Laboratory of Anelastic Spectroscopy and Thermal Treatments

 
 

Francesco Cordero  -

Paolo Massimiliano Latino -

Fabrizio Corvasce   -

 

Rome - Tor Vergata
 

Research Activities

The complex elastic modulus is measured as a function of temperature (1-1000 K) exciting resonances (0.1 -100 kHz) of samples of any solid material in order to obtain information on: any type of phase transitions; mobility and aggregation of ions, point and extended defects; small polarons; domain walls and structural, electric, magnetic excitations or inhomogeneities. With non—conducting materials the measurements are usually complemented with the dielectric ones (see LAB-DS). It is possible to treat the samples at high temperature in UHV o controlled gas atmosphere/flux for annealing, degassing, hydrogenation, oxygenation, reduction

Phase transitions and ionic mobilities in perovskites

Figures 1 shows the anelastic spectrum of SrTiO3-d with O vacancies (VO) introduced by heating in reducing atmosphere at >1100 C. The lower panel shows the elastic energy loss, 1/Q = E’’/E’, measured during the same cooling at two frequencies f. The Debye peaks that shift with varying f are due to jumps of VO and their complexes and are centered at the temperatures at which the respective jump frequencies, which rise with temperature, coincide with f. In this manner, it is possible to measure with precision the hopping or reorientation or tunnelling frequencies of point defects and their complexes. In this manner it has been demonstrated that in SrTiO3-d the activation energy of 1 eV, generally attributed to diffusion of free VO, is instead due to the reorientation of VO pairs, (peak at 550 K), whereas isolated VO are much more mobile with 0.6 eV (peak at 350 K). In the upper panel of Fig. 2 the elastic modulus increases linearly with decreasing T until the TiO6 octahedra that form the cubic perovskite rotate giving rise to a structural transition with marked steps in the anelastic response. This demonstrates how the technique is extremely effective for studying phase transitions. In complex perovskites for ionic conduction (e.g. fuel cells, gas purifiers) the anelastic spectra provide information on the mobilities of the various ionic species (VO e H), their complexes and the interplay between their ordering at high concentrations and the structural transitions.
 

 

 
Relevant Publications

 

 

 
 
 

Metal-organic perovskites for photovoltaics

In combination with dielectric measurements, information has been obtained on the phase transitions and the dynamics of the MA molecules in the metalorganic perovskite MAPbI3 and FA in FAPbI3.
The α→β transition consists mainly of a rotation of the PbI6 octahedra about the c axis and therefore gives rise to a steplike softening of the elastic modulus. The weak dielectric response (not shown in Fig. 2) demonstrates that a ferroelectric component is absent or extremely weak, contrary to many reports. Based on the Landau theory, it has been shown that a strong coupling between octahedral tilting and the reorientation of the polar MA molecules can depress the temperature of the ferroelectric transition and prevent its occurrence. Also the β→γ transition involves rotations of the octahedra, but the fact the elastic modulus rises, and far above the value in the α phase, indicates an important contribution from the freezing of the rotations of the MA molecules. Much of this freezing occurs gradually with thermally activated and strongly correlated dynamics, as demonstrated by the anelastic relaxation peak that shifts to higher T with increasing f and has a broad shape.

 
 
Relevant Publications

 

 

 
 
 

Piezoelectric response of ferroelectric materials from elastic measurements

The measurement of the piezoelectric coefficients of ferroelectric materials is affected by great uncertainties, mainly from the only partial polarization of the samples and from free charges. It has been shown that the softening of the elastic modulus at the ferroelectric transition is proportional to the square of the piezoelectric coefficient divided by the dielectric constant, so providing an effective tool for measuring the piezoelectric response without the need of poling ceramic samples and hence without the uncertainties and difficulties connected with poling. A quantitative test of the proposed method has been made on BaTiO3. Figure 3 shows the piezoelectric contribution to the elastic compliance s = 1/E below the Curie temperature. Moreover, since the elastic response is totally insensitive to free charges, a piezoelectric response may be revealed also in the metallic state, which is of both theoretical and practical interest and otherwise difficult to be probed. In this manner, it has been shown that BaTiO3-d doped with O vacancies remains ferroelectric even within the metallic state.
 

 

 
Relevant Publication

 

 

 
 

Instrumentation

Anelastic spectroscopy

 

Collaborations

  • Scientific collaboration and co-participation to projects with Floriana Craciun, CNR-ISM (Tor Vergata) for dielectric measurements
  • Scientific collaboration and co-participation to projects with the group of Carmen Galassi, CNR-ISTEC (Faenza) for the preparation and characterization of ceramic ferroelectric samples
  • Scientific collaboration with Francesco Trequattrini, Dip. Fisica Università di Roma “La Sapienza” for anelastic relaxation measurements
  • Scientific collaboration and co-participation to projects with Annamaria Paoletti, Gloria Zanotti, Patrizia Imperatori, Giovanna Pennesi, CNR-ISM (Montelibretti) for the preparation of samples of hybrid organic-inorganic perovskites for photovoltaic and with ferroelectric properties
  • Scientific collaboration with Barbara Paci e Amanda Generosi, CNR-ISM (Tot Vergata) for XRD analysis of hybrid organic-inorganic perovskites
  • Scientific collaboration and co-participation to projects with il gruppo di Victor Fruth, Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania, for the preparation and characterization of ferroelectric hybrid organic-inorganic and polymeric materials
  • Scientific collaboration and co-participation to projects with the group Paulo Sergio da Silva Jr., Dip. Fisica, UFSCar, São Carlo (SP), Brazil, for anelastic measurements, characterization and preparation of ceramic samples.
  • Start of scientific collaboration with prospective of common projects with Oktay Aktas, State Key Laboratory for Mechanical Behavior of Materials & School of Materials Science and Engineering, Xi'an Jiaotong University, China for studying the influence of defects on the ferroelectric fluctuations in the paraelectric phases of titanate perovskites.

 

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AnelSpec Projects

  • New phase transition in a 2D titanium carbide MXene: possible sliding ferroelectricity - A new paper +

    New phase transition in a 2D titanium carbide MXene: possible sliding ferroelectricity - A new paper MXene consists of metallic layers, that can easily slide with respect to each other like sheets in reams. A phase… Read More

  • Elastic precursor effects during the Ba1-xSrxTiO3 ferroelastic phase transitions +

    Elastic precursor effects during the Ba1-xSrxTiO3 ferroelastic phase transitions “Elastic precursor effects during the Ba1-xSrxTiO3 ferroelastic phase transitions”, published in Physical Review Research, is the result of a collaboration… Read More

  • Piezoelectric and Multiferroic Molecular Perovskites for Flexible and Wearable Devices +

    The project aims at studying and improving some classes of organic and metal-organic piezoelectric materials (in which a stress causes… Read More
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