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The investigation of the structural phase transition in the vicinity of the Curie temperature Tc$T_{\text{c}}$ of LiNb1−xTaxO3$\left(\text{LiNb}\right)_{1 - x} \left(\text{Ta}\right)_{x} \left(\text{O}\right)_{3}$ crystals is motivated by the expected combination of advantageous high‐temperature properties of LiNbO3$\left(\text{LiNbO}\right)_{3}$ and LiTaO3$\left(\text{LiTaO}\right)_{3}$, including high piezoelectric modules and remarkable high‐temperature stability, respectively. Tc$T_{\text{c}}$ marks the ultimate limit for exploiting the piezoelectric properties; however, transition‐related structural modifications might impact this and other properties even below Tc$T_{\text{c}}$. 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 Tc$T_{\text{c}}$ for both the end compounds LiNbO3$\left(\text{LiNbO}\right)_{3}$ and LiTaO3$\left(\text{LiTaO}\right)_{3}$, and might thus affect the electrical conductivity. Above Tc$T_{\text{c}}$, 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. [ABSTRACT FROM AUTHOR]

Ceramic dielectrics with particularly low levels of dielectric loss and low permittivity are of growing interest toward millimeter‐wave applications. Based on composition and structural considerations, lithic kosmochlor is expected to meet these requirements while exhibiting low densification temperatures, allowing its integration also in co‐fired circuits. Herein, it is found that the spark plasma sintering of LiCrSi2O6 ceramics facilitates densification at temperatures more than 200 °C lower than conventionally processed materials in only a fraction of the time. A spark plasma sintering duration of 10 min is necessary in order to achieve a controllable process with a repeatable high‐performance product material exhibiting a fine‐grained microstructure. Characterization millimeter‐wave frequencies reveal that LiCrSi2O6 materials produced in this manner exhibit excellent dielectric properties (Qf = 80 700 GHz and εr = 7.5 at 134.24 GHz) following densification at only 950 °C with a relative density of 97.4%. These results represent an unprecedented combination of low dielectric losses and processing temperatures for ceramics processed without the addition of sintering aids. The materials and methods explored here present a promising pathway toward high‐performance millimeter‐wave and co‐firable systems. [ABSTRACT FROM AUTHOR]

Laser‐flash measurements of the thermal conductivity λ(hkl)$\lambda _{(hkl)}$ of Czochralski‐grown β‐Ga2O3 single crystals are reported perpendicular to four independent crystallographic lattice planes (hkl)=(2¯01)$(hkl)=(\bar{2}01)$, (010), (001), and (100) between room temperature and 1200 K. λ is represented by a symmetric tensor of rank 2 and has four independent components. Tensor components λ11=λ(100)=12.13$\lambda _{11}=\lambda _{(100)}=12.13$ W m−1 K−1 and λ22=λ(010)=24.26$\lambda _{22}=\lambda _{(010)}=24.26$ W m−1 K−1 (values for 300 K) can be directly measured. λ33=14.09$\lambda _{33}=14.09$ W m−1 K−1 and λ13=−0.992$\lambda _{13}=-0.992$ W m−1 K−1, instead, are obtained by combining λ(001)=13.53$\lambda _{(001)}=13.53$ W m−1 K and λ(2¯01)=14.33$\lambda _{(\bar{2}01)}=14.33$ W m−1 K. The crystallographic b⃗$\vec{b}$‐axis is the direction with the highest thermal conductivity, λ22. The thermal conductivity assumes its lowest value (11.73 W m−1 K−1) in a direction close to the crystallographic a⃗$\vec{a}$‐axis. However, this direction does not coincide with a⃗$\vec{a}$, and changes with temperature. [ABSTRACT FROM AUTHOR]

