Structure and Properties of Tantalum Pentoxide

Tantalum pentoxide (Ta2O5) is a transition-metal oxide with a broad range of attractive characteristics such as high-dielectric and anti-reflection properties as well as wear resistance, thermal and chemical stability. It is used in the production of lithium tantalate single crystals. These single crystals are cut into wafers and are used as a substrate for manufacturing surface acoustic wave filters (SAW filters).

Tantalum pentoxide is disordered, being either amorphous or polycrystalline. It has at least 2 polymorphs exist. A low temperature form, known as L- or β-Ta2O5, and the high temperature form known as H- or α-Ta2O5. The transition between these two forms is slow and reversible; occurring between 1000 and 1360 ° C, with a mixture of existing structures at intermediate temperatures. The structures of the two polymorphs consist of chains constructed from octahedral TaO6 and pentagonal TaO7 bipyramidal polyhedra sharing opposite vertices; who are further joined by board sharing. The global crystalline system is orthorhombic in both cases, the β-Ta2O5 space group being identified by Pna2 by single-crystal X-ray diffraction.

The difficulty in forming a material with a uniform structure has led to variations in its reported properties. Like many metal oxides, tantalum pentoxide is an insulator and its band gap has been reported at various times between 3.8 and 5.3 eV, depending on the manufacturing process. In general, the more amorphous the material is, the greater the band gap will be. These observed values are significantly higher than those predicted by computational chemistry (2.3 – 3.8 eV).