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Conductive ferroelectric domain walls (DWs) represent a promising topical system for the development of nanoelectronic components and devices. DWs show very different properties as compared to their bulk counterparts. Of central interest here is the domain wall current (DWC) of charged DWs in 5mol\% Mg-doped lithium niobate single crystals; in contrast to former works, we extend the DWC study here to temperatures as high as 400$^\circ$C. Both the temporal stability and the thermal activation energies of 90 - 160 meV are readily deduced from current-voltage sweeps as recorded over multiple heating cycles. Our experimental work is backed up by atomistic modelling of the DWC. The latter suggests that a large band bending renders head-to-head and tail-to-tail DWs semimetallic. These detailed investigations underline the potential to extend DWC-based nanoelectronic applications even into the so-far unexplored high-temperature regime., Comment: 5 pages, 4 figures; with included supplementary material (6 pages, 9 figures)

The ferroelectric to paraelectric phase transition in LiTaO$_3$ and in pure as well as Mg doped LiNbO$_3$ is investigated theoretically by atomistic calculations in the framework of the density functional theory, as well as experimentally by calorimetry and electrical conductivity measurements. First principles models within the stochastic self-consistent harmonic approximation (SSCHA) allow to consider anharmonic effects and thus to obtain a realistic estimate of the Curie temperature $T_C$ of both ferroelectrics. \textit{Ab initio} molecular dynamics (AIMD) calculations performed on large supercells confirm the Curie temperatures estimated with the SSCHA approach. Moreover, they also suggest that the structural phase transition is a continuous process beginning at temperatures well below $T_C$. According to AIMD, significant ionic displacements occurr already at temperatures of about 100\,K and 300\,K below $T_C$ in LiTaO$_3$ and LiNbO$_3$, respectively. To asses whether and how far the ionic displacements affect the materials properties, the AIMD results are compared with measurements of the electrical conductivity and of the heat capacity across the phase transition. Our first principles calculations moreover show that Mg ions, a frequently employed dopant, raise the Curie temperature in LiNbO$_3$.

The investigation of the structural phase transition in the vicinity of the Curie temperature $T_c$ of LiNb$_{1-x}$Ta$_x$O$_3$ crystals is motivated by the expected combination of advantageous high-temperature properties of LiNbO$_3$ and LiTaO$_3$, including high piezoelectric modules and remarkable high-temperature stability, respectively. $T_c$ marks the ultimate limit for exploiting the piezoelectric properties, however transition related structural modifications might impact this and other properties even below $T_c$. Remarkably, the phase transition from the ferroelectric to the paraelectric phase, whose temperature strongly depends on the composition $x$, shows a significant drop in the activation energy of the electrical conductivity. The magnitude, temperature dependence and underlying mechanisms of this drop are discussed from a microscopic perspective. Molecular dynamics calculations in the framework of the density functional theory show that substantial displacements of the cations occur below $T_c$ for both the end compounds LiNbO$_3$ and LiTaO$_3$, and might thus affect the electrical conductivity. Above $T_c$, the migration of lithium ions is presumably facilitated by a shortened diffusion path for the most favorable jump of the lithium ions. Electronic contributions to the conductivity, which become important above 900 K, are explained within the polaronic picture by the formation and migration of free small polarons., Comment: I submitted it to The Physica Status Solidi A: Applications and Materials Science on 12/21/2023 and it's under review. It will be updated after receiving reviews

The current work is focused on the impact of the lithium stoichiometry on electrical conductivity, acoustic properties and high-temperature stability of single crystalline Li(Nb,Ta)O$_{3}$ at high temperatures. The crystals grown from Li-deficient melts were treated by the vapor transport equilibration (VTE) method, achieving near stoichiometric Li-content. It is shown, that the VTE-treated specimens generally exhibit lower conductivity at temperatures below 800 {\deg}C, which is attributed to the reduced number of Li-vacancies in near stoichiometric Li(Nb,Ta)O$_{3}$, provided that the Li-ion migration dominates the conductivity in this temperature range. Further, it is shown, that above 600-650 {\deg}C different mechanism increasingly contributes to the conductivity, which is consequently attributed to the electronic conduction. Further, it is shown that losses in LNT strongly increase above about 500 {\deg}C, which is interpreted to originate from conductivity-related relaxation mechanism. Finally, the thermal stability of Li(Nb,Ta)O$_{3}$ is evaluated by the measurement of the conductivity and resonance frequency as a function of time. It is found that during annealing at 700 {\deg}C for 350 hours, the resonance frequency of LiNbO$_{3}$ remains in a {\textpm} 100 ppm range of the initial value of 3.5 MHz., Comment: Submitted to Physica Status Solidi A: Applications and Materials Science on 21st December 2023

The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, ${\rm LiTaO}_3$ (LT), in order to address the question whether it is possible to localize electrons at interstitial ${\rm Ta_V}$:${\rm V_{Li}}$ defect pairs by strong, short-range electron-phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound ${\rm Ta}_{\rm Li}^{4+}$ (antisite) and ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ (interstitial site) electron polarons. As a result, two energetically well separated ($\Delta E \approx 0.5\,{\rm eV}$) absorption features with a distinct dependence on the probe light polarization and peaking at $1.6\,{\rm eV}$ and $2.1\,{\rm eV}$ are discovered. We contrast our results to the interpretation of a single small bound ${\rm Ta}_{\rm Li}^{4+}$ electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial ${\rm Ta}_{\rm V}^{4+}$:${\rm V_{Li}}$ small polarons in the framework of optical gating of ${\rm Ta}_{\rm V}^{4+}$:${\rm Ta}_{\rm Ta}^{4+}$ bipolarons. We can conclude that the appearance of carrier localization at $\mathrm{Ta_V}$:${\rm V_{Li}}$ must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to ${\rm Ta_{Li}}$, as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.