“Soft” Alkali Bromide and Iodide Fluxes for Crystal Growth

Abstract

In this review we discuss general trends in the use of alkali bromide and iodide (ABI) fluxes for exploratory crystal growth. The ABI fluxes are ionic solution fluxes at moderate to high temperatures, 207 to ∼1,300 • C, which offer a good degree of flexibility in the selection of the temperature profile and solubility. Although their main use is to dissolve and recrystallize "soft" species such as chalcogenides, many compositions with "hard" anions, including oxides and nitrides, have been obtained from the ABI fluxes, highlighting their unique versatility. ABI fluxes can serve to provide a reaction and crystallization medium for different types of starting materials, mostly the elemental and binary compounds. As the use of alkali halide fluxes creates an excess of the alkali cations, these fluxes are often reactive, incorporating one of its components to the final compositions, although some examples of non-reactive ABI fluxes are known.

Covalency in Actinide Compounds

“…Considering U as an example, one needs to achieve its highest +6 oxidation state while using low electronegative anions, such as sulfide or selenide, as counter‐ions. At first glance this appears to be a relatively straightforward approach, however, it is indeed challenging to stabilize uranium in the +6 oxidation state in a sulfide environment owing to the tendency of sulfides to reduce uranium from +6 to +4, [22, 23] complicating the rational design of inorganic solid‐state materials with covalent An−Q bonding (An=actinide, Q=chalcogenide).…”

“…The orbital degeneracy,o nt he contrary,c an be relatively easily tuned through the metal atom oxidation state and the nature of the atoms bound to it. To achieve higher covalency,o ne could take advantage of using am etal atom in its highest oxidation states to increasei ts electronegativity,a nd a" ligand" atom that haslow electronegativity.Considering Uasa nexample, one needs to achieve its highest + 6o xidations tate while using low electronegative anions, such as sulfide or selenide, as counter-ions.A tf irst glance this appearst ob ear elatively straightforward approach, however,i ti si ndeed challenging to stabilizeu ranium in the + 6o xidation state in as ulfide environment owing to the tendency of sulfides to reduce uranium from + 6t o+ 4, [22,23] complicating the rational design of inorganic solid-state materials with covalentA n ÀQb onding( An = actinide,Q= chalcogenide).…”

Synthesis-enabled exploration of chiral and polar multivalent quaternary sulfides

“…In order to grow single crystals of the reported and novel La6(TM)xSi2S14 compounds, potassium iodide (KI) flux. 5,53,54 was used in a 1:30 mass ratio for (precursor + sulfur) to KI using a similar temperature profile as for bulk powder synthesis. Depending on the transition metal and tetrel, different sizes of crystals were observed, ranging from thin hexagonal needles (1-3 mm in length)…”

“…Using metallic or salt fluxes allows the synthesis of many complex phases, including those with volatile components, but flux reactions depend on the solubility of the reactants and might be compromised by an unintentional incorporation of flux elements into the products. [2][3][4][5] One way of alleviating this problem is to use premixed reactants with similar reactivities that can be simultaneously introduced into the reaction mixture thus ensuring a full conversion to the product takes place. This can be achieved through the atomic mixing of components to achieve homogenous precursor reactants.…”

Semiconducting silicon–phosphorus frameworks for caging exotic polycations

“…Sn is a great choice of flux for P containing materials, but Si has a low solubility in Sn, making Sn an undesirable choice. A less traditional flux choice is the salt flux, 20 which has been employed in silicates, germanates, and P-containing thiophosphates. 21 Salt fluxes have been shown to promote the formation of highly complex crystal structures with multiple elements (quaternary and beyond).…”