Structural Changes and Ferroelectric Properties of BiFeO₃-PbTiO₃ Thin Films Grown via a Chemical Multilayer Deposition Method

Thin films of (1-x)BiFeO3-xPbTiO3 (BF-xPT) with x ~ 0.60 were fabricated on Pt/Si substrates by chemical solution deposition of precursor BF and PT layers alternately in three different multilayer configurations. These multilayer deposited precursor films upon annealing at 700{\deg}C in nitrogen show pure perovskite phase formation. In contrast to the equilibrium tetragonal structure for the overall molar composition of BF:PT::40:60, we find monoclinic structured BF-xPT phase of MA type. Piezo-force microscopy confirmed ferroelectric switching in the films and revealed different normal and lateral domain distributions in the samples. Room temperature electrical measurements show good quality ferroelectric hysteresis loops with remanent polarization, Pr, of up to 18 {\mu}C/cm2 and leakage currents as low as 10-7 A/cm2.Comment: 14 Pages and 6 figure

Temperature-induced isostructural phase transition, associated large negative volume expansion, and the existence of a critical point in the phase diagram of the multiferroic (1-x)BiFeO3-xPbTiO3 solid solution system

We report here an unusual ferroelectric to ferroelectric isostructural phase transition and associated giant negative thermal expansion (NTE) for the tetragonal composition x = 0.31 closest to the morphotropic phase boundary (MPB) of the multiferroic (1–x)BiFeO3–xPbTiO3 (BF-xPT) solid solution system. It is shown that the room temperature tetragonal phase (T1) of BF-0.31PT with extremely large tetragonality undergoes a first-order isostructural phase transition to another tetragonal phase (T2) with lower tetragonality without losing the P4mm space group symmetry and the occupied Wyckoff positions. The T2 phase finally transforms into the paraelectric cubic phase at still higher temperatures. Using group theoretical considerations, we show that the observed atomic displacements associated with this isostructural phase transition correspond to specific irreducible representations of the P4mm space group at its Brillouin zone center, and as such this transition may be phonon driven. Pronounced anomalies in the thermal displacement parameters at the T1 to T2 transition provide evidence for such a phonon-mediated isostructural phase transition. The high tetragonality ferroelectric phase (T1) of BF-0.31PT shows the largest NTE coefficient reported so far in the mixed BF-xPT system. The isostructural transition is shown to persist for tetragonal compositions of BF-xPT up to x = 0.60. A complete phase diagram of the BF-xPT system showing the existence of a critical point at T ∼ 677 K for x ≈ 0.63 is also presented

Absence of Morphotropic Phase Boundary Effects in BiFeO3-PbTiO3 Thin Films Grown via a Chemical Multilayer Deposition Method

Here, we report the unusual behaviour shown by the (BiFeO3)1-x-(PbTiO3)x (BF-xPT) films prepared using a multilayer deposition approach by chemical solution deposition method. Thin film samples of various compositions were prepared by depositing several bilayers of BF and PT precursors by varying the BF or PT layer thicknesses. X-ray diffraction showed that final samples of all compositions show mixing of the two compounds resulting in a single phase mixture, also confirmed by transmission electron microscopy. In contrast to bulk equilibrium compositions, our samples show a monoclinic (MA type) structure suggesting disappearance of morphotropic phase boundary (MPB) about x = 0.30 as observed in the bulk. This is accompanied by the lack of any enhancement of remnant polarization at MPB as shown by the ferroelectric measurements. Magnetic measurements show that the magnetization of the samples increases with increasing BF content. Significant magnetization of the samples indicates melting of spin spirals in the BF-xPT arising from random distribution of iron atoms across the film. Absence of Fe2+ ions in the films was corroborated by X-ray photoelectron spectroscopy measurements. The results illustrate that used thin film processing methodology significantly changes the structural evolution in contrast to predictions from the equilibrium phase diagram as well as modify the functional characteristics of BP-xPT system dramatically.Comment: 15 pages, 6 figure

