The selenium toner contains sodium selenite (Na2SeO3) as one of its active ingredients, which is the source of the selenide (Se2−) anion combining with the silver in the toning process.

This orthorhombic phase, stable at room temperature, is a narrow-gap semiconductor, with space group P212121.

For Ag2Se, this polymorph is increasingly unstable for larger crystallites, which explains its absence on the bulk phase diagram.

[7] It is thought that the influence of the surface energy plays a role stabilizing this phase on the nanoscale, thus allowing its experimental isolation.

Electronic structure calculations for this polymorph of Ag2Se suggest that it is a narrow-gap semiconductor,[6] which is supported by experimental evidence as well.