Low Temperature Hydrothermal Synthesis Research Articles

Zeolitization of glasses: a calorimetric study

The construction of calorimetric ampoules for work in hydrothermal conditions makes it possible to investigate the enthalpy of mineral crystallization. The potential of the equipment is demonstrated for the case of zeolite synthesis from glasses. Two synthetic Na- and NaK-glasses, and dacite pumice were used as starting materials. Aqueous solutions of 2 N and 6 N NaOH and KOH were prepared. The crystallization was performed in a Kalve calorimeter DAC-1A at 95°C. Three peaks are observed in the e.m.f. curves due to: wetting of the glass, glass hydration and dissolution, and crystallization of zeolites. Their position versus time and their intensity depend on the experimental conditions. The described calorimetric equipment is very useful for the low-temperature hydrothermal synthesis of zeolite minerals under isothermal conditions and provides a means of controlling the crystallization process.

The hydrothermal synthesis of new oxide materials

The development of advanced secondary lithium batteries depends on cathode structures that can reversibly intercalate lithium ions. The initial prototypical system Li/TiS2 showed the feasibility of high energy density systems with extensive reversibility. However, with the advent of the safe lithium-carbon anodes by Sony there is a need for higher voltage cathodes. LiCoO2 presently fulfills this need in small cells. However, for large systems where cost is an issue new oxides are needed. One approach to the formation of such oxide is low temperature hydrothermal synthesis. Mild hydrothermal reactions lead to the formation of new metastable transition metal oxide structures, not accessible by conventional high temperature methods, which have relatively open crystal structures. The nature of the cations present in solution (the “templating ion”) has a dramatic effect on the crystal structure of the phase formed, as also does the pH of the reaction medium and the particular transition metal. Thus, by appropriate choice of reaction medium new structures containing large tunnels or channels, similar to those found in aluminosilicate zeolites, can be formed that will offer unique properties for the Materials Scientist. In particular, it is expected to be possible to realize enhanced diffusion in such materials. Here, the hydrothermal synthesis of tungsten, molybdenum and vanadium oxides is considered. The role of the cation in the synthesis is described, as well as the key interfacial reactions that are taking place. Cases are described where the cation controls the structure formed and is retained in the structure, to instances where the cation critically controls the reactions occurring but is not retained in the lattice.

NH4FePO4F: Structural Study and Magnetic Properties

Hitherto unknown NH4FePO4F has been isolated by low-temperature hydrothermal synthesis. It is orthorhombic and acentric (space group Pna21 (No. 33); a = 12.993(3) Å, b = 6.468(1) Å, c = 10.640(3) Å, V = 894.2(6) Å3, Z = 8). This compound belongs to the KTiOPO4 (KTP) structure type, with a strict O-F ordering, the Ti-O short bond in KTP being replaced by a Fe-F bonding. The two iron sites, clearly evidenced by Mössbauer spectroscopy, correspond to two different local situations with 2 F- in cis position for Fe(1) and in trans position for Fe(2). With such a configuration, the main magnetic coupling probably occurs within the chains of octahedra via Fe-F-Fe superexchange interactions only, like in AFeF5 and A2FeF5 compounds in which the Néel temperature is close to 10 K. However, the high magnetic transition temperature of NH4FePO4F (45(1) K) shows that the influence of super-superexchange coupling is rather important in this compound.

Synthesis of novel compounds with the pyrochlore and hexagonal tungsten bronze structures

Low-temperature hydrothermal synthesis of acidified tungstate solutions leads to the formation of defect pyrochlores, M W 2 O 6 O 1/2 · x H 2 O, and compounds with the hexagonal tungsten bronze (HTB) structure, M x WO 3+x/2 , where M is normally a monovalent cation. The initial pH of the tungstate solution is critical to the structure obtained; a pH of 3.5 leads to the formation of a pyrochlore phase and pH 1.5 gives the hexagonal tungsten bronze structure. The HTB compound is formed with lithium or sodium in the hexagonal tunnel whereas the pyrochlore contains sodium, cesium, or rubidium inside the three-dimensional tunnels. Both phases are metastable and decompose around 500°C. The sodium ions in the pyrochlore phase can be exchanged for many monovalent cations and those in the hexagonal phase by hydrogen. Both structures readily intercalate lithium.

Low temperature hydrothermal synthesis of magnetite

Magnetite crystals have been grown hydrothermally for the first time at temperature as low as 250°C, at low pressures of 150–450 atm using formic acid as a good mineraliser and an effective reducing agent. This avoids the dependence upon hydrogen formation to provide the stability conditions for magnetite formation.

Low Temperature Hydrothermal Synthesis from Dolomite or Calcite, Quartz and Kaolinite

AbstractDolomite or calcite, quartz and kaolinite in various proportions were reacted at 250–300°C temperatures and pressures up to 90 bars in a hydrous environment. Reactions which approached completion produced talc, calcite, montmorillonite, anorthite (both metastable hexagonal and stable triclinic polymorphs) and the rare Ca-zeolite, garronite. These reactions are applicable to diagenesis, low-grade metamorphism and hydrothermal alteration.

Low temperature hydrothermal synthesis of montmorillonite. Ammonium-micas and ammonium-zeolites

Hydrothermal syntheses at 300°C indicate formation of montmorillonite-group minerals from several naturally occurring mineral combinations, such as (1) dolomite + kaolinite + quartz, (2) dolomite + illite + quartz, (3) dolomite + feldspar + quartz, and (4) dolomite + feldspar. The addition of ammonia or ammonium hydroxide to the system will produce such ammonium phases as NH4-mica and NH4-analcime. It is suggested that these reactions may take place in rocks under favourable diagenetic and hydrothermal conditions.