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A comparative spectroscopic study for and doped crystals has been realised at liquid nitrogen temperature. The single-doped sample shows principally blue emission corresponding to the transition, whereas the codoped sample gives rise to a very efficient green emission which is ascribed to the transition. The total integrated upconversion emission intensity (including blue and green emissions) for the codoped crystal is three times more intense than that of the doped one, while the ratio for green emission rises to 27. The green emission in the codoped sample results mainly from the mixed centre, whereas the blue emission is essentially due to the pair. Two energy transfer upconversion mechanisms have been proposed and are discussed in the paper.

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