Search

LiNb$_{1-x}$Ta$_x$O$_3$ solid solutions are investigated from first principles and by optical spectroscopy. The ground- and excited-state properties of the solid solutions are modelled within density functional theory as a function of the Ta concentration using special quasirandom structures spanning the entire composition range between LiNbO$_3$ and LiTaO$_3$. Deviations from a Vegard behavior are predicted for the lattice parameters, the heat capacity, the electronic bandgap, and consequently the absorption edge. The latter is measured for crystals of different composition by low temperature optical spectroscopy, qualitatively confirming the theoretical predictions. The LiNb$_{0.11}$Ta$_{0.89}$O$_3$ composition is found to be a highly unusual crystal with a permanent macroscopic electric polarization and nonetheless zero birefringence.

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

Femto-/nanosecond pulse-induced, red and near-infrared absorption is studied in $\mathrm{LiNb}_{1-x}\mathrm{Ta}_{x}\mathrm{O}_3$ (LNT) solid solutions with the goal to probe the intrinsic defect structure via the formation, transport and recombination of optically generated small bound electron polarons with strong coupling to the lattice. As a result, long-lived transients are uncovered for LNT which exceed lifetimes of LN and LT by a factor of up to 100 over the entire range of investigated compositions. At the same time, the starting amplitude varies in the range of $\alpha_\mathrm{li}^0\approx10-100\,\mathrm{m}^{-1}$ as a function of $x$ and exceed the ones of LN and LT by a factor of up to ten. The results are interpreted in the model of three-dimensional small polaron hopping transport considering the simultaneous presence of three different types of small bound polarons, in particular of small electron $\mathrm{Nb}_\mathrm{Li}^{4+}$ and $\mathrm{Ta}_\mathrm{Li}^{4+}$ antisite polarons, and of small electron $\mathrm{Ta}_\mathrm{V}^{4+}$ interstitial polarons. We conclude that the differences between LNT, LN, and LT may point to model systems that consist of one (LN), two (LT) and three (LNT) intrinsic defect centers for electron localization.

The phase diagram of the lithium niobate and lithium tantalate solid solutions was investigated using experimental data from differential thermal analysis (DTA) and crystal growth. We used XRF analysis to determine the elemental composition of crystals. Based on the Neumann-Kopp rule, essential data of end members lithium niobate (LN) and lithium tantalate (LT) was created. The heats of fusion of end members given by DTA measurements of LN (103 kJ/mol at 1531 K) and LT (289 kJ/mol at 1913 K) were given as input parameters to generate the data. This data served as the basis for calculating a phase diagram for LN and LT solid solutions. Finally, based on the experimental data and thermodynamic solution model, the phase diagram was optimized in the Calphad Factsage module. We also generated thermodynamic parameters for Gibb's excess energy of the solid solution. A plot of segregation coefficient as a function of Ta concentration was derived from the phase diagram.

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

In condensed-matter systems, competition between ground states at phase boundaries can lead to significant changes in material properties under external stimuli, particularly when these ground states have different crystal symmetries. A key scientific and technological challenge is to stabilize and control coexistence of symmetry-distinct phases with external stimuli. Using BiFeO3 (BFO) layers confined between layers of the dielectric TbScO3 as a model system, we stabilize the mixed-phase coexistence of centrosymmetric and non-centrosymmetric BFO phases with antipolar, insulating and polar, semiconducting behavior, respectively at room temperature. Application of in-plane electric (polar) fields can both remove and introduce centrosymmetry from the system resulting in reversible, nonvolatile interconversion between the two phases. This interconversion between the centrosymmetric insulating and non-centrosymmetric semiconducting phases coincides with simultaneous changes in the non-linear optical response of over three orders of magnitude, a change in resistivity of over five orders of magnitude, and a change in the polar order. Our work establishes a materials platform allowing for novel cross-functional devices which take advantage of changes in optical, electrical, and ferroic responses., Comment: L Caretta and Y-T Shao contributed equally. 64 pages, 4 figures, 25 supplemental figures, 1 supplemental table

Transmission electron microscopes use electrons with wavelengths of a few picometers, potentially capable of imaging individual atoms in solids at a resolution ultimately set by the intrinsic size of an atom. Unfortunately, due to imperfections in the imaging lenses and multiple scattering of electrons in the sample, the image resolution reached is 3 to 10 times worse. Here, by inversely solving the multiple scattering problem and overcoming the aberrations of the electron probe using electron ptychography to recover a linear phase response in thick samples, we demonstrate an instrumental blurring of under 20 picometers. The widths of atomic columns in the measured electrostatic potential are now no longer limited by the imaging system, but instead by the thermal fluctuations of the atoms. We also demonstrate that electron ptychography can potentially reach a sub-nanometer depth resolution and locate embedded atomic dopants in all three dimensions with only a single projection measurement., Comment: 13 pages and 4 figures in the main text. Supplementary materials included

