“Hydrothermal synthesis” usually refers to heterogeneous reactions in aqueous media above 100°C and 1 bar. The previously common distinction between hydrothermal conditions below and pneumatolytic conditions above the critical point is no longer made, since no discontinuities are observed upon exceeding the critical conditions. Under hydrothermal conditions, reactants otherwise difficult to dissolve go into solution as complexes, in whose formation water itself or very soluble “mineralizers” can participate. Thus, one can obtain the conditions of chemical transport reactions,[1] of which hydrothermal syntheses can be considered a special case. During recent decades in the geological sciences—in which the method is also historically rooted—it has received a strong impulse, whose effect on preparative solid state chemistry is discussed here.

The Role of Hydrothermal Synthesis in Preparative Chemistry

Chemical solution deposition of electronic oxide films

Oxidative nonstoichiometry in perovskites, an experimental survey; the defect structure of an oxidized lanthanum manganite by powder neutron diffraction

Future improvements in the development of highly volume-efficient multilayer ceramic capacitors (MLCC) require manufacturing processes that allow for a reduced dielectric thickness well below 1 μm. Obviously, such thin dielectric layers can only be produced if nanosized dielectric powders are applied and deposited by techniques more advanced than tape casting and screen printing. These processes require high-purity, homogeneous, weakly agglomerated ultrafine powders. Over the past few years, significant progress has been made in the synthesis of nanocrystalline BaTiO3-based powders for use in MLCC. The present review article aims to summarize the present state of the art with respect to some of the technically and industrially most relevant preparation routes, including methods based on solid-state reactions and, more importantly, solution-based approaches.

Progress in the Synthesis of Nanocrystalline BaTiO3 Powders for MLCC

Barium titanate (BaTiO3) powders of varying particle sizes were grown under hydrothermal conditions at a temperature of 240 °C. This was accomplished by varying the hydroxide concentration of the r...

Characterization of Submicron Particles of Tetragonal BaTiO3

Clear, transparent gels of BaTi acetate have been prepared from titanium tetrabutylate (TBT), Ba acetate aqueous solution, isopropanol and acetic acid. Infrared spectroscopy and transmission electron microscopy (TEM) of such gels revealed linked clusters of polymer Ti oxyacetate with surface adsorbed Ba acetate. Spray dried Ti oxyacetate has been found to have the stoichiometric composition Ti 2 O 3 [CH 3 COO] 2 . Calcination of BaTi acetate gels to BaTiO 3 in air turned out to be a less favourable preparative route, because stable intermediate carbonate phases were formed. Corresponding to the high surface area and fine dispersion of Ba and Ti, BaTi acetate gels are higly reactive precursors for hydrothermal synthesis of BaTiO 3 . Nearly quantitative (> 99·7%) chemical reactions were obtained without using Ba excess at processing. TEM provided interesting information about the chemical mechanisms of hydrothermal processing.

Hydrothermal preparation of barium titanate from barium-titanium acetate gel precursors

X-ray structure investigation of lanthanum modified lead titanate with A-site and B-site vacancies

Neun ternare Verbindungen wurden durch Hydrothermalsynthese in den entsprechenden Halogenwasserstoffsauren dargestellt. Sie liegen auf den pseudobinaren Schnitten, die Selen bzw. Tellur mit den jeweiligen Kupfer(I)-Halogeniden bilden. In den Selensystemen existieren: CuJSe3, rhomboedrisch; CuBrSe3, orthorhombisch und CuClSe2, monoklin. In den Tellursystemen bestehen zwei homologe Reihen: CuXTe, tetragonal und CuXTe2 monoklin (X Cl, Br, J). Fur die Selenverbindungen wurden thermodynamische Daten aus KNUDSEN-Messungen erhalten.



Suszeptibilitatsmessungen ergaben einen temperaturunabhangigen Diamagnetismus. Die Verbindungen lassen sich auch durch Reaktion von stochiometrischen Mischungen aus Kupfer(I)-halogenid und Selen (300°C) bzw. Tellur(350°C)herstellen. Entsprechende Sulfidhalogenide wurden nicht beobachtet.



Nine ternary chalcogenidehalides of copper could be synthesized under hydrothermal conditions from the resp. hydrogen halide solutions. They belong to the pseudobinary systems, formed between selenium and tellurium and the resp. CuI halide. In the selenium systems exist: CuISe3, rhombohedral; CuBrSe3, orthorhombic, and CuClSe2, monoclinic. Two homologous series of tellurium compounds exist: CuXTe, tetragonal, and CuXTe2, monoclinic (X Cl, Br, I). Thermodynamic data could be obtained for the selenium compounds using the Knudsen effusion method. The samples exhibit a temperature independant diamagnetism. All compounds can also be synthesized by reaction of stoichiometric amounts of CuI halide and selenium (300°C) or tellurium (350°C). The corresponding thiohalides could not be observed.

Über Chalkogenidhalogenide des Kupfers

The gold-selenium system has been investigated up to about 1000 ° C by DTA, X-ray powder, and single crystal diffraction patterns and, partially, vapour pressure measurements The existence of two modifications of AuSe, α and β, has been confirmed The unit cell relationships and relative stabilities of these—both monoclinic—are discussed No other binary compounds seem to exist The preparation of these phases, including hydrothermal synthesis, is dealt with Single crystals of α-AuSe showed metallic conductivity Besides the monotectic between gold and selenium at about 760 °C and 52 at% Se, a second monotectic is formed at 963 °C and 15 at% Se A completed phase diagram is given

Phase relationships in the gold-selenium system

The preparation of four gold telluride halides—including hydrothermal syntheses—is described. Chemical and physical properties, including lattice parameters and thermal and chemical stabilities, are dealt with. The compounds AuXTe 2 ( X = Cl , Br , I ) exhibit metallic conductivity whereas AuITe is a semiconductor with an optical band gap of about 0.9 eV. Only a formal resemblance seems to exist with the analogous copper compounds.

G. Rosenstein

Papers

Hydrothermal preparation of barium titanate from barium-titanium acetate gel precursors

X-ray structure investigation of lanthanum modified lead titanate with A-site and B-site vacancies

Über Chalkogenidhalogenide des Kupfers

Phase relationships in the gold-selenium system

Telluride halides of gold