Trouble looking up values of critical temperature for mercury

[slimjim]

I find values for the critical temperature of mercury, but two values are given;

one for Hg (α) and one for Hg (β).


Does anyone know what the alpha and beta denotes?


Here is the page I am referring to (under section titled "Properties of superconducting elements"):

http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_4.html

 

[haruspex]

Since this will be solid mercury, it could be two different crystalline forms, but I can't find any reference to back this up.