Thorough Raman and infrared (IR) reflectivity investigations of nominally pure ${\mathrm{SrTiO}}_3$ ceramics in the 10–300 K range have revealed a clear presence of the polar phase whose manifestation steeply increases on cooling. The Raman strengths of the Raman-forbidden IR modes are proportional to ${\omega }_{\mathrm{TO}1\mathrm{}}^{-\alpha }(\alpha \approx 1.6)$ where ${\omega }_{\mathrm{TO}1\mathrm{}}$ is the polar soft mode frequency. No pronounced permittivity dispersion is observed below the soft mode frequency so that, as in single crystals, the static permittivity is essentially determined by the soft mode contribution. A theory is suggested which assumes a frozen dipole moment connected with the grain boundaries which induces the polar phase in the grain bulk in correlation with the bulk soft-mode frequency. This stiffens slightly the effective soft mode response and reduces the low-temperature permittivity compared to that of single crystals. Moreover, the polar soft mode strongly couples to the $E_g$ component of the structural soft doublet showing that the polar axis is perpendicular to the tetragonal axis below the structural transition which is shifted to 132 K in our ceramics. Whereas the ${\mathrm{TiO}}_6$ octahedra tilt (primary order parameter) below the structural transition corresponds to that in single crystals, much smaller $A_{1g}-E_g$ splitting of the structural soft doublet shows that the tetragonal deformation (secondary order parameter) is nearly 10 times smaller, apparently due to the grain volume clamping in ceramics.

Bipolar resistive switching (BRS) cells based on the valence change mechanism show great potential to enable the design of future non-volatile memory, logic and neuromorphic circuits and architectures. To study these circuits and architectures, accurate compact models are needed, which showcase the most important physical characteristics and lead to their specific experimental behavior. If BRS cells are to be used for computation-in-memory or for neuromorphic computing, their dynamical behavior has to be modeled with special consideration of switching times in SET and RESET. For any realistic assessment, variability has to be considered additionally. This study shows that by extending an existing compact model, which by itself is able to reproduce many different experiments on device behavior critical for the anticipated device purposes, variability found in experimental measurements can be reproduced for important device characteristics such as I-V characteristics, endurance behavior and most significantly the SET and RESET kinetics. Furthermore, this enables the study of spatial and temporal variability and its impact on the circuit and system level.

The utilization of bipolar-type memristive devices for the realization of synaptic connectivity in neural networks strongly depends on the ability of the devices for analog conductance modulation under application of electrical stimuli in the form of identical voltage pulses. Typically, filamentary valence change mechanism (VCM)-type devices show an abrupt SET and a gradual RESET switching behavior. Thus, it is challenging to achieve an analog conductance modulation during SET and RESET. Here, we show that analog as well as binary conductance modulation can be achieved in a Pt/HfO₂/TiO_x/Ti VCM cell by varying the operation conditions. By analyzing the switching dynamics over many orders of magnitude and comparing to a fully dynamic switching model, the origin of the two different switching modes is revealed. SET and RESET transition show a two-step switching process: a fast conductance change succeeds a slow conductance change. While the time for the fast conductance change, the transition time, turns out to be state-independent for a specific voltage, the time for the slow conductance change, the delay time, is highly state-dependent. Analog switching can be achieved if the pulse time is a fraction of the transition time. If the pulse time is larger than the transition time, the switching becomes probabilistic and binary. Considering the effect of the device state on the delay time in addition, a procedure is proposed to find the ideal operation conditions for analog switching.

The transport properties of edge dislocations comprising a symmetrical 6° [001] tilt grain boundary in weakly acceptor-doped SrTiO₃ were investigated by means of various experimental and computational techniques. Oxygen transport along the dislocation array was probed by means of ¹⁸O/¹⁶O exchange experiments under (standard) oxidising conditions (pO₂ = 5 × 10⁻¹ bar) and also under reducing conditions (pO₂ = 7 × 10⁻²² bar) at T = 973 K. In both cases, isotope profiles obtained by Secondary Ion Mass Spectrometry (SIMS) indicated no evidence of fast diffusion along the dislocation array. Charge transport across the dislocation array was probed in equilibrium electrical conductivity measurements as a function of oxygen partial pressure, 10⁻²³ ≤ pO₂/bar ≤ 1 at temperatures of T/K = 950, 1050, 1100. A significant decrease in the conductivity of the bicrystal (relative to that of a single crystal) was observed under oxidising conditions, but not under reducing conditions. These studies were complemented by static lattice simulations employing empirical pair-potentials. The simulations predict, that the tilt boundary comprises two types of dislocation cores, that the formation of oxygen vacancies is energetically preferred at both cores relative to the bulk, and that the migration of oxygen ions along both cores is hindered relative to the bulk. Combining all results and literature reports, we present a comprehensive and consistent picture of the transport properties of dislocations in SrTiO₃.

A thermodynamic theory is used to calculate the dielectric properties of epitaxial thin films grown on cubic substrates imposing biaxial tension on the film prototypic cubic state. For such “tensile” substrates, the theory shows that, in a conventional plate–capacitor setup, the films should display only a monotonic variation or a broad maximum in the temperature dependence of the permittivity and an apparent absence of the hysteretic polarization behavior. Changes of the Curie–Weiss temperature and constant caused by the mechanical film/substrate interaction are also predicted. Theoretical results agree with the observed properties of columnar-structured ${\mathrm{BaTiO}}_3$ and ${\mathrm{SrTiO}}_3$ thin films grown on Si.