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, it is the densest known white stable material. (Although thorium dioxide ThO₂ is also white and has a higher density of 10 g/cm³, it is radioactively unstable; while not radioactive enough to make it unstable as a material, even its low rate of decay is still too much for certain uses such as phosphors for detecting ionising radiation.) The white color and high density of LuTaO₄ make it ideal for phosphor applications, though the high cost of lutetium is a hindrance.

Under standard conditions, LuTaO₄ has a monoclinic (labeled as M'; Pearson symbol mP12, Space group = P2/a, No 13) fergusonite-type crystal structure. This can be changed to an I2/a (M) 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 (YTaO₄) and gadolinium tantalate (GdTaO₄).

Lutetium tantalate itself is weakly fluorescent. Bright emission is achieved by incorporating small amounts (about 1%) of various rare earth dopants during the crystal growth process, for example with europium (sharp red line at 610 nm), samarium (red: 610 nm), terbium (green-yellow: 495 and 545 nm lines), praseodymium (red: 615 nm), thulium (blue: 455 nm), dysprosium (orange: 580 nm) or niobium (blue: 400 nm, broad peak). 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.

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