Phase transitions in silver tantalate niobate thin films

A large number of perovskite materials undergo cooperative rotations of the oxygen octahedral framework. To date, there has been relatively little work studying how strain might be used to modify these common phase transitions, especially in materials with complex tilt systems. This work aims to gain a fundamental understanding of the relationship between tilt and strain.;The first objective of this dissertation was to obtain structural details on relaxed epitaxial Ag(TaxNb1-x)O3 (ATN) films. These results provided a baseline for measurements on strained films. Ag(Ta0.5Nb0.5)O3 films grown on (001) oriented SrRuO3/LaAlO3 and LaAlO3 substrates were characterized by electron diffraction and high resolution X-ray diffraction (XRD). 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 was suppressed by ∼250 K due to the fact that the correlation length for rotations about cpc was significantly reduced (pc = pseudo-cubic). 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 ¼(00L)-type reflections. On cooling, pronounced ordering of B-cation displacements occurred at ∼60 K, which is significantly below 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 anti-phase rotations around c pc, thereby reducing the effective strength of interactions between the tilting and cation displacements.;The second objective was to perform structural studies on compressively strained Ag(Ta0.5Nb0.5)O3 films. Phase transitions in coherent Ag(Ta0.5Nb0.5)O3 films on SrTiO 3 (001) substrates were characterized by high resolution X-ray diffraction and transmission electron microscopy. The compressively strained films were found to undergo the same phase transition sequence as bulk materials: cubic (C) ↔ tetragonal (T) ↔ orthorhombic (O) ↔ orthorhombic (M 3). However, the biaxial in-plane strain stabilized the tetragonal and orthorhombic phases, expanding these phase fields by a total of ∼280 °C. The compressive strain state also favors domain states in which the c-axis is directed out-of-plane. Consequently, unit cell quadrupling in the M3 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 M3 phase at room temperature rather than in the M2 phase as is observed in bulk. These results demonstrate unambiguously that strain engineering in systems with complex tilt sequences such as Ag(Ta0.5Nb0.5 )O3 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.;The third objective was to examine coherent Ag(Ta0.5Nb 0.5)O3 films under tensile strain that had been deposited on (Ba0.4Sr0.6)TiO3/LaAlO3 (001) and KTaO3 substrates. Unlike the poly-domain nature of bulk materials or relaxed films, these films exhibited a domain structure with the c-axis aligned primarily along the in-plane axes. In addition, it is believed that the tilt angle of the oxygen octahedra was significantly reduced, as was evidenced by the weaker superlattice reflection intensity. It was determined that films under tensile strain undergo the same phase transition sequence as bulk materials and other thin films. Similar to films under compressive strain, an expansion of the tetragonal and orthorhombic phase fields was observed in ATN/(Ba0.4Sr0.6)TiO3/LaAlO3 due to the closer lattice match between the film and the pseudosubstrate in these temperature regimes. These films under tensile strain showed T and O phase fields that were collectively ∼270 °C wider than that of bulk materials. In addition, it was found that the films were in the M3 phase at room temperature rather than M2 observed in bulk. This work demonstrates that, in addition to compressive strain, tensile strain can be used to strain engineer materials with complex tilt systems and complicated phase transition sequences.;The fourth objective was to develop a measurement technique for the “roto”-electric effect and then test AgNbO3 as a candidate rotoelectric. In perovskite materials, the tilting of the oxygen octahedra can lead to new symmetries that include such rotations. This, in turn, can introduce new properties related to these symmetries. For example, in the “roto”-electric effect the rotation of the oxygen octahedra is altered by an applied electric field. An experiment was developed to test this relationship using AgNbO3 as a model system. The intensity of superlattice reflections were measured at the Advanced Photon Source as a function of electric field for a pure AgNbO 3 single crystal, a (Ag0.95Li0.05)NbO3 single crystal, and an AgNbO3 thin film. Model calculations demonstrated that if the tilt angle was modified by the electric field, a change in superlattice peak intensity should have been observed. However, in the samples tested, no change was observed, suggesting that the octahedra in AgNbO3 do not undergo a substantial change in tilt as a function of electric field. This experiment provides a foundation for similar measurements on other material systems.