Abstract
We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd+4 and Pd+2 substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices. This is discussed in terms of the general rules for Voronoi patterns (Dirichlet tessellations) in two and three dimensions. At high pressures Raman soft modes are observed, as in pure lead titanate, and X-ray diffraction (XRD) indicates a nearly second-order displacive phase transition. However, two or three transitions are involved: First, there are anomalies in c/a ratio and Raman spectra at low pressures (P = 1 − 2 GPa); and second, the c/a ratio reaches unity at ca. P = 10 GPa, where a monoclinic (Mc) but metrically cubic transition occurs from the ambient tetragonal P4 mm structure in pure PbTiO3; whereas the Raman lines (forbidden in the cubic phase) remain until ca. 17 GPa, where a monoclinic-cubic transition is known in lead titanate.
Highlights
The host material in this report, lead titanate, has been of considerable interest as a ferroelectric material for more than fifty years, stimulated in part by the Raman and infrared studies of its soft mode by Burns and Scott[1,2] and others[3,4]
Pd incorporation was achieved for La(Fe,Pd) O3±δ and La(Co,Pd)O3±δ that showed the XANES line of Pd to be in octahedral coordination
That other researchers have found it almost impossible to substitute Pd into perovskites such as lead magnesium niobate-tantalate (PMN-PT)[51]; Pd substitution is important for multiferroic applications, having the Pd exsolve to the surface rather than occupying the B-sites of the lattice is highly favorable for catalysis[18]
Summary
The host material in this report, lead titanate, has been of considerable interest as a ferroelectric material for more than fifty years, stimulated in part by the Raman and infrared studies of its soft mode by Burns and Scott[1,2] and others[3,4]. More recently great interest in the effects of hydrostatic pressure[5,6,7,8,9] and uniaxial stress, especially “negative” stress produced by chemical substitution[10], has arisen. Of interest in the present context is the substitution of palladium or other magnetic ions into lead titanate[11,12]. This affords the possibility of a room-temperature multiferroic. Pd in PbTiO3 is expected to go into the B-site, replacing Ti+4, where it is an exact fit to ionic size (Pd+4 is 0.615 Å; Ti+4 is 0.605 Å)[13,14] and valence. Magnetism in Pd is usually produced or enhanced by stress or electric fields, so Pd in Pb sites may facilitate that. It is important to note that the introduction of both Pd and Pt substitutional in perovskite oxides has recently been successful for catalysis[15,16,17]
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