Large bulk LaInO3 single crystals are grown from the melt contained within iridium crucibles by the vertical gradient freeze (VGF) method. The obtained crystals are undoped or intentionally doped with Ba or Ce, and enabled wafer fabrication of size 10 × 10 mm2. High melting point of LaInO3 (≈1880 °C) and thermal instability at high temperatures require specific conditions for bulk crystal growth. The crystals do not undergo any phase transition up to 1300 °C, above which a noticeable thermal decomposition takes place. The good structural quality of the crystals makes them suitable for epitaxy. The onset of strong optical absorption shows orientation‐dependent behavior due to the orthorhombic symmetry of the LaInO3 crystals. Assuming direct transitions, optical bandgaps of 4.35 and 4.39 eV are obtained for polarizations along the [010] and the [100], [001] crystallographic directions, respectively. There is an additional weak absorption in the range between 2.8 and 4 eV due to oxygen vacancies. Density‐functional‐theory calculations support the interpretation of the optical absorption data. Cathodoluminescence spectra show a broad, structured emission band peaking at ≈2.2 eV. All bulk crystals are electrically insulating. The relative static dielectric constant is determined at a value of 24.6 along the [001] direction. [ABSTRACT FROM AUTHOR]

Cubic rare‐earth sesquioxide crystals are strongly demanded host materials for high power lasers, but due to their high melting points investigations on their thermodynamics and the growth of large‐size crystals of high optical quality remain a challenge. Detailed thermal investigations of the ternary system Lu2O3–Sc2O3–Y2O3 revealing a large range of compositions with melting temperatures below 2200°C and a minimum of 2053°C for the composition (Sc0.45Y0.55)2O3 are presented. These reduced temperatures enable for the first time the growth of high optical quality mixed sesquioxide crystals with disordered structure by the conventional Czochralski method from iridium crucibles. An (Er0.07Sc0.50Y0.43)2O3 crystal is successfully grown and characterized with respect to its crystallographic properties as well as its composition, thermal conductivity and optical absorption in the 1 µm range. [ABSTRACT FROM AUTHOR]

The phase relations in the system with the overall chemistry Nd2O3:Lu2O3 around 1:1 were investigated by x-ray diffraction (XRD) of annealed powders and by differential thermal analysis (DTA). A mixture of rare earth oxide solid solutions in the cubic C and in the monoclinic B rare earth oxide structure was determined in the temperature range 1200 to 1650 °C. The C-phase is dominated by Lu content while the B-phase is Nd rich. With rising annealing temperature, the portion of Nd in the C-phase is increased and lowered for the B-phase. Different facilitation possibilities for P-NdLuO3 perovskite formation were discussed and examined but no evidence for stability of this phase could be found experimentally. Two thermal signals of phase transitions were detected. The first occurs at 1908 °C and disappears in second heating run if the temperature is raised too much. Furthermore, a second one was detected at 2121 °C. [ABSTRACT FROM AUTHOR]

Measurements of thermal diffusivity ? (using laser flash analysis) for temperatures from 100 °C to 1200 °C and of the inversion parameter x (using X-ray analysis) for the single crystal spinel MgGa2O4 have been performed. X-ray analysis showed a small change in x for samples annealed at 800 °C (x = 0.838) and 1200 °C (x = 0.834). Thermal diffusivity revealed a significant rise of ? for temperatures higher than 950 °C, deviating from the typical 1/T drop of ?. An additional systematic rise of ? with time is observed for temperatures between 950 °C and 1100 °C. A connection between cation disordering processes typical for spinels at elevated temperatures and the observed deviation of ? is proposed: Data indicate that cations changing their occupancy site at equilibration processes contribute significantly to ? and therefore represent a new type of heat transport. [ABSTRACT FROM AUTHOR]

