The thermal stability of amorphous ionic solids is usually attributed to kinetic considerations related to mass transport. However, there are a number of amorphous ionic solids, which have recently been described, whose unusual resistance to nucleation and subsequent crystallization cannot be explained by mass transport limitations. Examples have been found in a large variety of fields, spanning the range from thin solid films to biomineralization. This poses a question regarding a possible common mechanism for the stabilization of amorphous ionic solids. Here we present a model which explains the formation and thermal stability of quasi-amorphous thin films of BaTiO 3 , one of the amorphous systems recently described which exhibit unusual thermal stability. On the basis of the experimental evidence presented we suggest that nucleation of the crystalline phase can occur only if the amorphous phase undergoes volume expansion upon heating and transforms into an intermediate low density amorphous phase. If volume expansion is unobstructed by external mechanical constraints, nucleation proceeds freely. However, thin films are clamped by a substrate; therefore, volume expansion is restricted and the low-density intermediate phase is not formed. As a result, under certain conditions, nucleation may be completely suppressed and the phase which appears is quasi-amorphous. A quasi-amorphous film is under compressive stress and as long as the mechanical constraints are in place it remains stable at the temperatures that normally lead to crystallization of amorphous BaTiO 3 . Quasiamorphous thin films of BaTiO 3 exhibit pyroelectricity, the origin of which is also explained by the proposed model.

Spontaneous or stress-induced polarization, signifying pyroelectricity and piezoelectricity, respectively, can appear in ionic solids solely due to a non-centrosymmetrical spatial distribution of ions in a polar crystalline structure. Although theory does not impose strict limitations on the size of a polar crystallite, [1,2] the magnitude of pyroelectric and piezoelectric effects of some ceramics, particularly BaTiO 3 , rapidly decrease as grain size diminishes to a few nanometers. [1,3,4] Determination of the minimal number of periodically arranged unit cells for which a crystal retains pyroelectric and piezoelectric properties has become increasingly important due to the rapid incorporation of these materials into nanometer-scale devices. An intriguing complication to this quest arises from the fact that pyro-and piezoelectric effects may exist in structures that lack the spatial periodicity inherent for ionic crystals, but are composed of polar molecules with directional ordering. An example of such a material is a nematic liquid crystal. Spontaneous or stress-induced dipole ordering without fine-tuned positional order is theoretically possible in ionic solids [5] as well; however, only indirect experimental evidence supporting this theory has been presented so far.[6±9] The present work reports on the preparation and pyroelectric properties of quasiamorphous BaTiO 3 thin films that represent a polar ionic solid without spatial periodicity. Pyroelectric amorphous BaTiO 3 films were obtained by passing sputtered amorphous BaTiO 3 films through a steep temperature gradient. As-deposited BaTiO 3 films were stressfree (< ± 30 MPa) with refractive indices in the range of n^= 1.97±2.02, and a very small in-plane±out-of-plane birefringence of n i ± n^= ± (0.002±0.008), confirming film isotropy. A compressive stress of r e = 2.0±2.2 GPa developed in the films passed through the temperature gradient. The refractive index of these films decreased to n^= 1.89/1.94 but the birefringence grew to n i ± n^= 0.03±0.07, corroborating the existence of high in-plane compressive stress.X-ray diffraction (XRD) spectra of the films passed through the temperature gradient were indistinguishable from those of as-deposited films (Fig. 1a). No XRD peaks apart from those of the Si substrate were observed, indicating the absence of a crystalline phase. The volume detection limit of a crystalline phase by XRD calculated from the signal to noise ratio [10] was < 0.3 %. Furthermore, thorough transmission electron microscopy (TEM) investigation only rarely detected the presence of crystalline grains (Fig. 1b) [11] and the concentration of the crystallites estimated from TEM images was far below 0.3 %. In contrast to the similarity with respect to electron and X-ray diffraction analysis, as-deposited films and the films passed through the temperature gradient have distinctly different electrical properties. The films passed through the temperature gradient show a large pyroelectric effect p measured = (1±3) 10 ±9 C cm ±2 K ±1 (5±15 % that of...

A method for work-function evaluation is proposed, based on recording the shift of x-ray photoelectron signals from a surface irradiated by low-energy electrons. The method is capable of measuring samples with very low conductivity, poor back contacts, and high dielectric constants. The method is also applicable to magnetic materials and can be particularly effective for studies of multilayer and heterogeneous systems.

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Self-supported films of nanocrystalline BaTiO 3 exhibit a two orders of magnitude enhancement of the pyroelectric coefficient ͑Ϸ1 C / cm 2 K͒ with respect to the value measured for a single BaTiO 3 crystal. The enhancement strongly depends on film geometry and appears only in buckled films where ferroelectric grains undergo self-organization into polycrystalline macrodomains. The authors posit that the enhancement of the pyroelectric effect is related to 90°polarization switching and is, therefore, similar in nature to an "extrinsic" piezoelectric effect.