In this work, a first cryogenic characterization of a scintillating LiAlO$_{2}$ single crystal is presented. The results achieved show that this material holds great potential as a target for direct dark matter search experiments. Three different detector modules obtained from one crystal grown at the Leibniz-Institut f\"ur Kristallz\"uchtung (IKZ) have been tested to study different properties at cryogenic temperatures. Firstly, two 2.8 g twin crystals were used to build different detector modules which were operated in an above-ground laboratory at the Max Planck Institute for Physics (MPP) in Munich, Germany. The first detector module was used to study the scintillation properties of LiAlO$_{2}$ at cryogenic temperatures. The second achieved an energy threshold of (213.02$\pm$1.48) eV which allows setting a competitive limit on the spin-dependent dark matter particle-proton scattering cross section for dark matter particle masses between 350 MeV/c$^{2}$ and 1.50 GeV/c$^{2}$. Secondly, a detector module with a 373 g LiAlO$_{2}$ crystal as the main absorber was tested in an underground facility at the Laboratori Nazionali del Gran Sasso (LNGS): from this measurement it was possible to determine the radiopurity of the crystal and study the feasibility of using this material as a neutron flux monitor for low-background experiments.

Based on differential thermal analysis (DTA) and X-ray powder diffraction (XRD), a description of the system Nd$_2$O$_3$--Lu$_2$O$_3$--Sc$_2$O$_3$ was obtained by thermodynamic assessment. Four fields of primary crystallization could be identified; from melt compositions close to the Lu$_2$O$_3$--Sc$_2$O$_3$ edge the rare-earth oxide C-phase crystallizes first, which is stable down to room temperature. From Nd$_2$O$_3$ rich melts the X-phase forms, which is stable only at high temperatures. An additional field, where the alternative high-temperature phase H solidifies as primary product touches the Nd$_2$O$_3$--Lu$_2$O$_3$ edge of the concentration triangle. From melts close to the composition NdScO$_3$, the P-phase (perovskite) can be crystallized and mixed crystals with second end member NdLuO$_3$ have been grown from the melt. Crystals of this mixed perovskite were grown by the micro-pulling-down and Czochralski methods., Comment: 16 pages, 8 figures, submitted to J. Crystal Growth

When calcium titanate crystals are grown from stoichiometric melts, they crystallize in the cubic perovskite structure. Upon cooling to room temperature they undergo subsequent phase transitions to tetragonal and orthorhombic modifications. These phase transitions are disruptive and result in severely damaged crystals. This paper presents differential thermal analysis data for several prospective solvents, with the aim to identify a system offering the possibility to perform crystal growth of undistorted CaTiO$_3$ crystals by crystallizing them significantly below the melting point directly in the low temperature modification. From mixtures CaF$_2$:TiO$_2$:CaTiO$_3$ = 3:1:1 (molar ratio) the growth of undistorted, at least millimeter-sized CaTiO$_3$ crystals is possible., Comment: 12 pages, 6 figures, submitted to J. Crystal Growth

High-temperature thermal properties of three neighboring rare-earth scandates DyScO$_3$, TbScO$_3$ and GdScO$_3$ were compared to La$_{0.29}$Sr$_{0.71}$Al$_{0.65}$Ta$_{0.35}$O$_3$ (LSAT) and sapphire. To calculate thermal conductivity, heat capacity and thermal diffusivity were measured by differential scanning calorimetry and laser flash technique, respectively. DyScO$_3$ and TbScO$_3$ showed an untypical rise in the thermal conductivity above 900\,K, while for GdScO$_3$, LSAT and sapphire the expected decrease at elevated temperatures could be observed. These results lead to the proposal of a new type of heat transport by migrating ions., Comment: 15 pages, 4 figures, submitted to J. Alloys. Compd

Eutectic fibers consisting of an ordered arrangement of LiF fibrils inside a LiREF4 matrix (RE = Y, Gd) can be grown with the micro-pulling-down method at sufficiently large pulling rate exceeding 120 mm/h. The distance between individual fibrils could be scaled down to 1 micrometer at 300 mm/h pulling. LiF-LiYF4 has stronger tendency to form facetted eutectic colonies than LiF-LiGdF4, explained by the larger entropy of melting of the former., Comment: 6 pages, 5 figures, Talk on MRS Fall 2012 Boston

Small ruby crystals were grown by the Czochralski technique in different atmospheres and their actual chromium content was analysed by a wet chemical method. The chromium distribution coefficient k was found to be strongly dependent on oxygen partial pressure p(O2). It ranges from k=0.3 in a reducing atmosphere to k=1.2 in a slightly oxidizing atmosphere and to a good approximation k depends linearly on log(p(O2)). The experimental data are discussed on the basis of thermodynamic equilibrium calculations.