Bulk MgGa2O4 single crystals with inverse spinel structure were grown from the melt by different methods. The degree of inversion could be changed by suitable annealing, which was confirmed by differential scanning calorimetry analysis and corresponding changes of the specific heat capacity. MgGa2O4 is thermally much more stable at high temperatures than β-Ga2O3 despite of a higher melting point of about 1930 °C and could be grown under a neutral atmosphere. Melt-grown MgGa2O4 crystals were found to be either electrical insulators or n-type semiconductors depending on the presence of oxygen in the growth atmosphere and the growth method applied. Growing the crystals in the presence of oxygen resulted in electrically insulating crystals. For as-grown and intentionally undoped semiconducting crystals, the free electron concentration was in the range of 1017-1018 cm−3, but the electron mobility was relatively low, just a few cm2V−1s−1. The free electron concentration remained at a level of 1018 cm−3 after annealing in a hydrogen-containing atmosphere at 600-900 °C for 10 h. On the other hand, annealing in an oxygen-containing atmosphere above 600 °C for 10-40 h turns the crystals from the semiconducting to the insulating state. The optical bandgap at room temperature amounts to about 4.9 eV. It decreases with temperature at a rate of 1.35 meV/K. Cathodoluminescence spectra of as-grown crystals show a dominant band at 362 nm. The melt-grown crystals have sufficient size and structural quality to be used as substrates for epitaxy. [ABSTRACT FROM AUTHOR]

Epitaxial β-Ga2O3 layers have been grown on β-Ga2O3 (100) substrates using metal-organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano-crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β-Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β-Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β-Ga2O3 epitaxial layers were characterized by X-ray diffraction pattern and high resolution transmission electron microscopy. As-grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2 θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen-containing atmosphere the reflections of the layers do shift back to the position of the bulk β-Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment. [ABSTRACT FROM AUTHOR]

SnO2 is a semiconductor with a wide optical bandgap (3.5 eV), which makes it an attractive transparent semiconducting oxide (TSO) for electronic and opto-electronic applications. At elevated temperatures it is, however, much more unstable than other TSOs (such as ZnO, Ga2O3, or In2O3). This leads to a rapid decomposition even under very high oxygen pressures. Our experiments showed that stoichiometric SnO2 does not melt up to 2100 °C, in contradiction to earlier published data. Bulk SnO2 single crystals, that could provide substrates for epitaxial growth, have not been reported so far. Hereby we report on truly bulk SnO2 single crystals of 1 inch diameter grown by physical vapor transport (PVT). The most volatile species during SnO2 decomposition is, in addition to oxygen, SnO, which is stable in the gas phase at high temperature and reacts again with oxygen at lower temperatures to form SnO2. We identified a relatively narrow temperature window, temperature gradients and a ratio of SnO/O2 for providing the best conditions for SnO2 single crystal growth. X-ray powder diffraction (XRD) proved the single SnO2 phase. Moreover, by selecting a suitable SnO/O2 ratio it was possible to obtain either n-type conductivity with electron concentrations up to 2 × 1018 cm−3 and electron mobilities up to 200 cm2 V−1 s−1, or insulating behavior. The crystals exhibited an optical absorption edge located at 330-355 nm, depending on the crystal orientation, and a good transparency over visible and near infrared (NIR) spectra. [ABSTRACT FROM AUTHOR]

The thermal history of an impact glass from the El'gygytgyn impact structure, central Chukotka, Russia has been investigated with the method of relaxation geospeedometry. The cooling rate of the melt fragment has been quantified by heat capacity ( c p) measurements using differential scanning calorimetry. Cycles of thermal treatments applied to the melt fragment sample result in a set of unique kinetic parameters, which were then used to model heat capacity curves. The Tool-Narayanaswamy-Moynihan approach was used to quantify the natural cooling rate from changes in the normalized heat capacity curves. Cooling rates ranging from 0.06 to 0.40 K min−1 were found by modeling the structural relaxation within the glass transition interval. Because of the absence of thermal reheating, a single natural annealing process is thought to have been responsible for the modeled cooling rate, even though a heat buffering process that affects the cooling history might have been involved. The El'gygytgyn impact glass cooled significantly slower than previously studied tektites and faster than most volcanic glasses. The calculated viscosity of log η = 10.75 ± 0.50 Pa s at the glass transition temperature Tgpeak (1020 K) supports this assumption. Consequently, a slow cooling process is accompanied by a high viscosity η. [ABSTRACT FROM AUTHOR]

