On the polymorphism of tantalum diselenide

The structure of a new polymorph of tantalum diselenide, 4s(c)-TaSe2, has been determined by X-ray diffraction. In this form the metal has trigonal-prismatic coordination, as in 2s(a)-TaSe2, 3s-TaSe2 and 4s(a)-TaSe2. In Is-TaSe2 the coordination of the metal is octahedral, while in 4s(b)-TaSe2 and 6s-TaSe2 the two types of coordination coexist. The thermal relations between the various polymorphic forms have been studied. It is found that octahedral coordination is stable above 880 °C, trigonal-prismatic coordination below 800 °C, while the two coordinations coexist at intermediate temperatures. The transitions Is →3 6s →3 2s and Is →3 4s(b) →3 4s(c) are reversible, but the high-temperature forms can be retained at room temperature by quenching. Heating of quenched Is- or 6s-TaSe2 gives the metastable 3s form. A mechanism is proposed for the irreversible transitions (occurring at 200 °–225 °C) which explains these observations.