The tungstic acid calcium tungstate is a luminophore that emits blue light when exposed to X-ray radiation. It is used in fluorescent paint, photography using mask, medicine and X-ray photographs as well as in a variety of screens for plasma display panels and advertising signs. The luminescence varies according to the photon energy and is influenced by the tungsten(W) emission band.
To enhance the versatility of this phosphor, rare earth ions such as terbium (Tb3+) and europium (Eu3+) were doped into crystalline CaWO4 in order to increase its luminescent properties. This was achieved by a simple and fast method involving oxidative polymerization of aniline in the presence of calcium tungstate. The resulting PANI-CaWO4 nanocomposite was characterized by X-ray powder diffraction, Fourier-transformed infrared spectroscopy, UV-visible and photoluminescence. The X-ray and electronic structural analysis showed that the incorporation of the CaWO4 into the polyaniline resulted in the modification of its optical band gap.
The crystalline form of the tungstic acid CaWO4 is known as scheelite. The crystal structure of scheelite is tetragonal and its space symmetry is two formula units per primitive cell. The tungsten atoms in scheelite are covalently bonded with oxygen atoms and their d electrons are trapped by the O atoms. The tungsten can be converted to metallic tungsten by reducing it with hydrogen. The reduction can be performed under high temperatures in the presence of hydrofluoric acid or in aqueous solutions of various acids that are capable of forming water-soluble chelates with the calcium ions.