Stability of the various crystallographic phases of the multiferroic (1-x)BiFeO<SUB>3</SUB>-xPbTiO<SUB>3</SUB> system as a function of composition and temperature

In this report, we have studied the stability of various crystallographic phases of (1-x)BiFeO3-xPbTiO3 as a function of composition and temperature. The structure of BF-xPT is reconfirmed to be tetragonal and monoclinic in the P4mm and Cc space groups for x&gt;0.31 and 0.10&#8804;x&#8804;0.27, whereas the two phases coexist in the morphotropic phase boundary (MPB) region 0.27&lt;x&lt;0.31. A recent proposition of an orthorhombic phase in the MPB region has been ruled out comprehensively. From the high temperature powder x-ray diffraction (XRD) data it is shown that the MPB in this system is tilted towards the tetragonal side, a unique feature among the family of PbTiO3 based MPB systems. We have established accurately the room temperature phase diagram for this solid solution. By comparing the observed bond lengths between oxygen and other cations, obtained from Rietveld analysis of the room temperature powder XRD data with expected ionic bond lengths, we have shown that the very high c/a ratio in the tetragonal phase of this system is linked with the covalency effects for bonding between both A and B site cations with oxygen

Effect of stress induced monoclinic to tetragonal phase transformation in the multiferroic (1-x)BiFeO<SUB>3-x</SUB>PbTiO<SUB>3</SUB> system on the width of the morphotropic phase boundary and the tetragonality

The relative stability of the tetragonal and monoclinic phases in the (1-x)BiFeO3-xPbTiO3 solid solution system has been investigated under externally applied stress. It is shown that external stress can transform monoclinic compositions partially to the tetragonal phase in the vicinity of the morphotropic phase boundary (MPB) leading to an extension of the MPB region from Δx ≈ 0.03 for annealed samples to Δx ≈ 0.17 for the stressed samples toward the BiFeO3 richer end. The tetragonality of the stress-induced tetragonal phase for x = 0.20 is shown to be higher than the highest value reported so far for x = 0.31 composition in annealed samples

Morphotropic phase boundary in (1-x)BiFeO<SUB>3-x</SUB>PbTiO<SUB>3</SUB>: phase coexistence region and unusually large tetragonality

It is shown that the tetragonal (T) and rhombohedral (R) structures of (1-x)BiFeO3-xPbTiO3 are stable for xT ≥ 0.31 and xR ≤ 0.27, respectively, giving the narrowest width of 0.03, reported so far, for the morphotropic phase boundary region in this system. The Rietveld refined structure of the T phase for x = 0.31 reveals partial covalent character of the Ti/Fe-O and Pb/Bi-O bonds which may be responsible for the unusually large tetragonality

Unambiguous evidence for magnetoelectric coupling of multiferroic origin in 0.73BiFeO3–0.27PbTiO3

Magnetization, frequency dependent dielectric, and structural studies on 0.73BiFeO3–0.27PbTiO3 in the temperature range from 300 to 600 K reveal anomalies in the unit cell parameters and the intrinsic value of the dielectric constant, free from space charge contributions, at the antiferromagnetic transition temperature (TN). Our results provide unambiguous confirmation of magnetoelectric coupling of multiferroic origin at TN and evidence for monoclinic distortion of the ferroelectric phase

Simultaneous changes of nuclear and magnetic structures across the morphotropic phase boundary in (1-x)BiFeO<SUB>3-x</SUB>PbTiO<SUB>3</SUB>

We present here the results of a powder neutron diffraction study on the (1-x)BiFeO3-xPbTiO3 (BF-xPT) system, which reveal an abrupt change in magnetic structure from a noncollinear antiferromagnetic one to a collinear G-type antiferromagnetic as the nuclear structure of the ferroelectric phase changes from monoclinic in Cc space group for x ≤ 0.27 to tetragonal in P4mm space group for x ≥ 0.31 across the morphotropic phase boundary (MPB). This reflects the multiferroic character of the MPB in the BF-xPT system