Ferroelastic domain walls in ferroelectric materials possess two properties that are known to affect phonon transport: a change in crystallographic orientation and a lattice strain. Changing populations and spacing of nanoscale-spaced ferroelastic domain walls lead to the manipulation of phonon-scattering rates, enabling the control of thermal conduction at ambient temperatures. In the present work, lead zirconate titanate (PZT) thin-film membrane structures were fabricated to reduce mechanical clamping to the substrate and enable a subsequent increase in the ferroelastic domain wall mobility. Under application of an electric field, the thermal conductivity of PZT increases abruptly at ∼100 kV/cm by ∼13% owing to a reduction in the number of phonon-scattering domain walls in the thermal conduction path. The thermal conductivity modulation is rapid, repeatable, and discrete, resulting in a bistable state or a "digital" modulation scheme. The modulation of thermal conductivity due to changes in domain wall configuration is supported by polarization-field, mechanical stiffness, and in situ microdiffraction experiments. This work opens a path toward a new means to control phonons and phonon-mediated energy in a digital manner at room temperature using only an electric field.

Ferroelectric/ferroelastic domain reorientation was measured in a 1.9 m thick tetragonal{001} oriented PbZr0.3Ti0.70O3 thin film doped with 1% Mn under different mechanical boundary constraints. Domain reorientation was quantified through the intensity changes in the 002/200 Bragg reflections as a function of applied electric field. To alter the degree of clamping, films were undercut from the underlying substrate by 0%, ~25%, ~50%, or ~75% of the electrode area. As the amount of declamping from the substrate increased from 0% to ~75%, the degree of ferroelectric/ferroelastic domain reorientation in the films increased more than six fold at three times the coercive field. In a film that was ~ 75% released from the substrate, approximately 26% of 90° domains were reoriented under the maximum applied field; this value for domain reorientation compares favorably to bulk ceramics of similar compositions. An estimate for the upper limit of 90° domain reorientation in a fully released film under these conditions was determined to be 32%. It was also found that the different clamping conditions strongly influence the amount of reorientation upon removing the applied field, with higher remanence of preferred domain orientations observed in declamped films.

In recent years, there has been an increased interest in octahedral rotations in perovskite materials, particularly on their response to strain in epitaxial thin films. The current theoretical framework assumes that rotations are affected primarily through the change in inplane lattice parameters imposed by coherent heteroepitaxy on a substrate of different lattice constant. This model, which permits prediction of the thin-film rotational pattern using firstprinciples density functional theory, has not been tested quantitatively over a range of strain states. To assess the validity of this picture, coherent LaAlO 3 thin films were grown on SrTiO 3 , NdGaO 3 , LaSrAlO 4 , NdAlO 3 , and YAlO 3 substrates to achieve strain states ranging from +3.03% to-2.35%. The out-of-plane and in-plane octahedral rotation angles were extracted from the intensity of superlattice reflections measured using synchrotron x-ray diffraction. Density functional calculations show that no measurable change in intrinsic defect concentration should occur throughout the range of accessible strain states. Thus, the measured rotation angles were compared with those calculated previously for defect-free films. [Hatt and Spaldin, Phys. Rev. B 82, 195402 (2010)]. Good agreement between theory and experiment was found, suggesting that the current framework correctly captures the appropriate physics in LaAlO 3 .

Direct evidence of ferroelectric/ferroelastic domain reorientation is shown in Pb(Zr0.30Ti0.70)O3 (PZT30/70) thin films clamped to a rigid silicon substrate using in situ synchrotron X-ray diffraction during application of electric fields. Both dense films and films with 3 to 4 vol% porosity were measured. On application of electric fields exceeding the coercive field, it is shown that the porous films exhibit a greater volume fraction of ferroelastic domain reorientation (approximately 12 vol% of domains reorient at 3 times the coercive field, Ec) relative to the dense films (~3.5 vol% at 3Ec). Furthermore, the volume fraction of domain reorientation significantly exceeded that predicted by linear mixing rules. The high response of domain reorientation in porous films is discussed in the context of two mechanisms: local enhancement of the electric field near the pores and a reduction of substrate clamping resulting from the lowering of the film stiffness as a result of the porosity. Similar measurements during weak-field (subcoercive) amplitudes showed 0.6% volume fraction of domains reoriented for the porous films, which demonstrates that extrinsic effects contribute to the dielectric and piezoelectric properties.

