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M' structure of LuTaO4. Atoms are: O(red), Ta(blue) and Lu(green)
An example of strontium silicate-aluminate oxide powder (left), its luminescence under UV light (center) and phosphorescence after switching off the light (right)

Lutetium tantalate is a chemical compound of lutetium, tantalum and oxygen with the formula LuTaO4. With a density of 9.81 g/cm3,[1] 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.) The white color and high density of LuTaO4 make it ideal for phosphor applications, though the high cost of lutetium is a hindrance.[2][3]

Properties

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.[4] 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).[5]

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.[1]

Preparation

To prepare a sample of lutetium tantalate, powders of lutetium and tantalum oxides (Lu2O3 and Ta2O5) 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 micron-sized particles of LuTaO4.[1]

References

  1. ^ a b c Blasse, G. (1994). "Luminescence of materials based on LuTaO4". Journal of Alloys and Compounds 209: 1–2. doi:10.1016/0925-8388(94)91069-3.   edit
  2. ^ Shigeo Shionoya (1998). Phosphor handbook. CRC Press. p. 846. ISBN 0849375606. http://books.google.co.jp/books?id=lWlcJEDukRIC&pg=PA846.  
  3. ^ C. K. Gupta, Nagaiyar Krishnamurthy (2004). Extractive metallurgy of rare earths. CRC Press. p. 32. ISBN 0415333407. http://books.google.co.jp/books?id=F0Bte_XhzoAC&pg=PA32.  
  4. ^ Liu, W.; Zhang, Q.; Ding, L.; Sun, D.; Luo, J.; Yin, S. (2009). "Photoluminescence properties of LuTaO4:RE3+ (RE3+=Eu3+, Tb3+) with M′-type structure". Journal of Alloys and Compounds 474: 226–228. doi:10.1016/j.jallcom.2008.06.059.   edit
  5. ^ Guokui Liu, Bernard Jacquier (2005). Spectroscopic properties of rare earths in optical materials. Springer. p. 505. ISBN 3540238867. http://books.google.co.jp/books?id=HDleGQrKuEgC&pg=PA505&lpg=PA505.  
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