Your metals trivia of the day.
OK, so everyone knows that the rare earths are used as phosphors. For they luminesce.
Yes, everyone does know that, yes?
Quite, everyone does, for example, europium, terbium and so on have been used for decades to make up the coloured dots on the inside of cathode ray tubes. That\’s what gives us the pretty pictures…as, of course, everyone knows.
However, here\’s something that not everyone knows. Euro banknotes luminesce. It\’s one of the anti-fraud devices built in.
So, which of the rare earths do you think they used to provide that feature?
Euro notes luminesce in the red, green and blue (excited by 254 nm). The red light is clearly linked to europium and most likely to a Eu3+-?-diketone complex – according to the Utrecht researchers. They found it less easy to identify the source of the blue and green luminescence.
Still, they say that a likely candidate for the source of the green color is SrGa2S4:Eu2+.
There are many candidates for the blue color. Suyver and Meijering suspect, however, that the designers of the Euro notes were really inspired. So the blue color may be caused by (BaO)x.6Al2O3:Eu3+.
You wouldn\’t normally use Eu to provide all three colours, there are better candidates for two of them. But, hey, come on, we\’re designing Euro note for use in the European Union. So of course we\’re preferentially going to use europium, even if it is less efficient.
In one way this is really rather fun. In another it\’s quite possibly the most extreme example of European twatishness even I\’ve heard of.