Structural Complexities of PbTi_0.5Fe_0.5O₃ Nanocrystals Revealed by HRTEM

Abstract

Multiferroic materials, i.e. materials that possess more than one ferroic order (ferro-or antiferromagnetic, ferroelectric, ferroelastic, and so on) are of great current interest because they provide means to combine electronic and magnetic device functions into a single class of materials. One approach, in the search for new materials, has been to substitute Fe into a ferroelectric lattice, like the PbTiO 3 perovskite structure, in an attempt to generate multiferroic coupling. Among the many questions, in ongoing efforts to understand multiferroic behavior, is whether multiple ferroic order is a result of the subtle effects of the coexistence of two types of structural phases (not necessarily "chemical" phases) fueling intense structural investigations at the micro-and nano-level. This structural study focuses on PbTi 0.5 Fe 0.5 O 3 (PTFO), a material reported to display room temperature magnetoelectric behavior1 .The present HRTEM study is performed using a Hitachi H9000NAR instrument operated at 300keV. It is part of systematic synthesis and multi-technique characterization of PbTi 1-x Fe x O 3 (PTFO) in the range 0<x<0.5. Samples were synthesized using a modified Pechini sol-gel process based on a citric acid-glycerol route, followed by pulverization, calcination, and annealing, yielding nanopowders 2 that were then suspended ultrasonically in methanol and pipetted onto TEM grids covered with ultrathin and/or holey amorphous carbon supporting films. Lead(II) Nitrate, di-hydroxy bis(ammonium lactato) titanium (IV) solution, and Iron(III) nitrate nonahydrate were used as sources of Pb, Ti and Fe ions. Figure 1 shows a low magnification TEM image, and a corresponding SAD pattern from a collection of particles in the x=0.5 PTFO sample. The particle size is in the 20-50nm range, and the polycrystalline SAD rings, in agreement with the powder X-ray diffraction pattern in Figure 2, can be indexed based on the allowed PbTiO 3 reflections 2 . Rietveld refinement of the XRD data, under the single-phase approximation, finds better fit for the orthorhombic Pmmm space group than for the tetragonal P4/mmm group of pure PbTiO 3 . The XRD-determined unit cell parameters for the nano PTFO samples are a=0.3943nm, b=0.3929nm and c=0.3998nm (the bulk PbTiO 3 parameters are a=b=0.3999nm and c= 0.4143nm) 3 .HRTEM studies were undertaken to look into the structure of individual nanocrystals. Figure 3 shows images of a dominant (a) and a minority (b) phase nanocrystal in <100> type orientations. The minority phase has straight fringes with some lattice spacings that coincide with the majority phase, but also spacings that are unique. This is similar to a rare tetragonal form of PbTiO 3 with unit cell parameters of a=b=1.236nm and c=1.4534nm. 3 The contrast of the dominant phase appears complicated. After Fourier filtering (inset in Fig. 4a) and superposition of a regular grid (Fig 4b), it becomes apparent that even the individual nanocrystals are composed of sub-domains. We find that the shift of the lattice fringes between the nei...