Lutetium tantalate


Lutetium tantalate is a chemical compound of lutetium, tantalum and oxygen with the formula LuTaO4. With a density of 9.81 g/cm3, this salt is the densest known white stable material. The white color and high density of LuTaO4 make it ideal for phosphor applications, though the high cost of lutetium is a hindrance.

Properties

Under standard conditions, LuTaO4 has a monoclinic fergusonite-type crystal structure. This can be changed to an I2/a structure by annealing at 1600 °C. Both structures are stable under standard conditions. In the M' structure, the lutetium atom is 8-fold coordinated with oxygen and forms a distorted antiprism with a C2 site symmetry. The structure of lutetium tantalate is identical to that of yttrium tantalate and gadolinium tantalate.
Lutetium tantalate itself is weakly fluorescent. Bright emission is achieved by incorporating small amounts of various rare-earth dopants during the crystal growth process, for example, with europium, samarium, terbium, praseodymium, thulium, dysprosium or niobium. The emission is best excited by electrons, X-rays or ultraviolet light at 220 nm. The high density of LuTaO4 favors X-ray excitation, which has relatively more efficient, stronger absorption in LuTaO4, compared to other materials. LuTaO4 also exhibits thermoluminescence — it glows in the dark when heated after illumination.

Preparation

To prepare a sample of lutetium tantalate, powders of lutetium and tantalum oxides are mixed and annealed at a temperature above 1200 °C for several hours. To prepare a phosphor, a small fraction of appropriate material, such as an oxide of another rare-earth metal, is added to the mixture before annealing. After cooling, the product is leached with water, washed, filtered and dried, resulting in a white powder consisting of micrometre-sized particles of LuTaO4.