CALCIUM SELENIDE

Calcium selenide has the molecular formula of CaSe and the molecular weight of 119.0452 g/mol. Its CAS number is 1305-84-6. It is a white to brown cubic crystal with a melting point of 1408°C (where it decomposes, in air, to CaO and SeO2). Its density is 3.81 g/cm3 and it is unstable in air or moisture. Its refractive index is 2.274. Because of its instability in the presence of water, it cannot be prepared in an aqueous solution.
CaSe can be prepared by the reaction of calciumoxide and selenium:
CaO(solid)+ Se(solid)+ heat ? CaSe(solid)+ SeO2(gas)
Alternately, the two elements can be reacted together to form the selenide:
Ca(solid)+ Se(solid)+ heat ? CaSe(solid)
In the first reaction, Se melts at 225°C while the CaO is stable. Selenium dioxide sublimes at 320°C. The reaction is carried out in an inert atmosphere at about 550°C. If a tube furnace is used, crystals of SeO2 appear at the cool end of the tube. In the second reaction, an inert atmosphere is mandatory and the reaction temperature is also 550°C (which is below the melting point of Ca metal). However, the reaction does not go to completion and only about 50% of theoretical CaSe is formed. To achieve close to 100% reaction, the selanate can be reduced to form CaSe:
CaSeO4+H2+ heat ? CaSe+H2O

The major usage of CaSe has been as a phosphor. Lenard and Pauli first described this phosphor, CaSe:- Sm³⁺ , as an infrared stimulable material about 100 years ago. CaSe:Ce³⁺ is a blue-emitting (～4500? ) phosphor. However, due to its instability, CaSe was never used in the lighting industry. More recently, it has been combined with the strontium compound to form phosphors.
(Ca_1-x,Sr_x)Se:Eu²⁺ , which is an efficient red-emitting phosphor, was prepared by solid-state reaction at high temperatures in CO atmosphere. These phosphors can be excited efficiently by visible light from 430 to 490 nm and emit red light with broadband excitation from blue LED chips. If the Sr/Ca ratio is decreased, the lattice parameters get smaller, and the emission wavelength shows a red shift. A suitable wavelength can be obtained by adjusting the Sr/Ca ratio. Because of the commercial afterglow exhibited by calcium sulfide phosphor, these synthesized phosphors have higher emission efficiency and are a good choice for manufacturing white LED lamps. These phosphors are promising candidates for emitting red light for LEDs. When mixed with a green phosphor on a InGaN-based blue chip, the as-synthesized red phosphors generated white light for LEDs.