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 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. Kinds. Mixed valency and oligomers. Some elements can form several different kinds of anions, and compounds may exist with more than one. Examples include the iodate periodates, sulfite sulfates and selenite selenates. These kinds also include different oligomeric forms such as phosphates or  fluorotitanates, such as [TiF] and [TiF].