, with the fraction of M prevailing near the MPB. A large frequency independent softening at the MPB indicates a genuine M phase over only finely twinned R phase. A new phase transition is found in both the anelastic and dielectric spectra at x = 0.1, at a temperature T_IT between the Curie temperature T_C and the boundary T_T to the phases with tilted octahedra. Such a diffuse transition is interpreted as onset of disordered tilts, which finally become ordered below T_T. In this manner, the phase diagram of PZT is rationalised with respect to the common tendency of perovskites to undergo tilting when the mismatch between the cation sizes exceeds a threshold.

Abstract: From neutron diffraction it is known that the BaCeO₃ perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y³⁺ on the Ce⁴⁺ site introduces charge compensating O vacancies (V_O) that may be partially filled with OH complexes with exposition to H₂O, so making the material an ionic conductor. Anelastic relaxation experiments have been carried out on samples doped with 2%Y and 10%Y; the real part s’(T) of the complex elastic compliance presents softenings at the transitions, and the loss s’’/s’ curves allow the content of V_O and H to be monitored. Doping has a strong effect on the temperature of the Pnma/R transition: with 10%Y in the fully hydrated state T_O1-R increases up to 750 K while after full outgassing falls to 500 K, meaning that the introduction of ~5% V_O shifts the transition of 250 K. While the effect of cation substitution on the transitions temperature is easily explained in terms of simple arguments usually valid for perovskites based on bond length considerations, the remarkable stabilization of the R phase by V_O requires to take into account the anomalous sequence of phase transitions of undoped BaCeO₃, where the R structure transforms into orthorhombic Pnma on cooling with the loss of an octahedral tilt system.

Abstract: The perovskite (Na_1/2Bi_1/2)TiO₃ (NBT) undergoes a series of structural and polar transitions starting from the high temperature paraelectric phase: tetragonal paraelectric, tetragonal antiferroelectric, rhombohedral antiferroelectric and finally rhombohedral ferroelectric, according to neutron diffraction and dielectric spectroscopy. In solid solution with BaTiO₃ (BT) the ferroelectric phase changes from rhombohedral to tetragonal, at the so-called morphotropic phase boundary, and the phases at higher temperature become ill-defined, also because of the large lattice disorder induced by the coexistence of differently charged cations in the same sublattice. Combined dielectric and anelastic spectroscopy measurements are presented, which clarify some issues related to the phase transitions in NBT-BT. The influence of Ba substitution on the tetragonal antiferroelectric phase is determined for the first time, and the possibility that a monoclinic phase, although with very short coherence length, exists near the morphotropic phase boundary is discussed in view of a large maximum of the elastic compliance.

Abstract: We present extended X-ray absorption fine structure (EXAFS) and anelastic spectroscopy measurements performed on (La0.63Ca0.37)(Mn1-yMy)O3 manganites (M = Cr or Ni and y = 0.03, 0.08). The ferromagnetic metallic phase maintains its long-range character even after Mn substitution with non- Jahn-Teller ions, but both the doping species (Ni or Cr) lower the Curie temperature and broaden the magnetic transition. EXAFS suggests the presence of distorted insulating zones in the framework of phase inhomogeneity. Moreover, a strong dependence of the real part of the complex dynamic Young’s modulus and of the elastic energy-loss coefficient on the Mn substitution is evidenced by the anelastic spectroscopy spectra. These results confirm the correlation among local lattice disorder, dynamic and magnetic degrees of freedom and also evidence the possibility to tune manganites physical properties acting on their chemical composition.