Lanthanum hexaboride

[ellicot]

I am trying to build Gibbs diagram on a compositon of 2 w/o LaB6 in W matrix. Unfortunately, it doesn't show LaB6 at all. Instead it nicely plots WB and W2B as well as La and all variations of boron. I thought LaB6 exists up to at least 2000°C. Can anyone explain what is wrong?