Mixed-anion compounds

Mixed-anion compounds, heteroanionic materials or mixed-anion materials are chemical compounds containing cations and more than one kind of anion. The compounds contain a single phase, rather than just a mixture.

Use in materials science
By having more than one anion, many more compounds can be made, and properties tuned to desirable values. In terms of optics, properties include phosphorescence, photocatalysis, laser damage threshold, refractive index, birefringence, absorption particularly in the ultraviolet or near infrared, non-linearity. Mechanical properties can include ability to grow a large crystal, ability to form a thin layer, strength, or brittleness.

Thermal properties can include melting point, thermal stability, phase transition temperatures, thermal expansion coefficient.

For electrical properties, electric conductivity, band gap, superconducting transition temperature piezoelectricity, pyroelectricity, ferromagnetism, dielectric constant, charge-density wave transition can be adjusted.

Production
Many of the non-metals that could make mixed-anion compounds may have greatly varying volatilities. This makes it more difficult to combine the elements together. Compounds may be produced in a solid state reaction, by heating solids together, either in a vacuum or a gas. Common gases used include, oxygen, hydrogen, ammonia, chlorine, fluorine, hydrogen sulfide, or carbon disulfide. Soft chemical approaches to manufacture include solvothermal synthesis, or substituting atoms in a structure by others, including by water, oxygen, fluorine, or nitrogen. Teflon pouches can be used to separate different formulations. Thin film deposits can yield strained layers. High pressures can be used to prevent evaporation of volatiles. High pressure can result in different crystal forms, perhaps with higher coordination number.

Elemental

 * pnictochalcogenides
 * oxypnictides, including oxynitrides, oxyphosphides, oxyarsenides, oxyantimonides, oxybismuthides
 * chalcohalides or chalcogenide halides
 * oxohalides, including oxyfluorides, oxychlorides, oxybromides, oxyiodides
 * fluorosulfides
 * sulfide chlorides, selenide chlorides, telluride chlorides
 * sulfide bromides, selenide bromides, telluride bromides
 * sulfide iodides, selenide iodides, telluride iodides
 * oxysulfides, oxyselenides
 * oxyhydrides
 * halopnictides
 * fluoropnictides, including fluorophosphides, fluoroarsenides, fluoroantimonides, fluorobismuthides, arsenide chlorides

Molecular anions

 * borohydride-chloride
 * disulfide dithioorthovanadate

Oxyanions

 * halocarbonates, including carbonate fluorides, carbonate chlorides, carbonate bromides
 * phosphates, including fluoride phosphates, chloride phosphate, phosphate molybdates, phosphate arsenates
 * borates
 * halide borates, including fluoride borates borate chlorides, borate bromides, borate iodides
 * chalcogenide borates, including sulfide borates
 * borate carbonates, borate nitrates, borate sulfates, borate phosphates
 * borate acetates
 * Condensed borates: borosulfates, boroselenates, borotellurates, boroantimonates, borophosphates, boroselenites
 * sulfates
 * sulfate fluorides, sulfate chlorides
 * sulfate arsenate
 * selenite fluorides
 * iodate fluorides
 * Silicates
 * sulfide silicates

Mixed valency and oligomers
Some elements can form several kinds of anions, and compounds may exist with more than one. Examples include the iodate periodates, sulfite sulfates, selenate selenites, tellurite tellurates, nitrate nitrites, phosphate phosphites, and arsenate arsenites.

These kinds also include different oligomeric forms such as phosphates or fluorotitanates, such as [Ti4F20]4- and [TiF5]−.

Organic

 * borate acetate
 * oxalate formate