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4.2.1. BiMnO

3

rich Compositions

The structural characterization (previously shown in Chapter 3, section 3.2.1) has clearly indicated the coexistence of two perovskite polymorphs for the BiMnO3rich

compositions, thus suggesting a MPB to exist. However, this MPB would be between a ferroelectric tetragonal P4mm phase and a non-ferroelectric orthorhombic Pnma one. Note that Pmna is a centrosymmetric space group and thus ferroelectricity is not allowed; so it would be a distinctive polar/non-polar MPB. Similar MPBs have been reported for other perovskite solid solutions containing BiFeO3, such as rare-earth (RE)

substituted BiFeO3 (RE = Sm, Dy, Gd)142 and BaTiO3BiFeO3 among others.143 For

instance, in the former case, a boundary between a rhombohedral R3c and an orthorhombic Pnma phase has been established with increasing RE-substitution, which was claimed to be antiferroelectric. Although latter on, the emergence of a double hysteresis loop behavior was associated to an electric-field-induced structural transformation from Pnma to R3c phases.142 Indeed, Pnma could be antiferroelectric in the case where the local dipolar distortions are driven by the Asite Pb or Bi displacements.144 On the other hand, in the case of BaTiO3BiFeO3, an MPB was stated

between the tetragonal P4mm and a polymorph whose crystal structure seems to be cubic Pm-3m, at least in an average sense, and then centrosymmetric. In this case, the average cubic symmetry is broken locally by the off-centering of the Bi3+ ions, and thus the material exhibits a nonrelaxor-type diffuse ferroelectric phase transition.143

First of all, with the aim of studying the distinctive properties exhibited by this polar/non-polar phase coexistence, the complex dielectric permittivity was measured at several frequencies during a heating/cooling cycle, for ceramics with composition 0.4BiMnO30.6PbTiO3 (sample K) and Bi0.47Pb0.53Fe0.17Mn0.3Ti0.53O3 (sample I), (see

also Table 3.1 and Figure 3.1); both sintered at 900 ºC with a subsequent slow cooling treatment from 750 ºC (see Figure 4.5a and Figure 4.5b, respectively). A logarithmic scale is used on the permittivity axis for a better view of the overall behavior. A first noteworthy aspect is the large conductivity for all these BiMnO3rich compositions, so

dielectric anomalies that could be associated with structural phase transitions are markedly overlapped with strong dielectric relaxations that make difficult the dielectric

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analysis. The high electrical conductivity of these samples is also evident in the plots of dielectric loss factor, with values quite above 0.1 even at room temperature and large frequency dispersion in the whole temperature range. Indeed, this is very disadvantageous not only for the proper characterization of the ferroelectric properties but also for their use in current and novel applications.

Figure 4.5: Temperature dependences of the (left) real permittivity(K’) and (right) tangent of dielectric loss(Tan δ), at several frequencies (0.1-1000 kHz) during a heating/cooling cycle for a) 0.4BiMnO30.6PbTiO3 (sample K) and b)Bi0.47Pb0.53Fe0.17Mn0.3Ti0.53O3 (sample I).

The temperature dependence of the DC conductivity (VDC) confirms the same trend for all compositions at the BiMnO3rich region up to the intermediate ones in the line of

MPBs, as shown in Figure 4.6 for samples K, I and F (composition Bi0.57Pb0.43Fe0.4Mn0.17Ti0.43O3 ). This VDC is obtained from the analysis of the Nyquist

diagrams (representation of the imaginary Z’’ versus the real Z’ components of impedance with varying frequency) at different temperatures, and by using the circuit

100 200 300 400 500 600 10000 100000 1000000

K'

Temperature ( ºC ) 100 200 300 400 500 600 1 10 100 1000 10000 7DQ G Temperature ( ºC ) 100 200 300 400 500 600 1000 10000 100000 1000000

K'

Temperature ( ºC ) 100 200 300 400 500 600 1 10 100 1000 10000 Tan G Temperature ( ºC ) a) b) Sample K Sample I

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analysis of the impedance spectroscopy data.145 All materials presented the same Arrhenius-type behavior of VDC with very similar RT resistivity below ~106 _{Ω cm. This} value is effectively very low for dielectric ferroelectric materials, for which RT resistivity must be at least above ~109 _{Ω·cm, and indeed reaches up to 10}11 _{Ω·cm in} most known ferroelectrics.146 This high conductivity prevents the application of high electric fields to obtain ferroelectric hysteresis loops, and the measurement of the true permittivity free of conduction artifacts. For all these compounds the onset of conductivity is below room temperature. This is one of the main challenges in the research on multiferroics that makes usually very difficult the assignment of the ferroelectric nature of a so-called multiferroic compound.72

In ferroelectric perovskites, conductivity is usually associated with the presence of oxygen and/or cation vacancies along with mixed-valence states of complex ions, so as both charge transfer by electron hopping and ionic conduction mechanisms are usually present and accounts for conductivity. However, only one mechanism seems to dominate the total conductivity in the whole temperature range in the materials under study, with activation energies (Ea) of about 0.4 to 0.45 eV. Note the slightly change in

the slope at about 200 ºC in all samples, which indicates a small change of activation energy below and above this temperature. We will come back on this issue latter on mechano-elastic properties. These values for the activation energy are typically of electron hopping mechanisms linked to the presence of cations with mixed-valence states (e.g., Fe3+/Fe2+ and/or Mn3+/Mn4+), which seems to be the origin of the large conductivity here, as confirmed by the XPS analysis described and discussed in the next Chapter 5.‡

In the case of the BiMnO3-PbTiO3 system (composition K), XPS indicates the

presence of Mn4+/Mn3+ with a ratio of 0.26, which might be explained by considering the presence of either Bi/Pb (A-site) or Mn (B-sites) vacancies, or both, that give rise to an oxygen hyperstoichiometry, in agreement with previous reports in BiMnO3.56

Besides, the presence of mixed-valence states for both Fe and Mn cations was also confirmed in composition I, for which the Mn4+/Mn3+ ratio increases slightly up to 0.31 and that for Fe2+/Fe3+ was of 0.57 in the ternary system.

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Figure 4.6: Arrhenius plots for the total conductivity (VDC) of samples K, I and F.

By comparing the three samples in Figure 4.6 a distinctive feature is evident, conductivity decreases yet very little from sample K to sample I, that is, by increasing the amount of Fe substituting Mn, in spite of a larger amount of total charge carriers associated to the mixed-valence states of both Fe and Mn. Note that for sample K, the Mn4+/Mn3+ ratio obtained corresponds to a total 8% of the B-sites being occupied by Mn4+, which is shared by Mn and Ti ions, whereas for sample I, the amount of Mn4+ at

B-sites slightly decreases to a total 7% but there is also a 6% of Fe2+.§ So why the VDC

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