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3 and In2O3., Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Boston

The quasi-binary phase diagram lithium oxide - molybdenum(VI) oxide was investigated by differential scanning calorimetry and X-ray diffraction. The four intermediate phases Li4MoO5, Li2MoO4, Li4Mo5O17, and Li2Mo4O13 show incongruent melting. The system has one eutectic point at 50.5 mol% MoO3 and 49.5 mol% LiO0.5 with a eutectic temperature of 524.6 deg C. At this point the melt is in equilibrium with Li2MoO4 and Li4Mo5O17., Comment: excerpt from a published article, with 3 figures and 1 table

The binary phase diagram of the YAlO3 (YAP) - NdAlO3 (NAP) system was determined by differential thermal analysis (DTA) and X-ray powder diffraction (XRD) measurements. High purity nanocrystalline powders and small single crystals of Y_{1-x}Nd_{x}AlO_3 (0 \leq x \leq 1) have been produced successfully by modified sol-gel (Pechini) and micro-pulling-down methods, respectively. Both end members show high mutual solubility >25% in the solid phase, with a miscibility gap for intermediate compositions. A solid solution with x \approx 0.2 melts azeotropic ca. 20 degrees below pure YAP. Such crystals can be grown from the melt without segregation. The narrow solid/liquid region near the azeotrope point could be measured with a "cycling" DTA measurement technique., Comment: 12 pages, 8 figures, submitted to J. Alloys. Compd

Bulk crystals of zinc oxide can be grown from the melt by a Bridgman technique under pressure. This new technology using an iridium crucible shows the potential to yield large single crystals of good crystalline perfection. Crystals with diameters up to 33 mm and a length of up to 50 mm have been demonstrated. The impurity content can be strongly reduced by using the crucibles repeatedly., Comment: 6 pages, 2 figures, 1 table, talk on MRS Fall 2009 Boston, MA

Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10^{-6} bar, independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga2O3, or Fe{1-x}O can be grown from the melt., Comment: 4 pages, 3 figures, talk on CGCT-4 Sendai, May 21-24, 2008

Neodymium doped yttrium aluminum garnet powders (Nd:YAG or (Y{1-x}Nd{x})3Al5O12, x=0.15, 0.25, or 0.30, respectively) were prepared by a sol-gel technique. By DTA measurements up to 2000 deg C eutectic and liquidus temperatures could be determined. Exothermal peaks in the second and subsequent DTA heating runs indicate that the crystallized DTA samples are not in equilibrium. The section Y3Al5O12-Nd3Al5O12 of the concentration triangle Al2O3-Nd2O3-Y2O3 is proposed on the basis of thermodynamic calculations that allows to explain the experimental results by the balance of metastable phase states in the previously crystallized DTA samples., Comment: 6 pages, 4 figures

Attempts to grow terbium aluminium garnet (Tb3Al5O12, TAG) by the Czochralski method lead to crystals of millimeter scale. Larger crystals could not be obtained. DTA measurements within the binary system showed that TAG melts incongruently at 1840 deg. C. The perovskite (TbAlO3, TAP) with a congruent melting point of 1930 deg. C is the most stable phase in this system. The region for primary crystallization of TAP covers the chemical composition of TAG and suppresses the primary crystallization of the terbium aluminium garnet., Comment: 6 pages, 2 figures

The peculiar properties of zinc oxide (ZnO) make this material interesting for very different applications like light emitting diodes, lasers, and piezoelectric transducers. Most of these applications are based on epitaxial ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the thermochemical properties of ZnO make the growth of single crystals difficult: the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals for technical applications can only be grown either by a hydrothermal method or from "cold crucibles" of solid ZnO. Both methods are known to have significant drawbacks. Our thermodynamic calculations and crystal growth experiments show, that in contrast to widely accepted assumptions, ZnO can be molten in metallic crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result is believed to offer new perspectives for ZnO crystal growth by established standard techniques like the Bridgman method., Comment: 6 pages, 6 figures, accepted for J. Crystal Growth

Crystal growth experiments (micro-pulling down or Czochralski methods, respectively) and DTA/TG measurements with Fe2+ containing olivines (fayalite--forsterite solid solution) and with FeO (wustite) are performed. For both substances the oxygen partial pressure p_O2 of the growth atmosphere had to be adjusted within the stability region of Fe2+ for all temperatures ranging from room temperature to the melting point. The formation of Fe3+ (Fe3O4, Fe2O3) had to be avoided. The adjustment of p_O2 could be obtained by a mixture of argon, carbon dioxide and carbon monoxide. Thermodynamic equilibrium calculations show, that mixtures of an inert gas (e.g. argon) with another gas or gas mixture that supplies oxygen at elevated temperature (e.g. CO2/CO) are superior to the use of inert gases with constant oxygen admixture. The reason is that the Ar/CO2/CO mixture adjusts its oxygen concentration with temperature in a way similar to that needed for the stabilization of Fe2+., Comment: talk on 14th International Conference on Crystal Growth (ICCG14-ICVGE12) 9-13 August 2004, Grenoble, France