Yet, as of today, the microscopic origin of the remarkable charge carrier dynamics in perovskite solar cells remains unclear. An often proposed idea to explain the enhanced separation and transport of photogenerated charge carriers describes the formation of ferroelectric domains, i.e., domains with alternating polarization.Ferroelectric domains would provide local internal electric fields that assist with the separation of charge carriers and hence reduce recombination. [2][3][4] However, whether or not OMH perovskites are ferroelectric materials or exhibit other ferroic properties, and which effects might result from these properties have been a subject of debate for several years. A multitude of measurement techniques was employed in order to probe ferroic properties of single crystal samples, [5,6] pressed powder pellets, [7] and polycrystalline thin-films [8] of OMH perovskites. Scanning microscopy techniques such as atomic force microscopy (AFM), [9][10][11][12] scanning electron microscopy (SEM), [13] and tunneling electron microscopy (TEM) [14] were extensively used in order to reveal the microstructure of OMH-perovskite thinfilm samples with subgrain resolution. Some of these studies revealed ordered domains within crystal grains, but no consensus has been reached about the interpretation of their ferroic and crystallographic origins. Claims included piezoelectricity, [15][16][17] pyroelectricity, [5] ferroelasticity, [9,12,18] antiferroelectricity [19] as well as ferroelectricity, [10,11,15,16,20] and the seemingly contradictory reports on these ferroic properties have heated up the discussion. Techniques that probe large areas of specimens such as X-ray diffraction (XRD), [21] impedance spectroscopy, [22,23] and J-V characterization [5,7] added a rather spatially averaged picture of the samples' microstructure. For example, second harmonic generation (SHG) measurements were employed in order to identify whether or not OMH perovskites form polar crystals, but their interpretation led to ambiguous results. [5,24] In order to clarify some of the confusion and seemingly contradictory reports on the ferroic and, in particular, ferroelectric features of OMH perovskites, we review and discuss some of the fundamental properties of this material class and correlate them with our own observations on perovskite thin-films. We deliberately focus on the ferroic properties of archetypical methylammonium lead iodide (MAPbI 3 ), representing the class of light-harvesting perovskites.Whether or not methylammonium lead iodide (MAPbI 3 ) is a ferroelectric semiconductor has caused controversy in the literature, fueled by many misunderstandings and imprecise definitions. Correlating recent literature reports and generic crystal properties with the authors' experimental evidence, the authors show that MAPbI 3 thin-films are indeed semiconducting ferroelectrics and exhibit spontaneous polarization upon transition from the cubic high-temperature phase to the tetragonal phase at room temperature. The polarization is predomina...

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The microstructure of absorber layers is pivotally important for all thin‐film solar technologies. Using electron backscattered diffraction (EBSD), the crystal orientation in methylammonium lead iodide thin films with submicrometer resolution is reported. For the vast majority of (110) oriented grains, the c‐axis of the perovskite unit cell is oriented in‐plane. Although some adjacent grains exhibit the same in‐plane horizontal orientation of the c‐axis, no universal horizontal orientation of the c‐axis within the sample plane exists. The (110) crystal orientation correlates with an in‐plane orientation of the ferroelectric polarization as investigated by vertical and lateral piezoresponse force microscopy (PFM). The individual grains with different crystal orientations that exhibit different ferroelectric patterns and surface potentials are identified. The strong correlation between crystal orientation and ferroelectric polarization allows conclusions to be drawn about the microstructure from PFM measurements and, likewise, the ferroelectric polarization to be derived from crystallographic observations by EBSD.

Differences in coping styles could be mainly related to age and social factors. Individual coping strategies seem only to be little related to the type of brain pathology.

Niobium-alumina aggregate fractions with particle sizes up to 3150 µm were produced by crushing pre-synthesised fine-grained composites. Phase separation with niobium enrichment in the aggregate class 45–500 µm was revealed by XRD/Rietveld analysis. To reduce the amount of carbon-based impurities, no organic additives were used for the castable mixtures, which resulted in water demands of approximately 27 vol.% for the fine- and coarse-grained castables. As a consequence, open porosities of 18% and 30% were determined for the fine- and coarse-grained composites, respectively. Due to increased porosity, the modulus of rupture at room temperature decreased from 52 MPa for the fine-grained composite to 11 MPa for the coarse-grained one. However, even the compressive yield strength decreased from 49 MPa to 18 MPa at 1300 °C for the fine-grained to the coarse-grained composite, the latter showed still plasticity with a strain up to 5%. The electrical conductivity of fine-grained composite samples was in the range between 40 and 60 S/cm, which is fifteen magnitudes above the values of pure corundum.

Lead-free piezoceramic potassium sodium niobate in its morphotropic composition was synthesized with abnormal grain growth. Ferroelectric domain patterns were imaged with piezoresponse force microscopy. Analysis of the domain structure at the morphotropic phase boundary revealed a coexistence of tetragonal and orthorhombic polarized domains in a single grain.

The strain behavior of morphotropic, 1La2Sr-doped lead zirconate titanate was investigated. Particular attention was paid to the electric field induced strain ͑S-E characteristic͒ in the high field range for morphotropic materials having different Zr/ Ti ratios. Unipolar electrical cycles with maximum fields between 1 and 3 kV/ mm were applied to the poled materials under load-free conditions. Strain curves were recorded and the data were evaluated with respect to field dependence of strain and strain hysteresis. Nonlinearity of strain was detected for all materials. Depending on the composition, marked differences were found with respect to the types of nonlinearity. The corresponding data on strain hysteresis indicate that the nonlinearity was due to a loss generating strain mechanism. Therefore the nonlinearity was attributed to domain switching. A phenomenological model is suggested, which is based on the assumptions of a distribution of domain switching over a field range. This field range is characterized by the activation field parameter E a , which is specific for each composition. The model is capable of explaining the types of nonlinearity in strain that have been observed.

From the work of Le Chatelier [1884], it is well known that chemical reactions that exhibit a change in volume are sensitive to the ambient pressure of the reaction. Increasing the pressure will alter the ratio of reaction products to reactants. If the change in volume is constrained to occur at a surface, then such reactions can result in residual stresses that affect the strength of the solid. These effects are applicable to silica glass, which increases in volume when reacting with water. In this paper, we discuss the possibility of using this effect to strengthen silica glass. Using a modification of Le Chatelier's theory to handle applied stresses, we show that water penetration into the surface of silica glass can yield sufficient residual stress to increase the strength of silica glass into the GPa range. Applying these ideas to recent data published by Lezzi et al., we are able to attribute the strengthening they observe to a water/silica reaction under an applied tensile stress.