The hydroflux method is a promising new synthesis approach for explorative crystal growth. Various new compounds were synthesized during the preparation of this PhD thesis, doubling the number of substances discovered to date via the hydroflux approach. The product range consists primarily of oxides, hydroxides or a mixture of both, with oxygen-free compounds being obtained for the first time in form of various chalcogenides. The so far barely explored redox chemistry of the hydroflux was elucidated in more detail and novel preparation procedures were developed to intentionally introduce reductive and oxidative conditions. Thus, chalcogenides and highly oxidized cations were obtained. In addition, important reaction parameters of the hydroflux method were derived based on the developed syntheses procedures and properties of the various new compounds.
The two largest compound classes within the product range are the hydrogarnets and the oxo(hydroxo)ferrates. Among the various interesting properties of the latter compounds, the potassium ion conductivity stands out, which is closely related to their structure and chemical stability. The structure of the oxohydroxoferrates K2–x(Fe,M)4O7–y(OH)y (M = Fe, Si, Ge, Ti, Mn, Ir) can be described as a parking garage. Honeycomb layers consisting of edge-sharing [FeO6] octahedra form the floors, which are connected by pairs of vertices-sharing [FeO4] tetrahedra representing the pillars. In the pictorial representation of this two-dimensional ion conductor, the potassium ions represent the cars that are mobile within one floor because not all parking lots are occupied, i.e., the structure has a potassium deficit. The substituted elements M influence the potassium content and thus the ion conductivity, which tends to increase with higher potassium deficits. The oxohydroxoferrates hydrolyze slowly in moist air under segregation of potassium hydroxide, which significantly increases the mobility of the potassium ions due to the hygroscopic nature and thus the ion conductivity.
The three-dimensional ion conductor K12+6xFe6Te4–xO27 consists of a cubic labyrinth of potassium channels, which are surrounded by an open framework of [FeO5] pyramids and [TeO6] octahedra. Every potassium position is connected with eight large cavities acting as nodes for the potassium channels. However, the potassium positions within the channels are fully occupied, which hinders mobility within the labyrinth to the disadvantage of the ion conductivity. Similar to K2–x(Fe,M)4O7–y(OH)y, K12+6xFe6Te4–xO27 hydrolyzes under ambient conditions decreasing the potassium content within the structure. However, only a slight amount of potassium can be removed before the open framework collapses.
Hydrogarnets crystallize in the flexible garnet structure-type and adapt the general formula AE3[M(OH)6]2. The crystal structure consists of a complex three-dimensional framework, in which [MO6] octahedra and empty (O4H4)4– tetrahedra are connected via their vertices and the larger alkaline earth metal cations AE filling the remaining voids. In contrast to garnets (nesosilicates), the hydrogarnets have a lower thermal stability and hardness. For many applications, this instability might be a drawback, but at the same time, it qualifies them for a low temperature and resource efficient application as carbon-free single-source precursors. In case of the rare earth hydrogarnets (AE = Sr, Ba; M = Sc, Y, Ho–Lu), the dehydration at about 550 °C leads to the formation of AEM2O4, which were previously obtainable only at reaction temperatures above 1300 °C.[86–89]
Redox chemistry in hydroflux systems had been barely investigated so far, with neither equations nor possible mechanisms discussed to explain redox phenomena. In more than half of the published articles of this thesis, redox reactions were observed, often involving molecular oxygen or nitrate as oxidant. Similar to alkali metal hydroxide melts, molecular oxygen is expected to react with hydroxide ions to form peroxides or even superoxides, while nitrates might be reduced to nitrites. Moreover, higher oxidations states seem to be preferred in the hydroflux medium, as, for example, tellurium(IV), chromium(III) and arsenic(III) were readily oxidized to their maximum oxidation states. Additionally, the partial replacement of KOH by KO2 in the hydroflux medium introduced a high oxygen partial pressure, resulting in the oxidation of iodide(–I) ions to orthoperiodate(VII) ions. This preparation procedure has a great potential to yield compounds with elements in unusual high oxidation states, especially transition metals.
The tendency of some elements to prefer higher oxidation states than usual was utilized to intentionally introduce reductive conditions. With this approach, reduction of selenium(IV) and tellurium(IV) oxides to their chalcogenides was achieved by using arsenic(III) oxide as reducing agent. In solution, monochalcogenide and dichalcogenide anions as well as the new (SeTe)2– anions were obtained. In addition, millimeter-sized crystals of the chalcogenides K2Se3 and K2Te3 and the previously unknown K2Se2Te were crystallized. This unexpected redox chemistry is far from what the standard potentials would suggest. The activity of water is considerably reduced by the ultra-alkaline conditions, which does not only decrease its vapor pressure and drives the reaction but obviously prevents the hydrolysis of the water sensitive chalcogenides. Overall, a preparatively simple, time-saving and secure approach compared to traditional methods like the synthesis in liquid ammonia was developed. Moreover, this method allows known and new potassium trichalcogenides to be obtained in larger amounts and in form of millimeter-sized single-crystals. A transfer of the approach to other systems should be promising.
Reaction parameters described in literature were mostly confirmed and some details were added. For example, the selection of mineralizers was extended, reaction times and temperatures were specified, and a method for purifying the reaction products was added. With the exception of base concentration and concentration-dependent product formation, both of which have barely been studied so far. An example is the iron(III)-KOH hydroflux system, where four different products are accessible with increasing base-concentrations: α-Fe2O3, K2–xFe4O7–x(OH)x, K2Fe2O3(OH)2 and KFeO2. Overall, two trends are evident with increasing base concentration. First, the alkali metal content within the product rises or the alkali metal is incorporated in the structure in the first place. Second, the hydrogen content of the products constantly decreases. The latter is attributed to the increasing hygroscopicity of the reaction medium at higher hydroxide concentrations, which also reduces the activity of water in the hydroflux medium, so that water-sensitive compounds are stabilized.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:78243 |
| Date | 01 March 2022 |
| Creators | Albrecht, Ralf |
| Contributors | Ruck, Michael, Grin, Juri, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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