The thermodynamic and crystallographic background for the development of substrate crystals that are suitable for the epitaxial deposition of biaxially strained functional perovskite layers is reviewed. In such strained layers the elastic energy delivers an additional contribution to the Gibbs free energy, which allows the tuning of physical properties and phase transition temperatures to desired values. For some oxide systems metastable phases can even be accessed. Rare‐earth scandates, REScO3, are well suited as substrate crystals because they combine mechanical and chemical stability in the epitaxy process with an adjustable range of pseudo‐cubic lattice parameters in the 3.95 to 4.02 Å range. To further tune the lattice parameters, chemical substitution for the RE or Sc is possible. [ABSTRACT FROM AUTHOR]

The ternary phase diagram of copper–iron–aluminium delafossite is established with FactSage calculations to improve the conditions for solution growth and the electrical conductivity of the transparent p‐type semiconductor copper–aluminium delafossite by addition of iron oxide. Initial growth experiments with the optical float zone method show that the growth of single crystals with commercially relevant dimensions is not expected in the near future. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses various reports within the issue on topics including point of crystal defects in semiconductors and dielectrics, crystal growth twinning, and the properties and the production of crystalline materials.

Ceramic dielectrics with particularly low levels of dielectric loss and low permittivity are of growing interest toward millimeter‐wave applications. Based on composition and structural considerations, lithic kosmochlor is expected to meet these requirements while exhibiting low densification temperatures, allowing its integration also in co‐fired circuits. Herein, it is found that the spark plasma sintering of LiCrSi2O6 ceramics facilitates densification at temperatures more than 200 °C lower than conventionally processed materials in only a fraction of the time. A spark plasma sintering duration of 10 min is necessary in order to achieve a controllable process with a repeatable high‐performance product material exhibiting a fine‐grained microstructure. Characterization millimeter‐wave frequencies reveal that LiCrSi2O6 materials produced in this manner exhibit excellent dielectric properties (Qf = 80 700 GHz and εr = 7.5 at 134.24 GHz) following densification at only 950 °C with a relative density of 97.4%. These results represent an unprecedented combination of low dielectric losses and processing temperatures for ceramics processed without the addition of sintering aids. The materials and methods explored here present a promising pathway toward high‐performance millimeter‐wave and co‐firable systems. [ABSTRACT FROM AUTHOR]

The phase relations in the system with the overall chemistry Nd2O3:Lu2O3 around 1:1 were investigated by x-ray diffraction (XRD) of annealed powders and by differential thermal analysis (DTA). A mixture of rare earth oxide solid solutions in the cubic C and in the monoclinic B rare earth oxide structure was determined in the temperature range 1200 to 1650 °C. The C-phase is dominated by Lu content while the B-phase is Nd rich. With rising annealing temperature, the portion of Nd in the C-phase is increased and lowered for the B-phase. Different facilitation possibilities for P-NdLuO3 perovskite formation were discussed and examined but no evidence for stability of this phase could be found experimentally. Two thermal signals of phase transitions were detected. The first occurs at 1908 °C and disappears in second heating run if the temperature is raised too much. Furthermore, a second one was detected at 2121 °C. [ABSTRACT FROM AUTHOR]

Cubic rare‐earth sesquioxide crystals are strongly demanded host materials for high power lasers, but due to their high melting points investigations on their thermodynamics and the growth of large‐size crystals of high optical quality remain a challenge. Detailed thermal investigations of the ternary system Lu2O3–Sc2O3–Y2O3 revealing a large range of compositions with melting temperatures below 2200°C and a minimum of 2053°C for the composition (Sc0.45Y0.55)2O3 are presented. These reduced temperatures enable for the first time the growth of high optical quality mixed sesquioxide crystals with disordered structure by the conventional Czochralski method from iridium crucibles. An (Er0.07Sc0.50Y0.43)2O3 crystal is successfully grown and characterized with respect to its crystallographic properties as well as its composition, thermal conductivity and optical absorption in the 1 µm range. [ABSTRACT FROM AUTHOR]