Piezoelectric PbZr(0.52)Ti(0.48)O(3) (PZT) thin films deposited on thin glass substrates have been proposed for adjustable optics in future x-ray telescopes. The light weight of these x-ray optics enables large collecting areas, while the capability to correct mirror figure errors with the PZT thin film will allow much higher imaging resolution than possible with conventional lightweight optics. However, the low strain temperature and flexible nature of the thin glass complicate the use of chemical-solution deposition due to warping of the substrate at typical crystallization temperatures for the PZT. RF magnetron sputtering enabled preparation of PZT films with thicknesses up to 3 μm on Schott D263 glass substrates with much less deformation. X-ray diffraction analysis indicated that the films crystallized with the perovskite phase and showed no indication of secondary phases. Films with 1 cm(2) electrodes exhibited relative permittivity values near 1100 and loss tangents below 0.05. In addition, the remanent polarization was 26 μC/cm(2) with coercive fields of 33 kV/cm. The transverse piezoelectric coefficient was as high as -6.1±0.6 C/m(2). To assess influence functions for the x-ray optics application, the piezoelectrically induced deflection of individual cells was measured and compared with finite-element-analysis calculations. The good agreement between the results suggests that actuation of PZT thin films can control mirror figure errors to a precision of about 5 nm, allowing sub-arcsecond imaging.

Octahedral tilt transitions in epitaxial AgTa0.5Nb0.5O3 (ATN) films grown on (001)p (where p=pseudocubic) oriented SrRuO3/LaAlO3 and LaAlO3 substrates were characterized by electron diffraction and high resolution x-ray diffraction. It was found that the ATN films exhibited octahedral rotations characteristic of the Pbcm space group, similar to those seen in bulk materials; however, the temperature of the M3↔M2 phase transition has been suppressed by ∼250 K due to the fact that the correlation length for rotations about cp was significantly reduced. The average off-center B-cation displacements, which signify the degree of long-range order for these local cation positions, were negligibly small compared to bulk materials, as inferred from the near-zero intensity of the 1/4(00l)p-type reflections. On cooling, pronounced ordering of B-cation displacements occurred at ≈60 K which is significantly lower compared to bulk (≈310 K). The onset of this ordering coincides with a broad maximum in relative permittivity as a function of temperature. It is believed that point and planar defects in thin ATN films disrupt the complex sequence of in-phase and antiphase rotations around cp thereby reducing the effective strength of interactions between the tilting and cation displacements.

Phase transitions in coherently strained epitaxial AgTa ½ Nb ½ O 3 films grown on SrTiO 3 (001) substrates were characterized by high resolution X-ray diffraction and transmission electron microscopy. Coherently strained films were found to undergo the same phase transition sequence as bulk materials:. However, the compressive in-plane strain stabilized the tetragonal and orthorhombic phases, expanding these phase fields by ≈ 280 °C. The compressive strain state also favors c-axis domain texture. Consequently, unit cell quadrupling in the M 3 phase and the in-phase tilt of the T phase both occur around the out-of-plane direction. In contrast, bulk materials and relaxed films are poly-domain, with the complex tilt system occurring along all three of the orthogonal axes. Compressively strained films are in the M 3 phase at room temperature rather than in the M 2 phase as is observed in bulk. This suggests that strain not only modifies octahedral rotations but also disrupts the ordering of local cation displacements. These results demonstrate unambiguously that strain engineering in systems with complex tilt sequences such as AgTa ½ Nb ½ O 3 is feasible and open up the possibility of modifying properties by manipulation of the pertinent octahedral tilt transition temperature in a wide range of functional ceramics.

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