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La-doped SrTiO3thin films with high structural quality were homoepitaxially grown by the metal–organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal–organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal–organic compound La(tmhd)3and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ions on the Sr2+perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden–Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3thin films grown by MOVPE for thermoelectric applications.

Iron(II) oxide, wüstite, is the iron oxide with the lowest oxygen content. Under ambient conditions it is metastable for two reasons: (1) it undergoes eutectoid decomposition to Fe and Fe3O4below ≈570° C, and (2) depending on temperature, it is thermodynamically stable only for very low oxygen partial pressures, down to below 10−20bar. Hence, for the growth of single crystals from the melt, the growth atmosphere must contain reducing components to keep the oxygen partial pressure on the required low level. With Ar/CO2/CO mixtures this aim can be reached. It is shown experimentally and by thermodynamic calculation, that the grown crystals contain carbon inclusions. Theoretically it is shown that wüstite crystals without carbon inclusions could be grown in humid N2/H2mixtures. First experiments are presented in this article, but a further adjustment of experimental parameters is required.

Pb2Sc2Si2O9and Pb2In2Si2O9, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna21space group and undergo phase transitions to the centrosymmetric Pbcnspace group at elevated temperatures. The Pbcninto Pna21phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.

Abstract: We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015and 5 × 1020cm−3and Hall electron mobilities (μH) up to 240 cm2V−1s−1. The widest range of nevalues was achieved for β-Ga2O3and In2O3. The most electrically conducting bulk crystals are InGaZnO4and ZnSnO3with ne> 1020cm−3and μH> 100 cm2V−1s−1. The highest μHvalues > 200 cm2V−1s−1were measured for SnO2, followed by BaSnO3and In2O3single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4crystals were always conducting, while others could be turned into electrical insulators. Graphic abstract:

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 LiCrSi2O6ceramics 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 LiCrSi2O6materials 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. Spark plasma sintering is shown to be an effective approach toward enhancing the low‐temperature densification characteristics of LiCrSi2O6dielectric ceramics for millimeter‐wave applications. The results obtained here represent a unique combination of low dielectric losses and low processing temperatures for ceramics processed without the addition of sintering aids.

We present a new approach for scaling-up the growth of b-Ga2O3 single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.

Bulk MgGa2O4single 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. MgGa2O4is thermally much more stable at high temperatures than β‐Ga2O3despite of a higher melting point of about 1930 °C and could be grown under a neutral atmosphere. Melt‐grown MgGa2O4crystals 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 1017–1018cm−3, but the electron mobility was relatively low, just a few cm2V−1s−1. The free electron concentration remained at a level of 1018cm−3after 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.

We describe a low-cost, compact, and efficient Cr^3+:LiCaAlF_6 (Cr:LiCAF) laser that is broadly tunable in the near infrared (746–887 nm) and blue/ultraviolet (375–433 nm) regions of the optical spectrum. The pump source is a 660 nm single-mode-diode with 145 mW of output power. A melt-grown Cr:LiCAF crystal with an extremely high figure of merit above 2000 was used as the gain medium. This enabled the construction of a high-Q-cavity with continuous wave (cw) lasing thresholds as low as 3 mW, slope efficiencies as high as 54%, and output powers up to 63 mW. The stored intracavity power levels were above 30 W. By placing a beta-barium-borate crystal at a second intracavity focus, we could obtain tunable cw blue/ultraviolet radiation with output powers up to 3.5 mW around 400 nm. When mode-locked using a saturable Bragg reflector around 800 nm, the Cr:LiCAF laser produced 95 fs pulses with average powers of 33 mW and peak powers of 3.58 kW at a repetition rate of 85.5 MHz. Extracavity second harmonic generation enables generation of femtosecond pulses around 400 nm. These results demonstrate the possibility to grow Cr:LiCAF crystals with passive losses below 0.15% per cm, which enables construction of low-cost and complexity systems that are pumped only with one low-power single-mode-diode.

The potential applications of transition metal-containing olivines, (M,Mg)2SiO4, in environmental sustainability and renewable energy rely on a better understanding of their internal structure, defect chemistry, and sublattice processes at high temperatures. In the olivine crystal structure, divalent cations occupy two nonequivalent octahedral sites. The kinetics of octahedral cation exchange between the two sites in a (Co0.21Mg0.79)2SiO4single crystal has been studied from 500 to 700 °C by means of time-resolved optical relaxation spectroscopy upon rapid temperature-jumps. Our experiments show that the cation distribution in the two octahedral sites changes toward a random distribution with increasing temperature and that the exchange kinetics is strongly temperature dependent. Modeling of experimental relaxation data using a kinetic equation of cation exchange yields relaxation times of about 12 300 s at 500 °C and about 6 s at 700 °C, respectively, and an activation energy of about 230 ± 12 kJ/mol for the cation exchange reaction in the (Co0.21Mg0.79)2SiO4olivine. Calculations of vacancy concentrations based on a defect model and impurity levels in the (Co0.21Mg0.79)2SiO4single crystal have been used to interpret the experimentally observed dependence on oxygen activity. We developed as well a formula to correlate experimental relaxation times to the cation diffusion coefficient along the b-axis in olivines. Such a relation allows one to estimate Mg self-diffusion coefficients DbMgas well as Co−Mg interdiffusion coefficients Dbalong the b-axis, especially at low temperatures, for example, DbMg= 4.78 × 10−23m2/s and Db= 9.33 × 10−23m2/s at 600 °C. Cation interdiffusion coefficients from the extrapolation of our diffusion data to high temperatures are in agreement with available literature data from Co−Mg interdiffusion experiments.

Large bulk LaInO3single 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 LaInO3crystals. 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. Large LaInO3single crystals are grown from the melt by the vertical gradient freeze method. The obtained crystals are either undoped or intentionally doped with Ba or Ce and enable wafer fabrication of size 10 × 10 mm2. The structural quality, thermal stability, and physical properties of the obtained crystals are evaluated.

Mullite is a promising material with its competitive thermochemical and mechanical properties. Although mullite could be obtained by several synthesis methods, the flux method emerges with its advantages over other methods. However, obtaining mullite whiskers with a high aspect ratio and length for ceramic reinforcements is still challenging. In this work, mullite whiskers were grown from AlF3-assisted flux. The addition of AlF3to flux salt not only decreases the formation temperature of mullite to as low as 700 °C and suppresses the formation of corundum side phase, but also increases the length and aspect ratio of the whiskers. The obtained mullite whiskers were used as reinforcement for porous alumina monoliths prepared by the freeze casting route and subsequent sintering at 1500 °C. The fabricated mullite-alumina monoliths show competitive compressive strength of 25.7 MPa while having as high as 70.6 % porosity, which makes them a potential candidate for membrane applications.

We present a new approach for scaling-up the growth of β-Ga2O3single crystals grown from the melt by the Czochralski method, which has also a direct application to other melt-growth techniques involving a noble metal crucible. Experimental and theoretical results point to melt thermodynamics as the crucial factor in increasing the volume of a growing crystal. In particular, the formation of metallic gallium in the liquid phase in large melt volumes causes problems with crystal growth and eutectic or intermetallic phase formation with the noble metal crucible. The larger crystals to be grown the higher oxygen concentration is required. The minimum oxygen concentration ranges from about 8 to 100 vol.% for 2 to 4 inch diameter cylindrical crystals, challenging the use of iridium crucibles in a combination with such high oxygen concentrations. A specific way of oxygen delivery to a growth furnace with the iridium crucible allows to minimize the formation of metallic gallium in the melt and thus obtaining large crystal volumes while decreasing the probability of the eutectic formation.