About that funny story I told my son some years ago about the plutonium/gallium phase diagram.

I don't recall exactly where I read it, but for about 15 or more years after the announcement of the discovery of plutonium - the announcement being the destruction of Nagasaki, more or less - scientists embarked on a vast investigation of the metallurgy of plutonium.

By its nature, pure plutonium has one of the most complex phase diagrams of any element in the periodic table, with multiple allotropes, more in fact, than any pure element other than carbon. (An example of allotropes is graphite, diamond, and "Buckminsterfullerene" which are all pure carbon but have very different properties such as hardness, density, compressibility, etc.)

This "connected phase diagram" of the actinide elements shows the complexity of plutonium:



Hecker, Number 26 2000 Los Alamos Science pg 305.

My joke to my son, which I read somewhere - where I don't remember - is that when scientists began publishing the phase diagrams of alloys of plutonium, a scientific issue, they published every possible metallic element except that of gallium.

The reason: The plutonium/gallium phase diagram stabilizes the delta phase, which is generally not stable at room temperature but is when alloyed with gallium, the delta phase being necessary for use in nuclear weapons.

All of the nuclear weapons states, either through spying or from independent discovery were aware of this, and so no one published this diagram since all nuclear weapons states regarded it as a "classified state secret."

Eventually, around the mid 1960's everyone knew about why the phase diagram wasn't published, and confessed as much at an international scientific meeting designed to increase scientific exchanges.

My son gave a talk at a conference this week where one of the announced talks was a discussion of this phase diagram. Intrigued my son went to the room where it was scheduled. The moderator announced, without giving a reason, that the talk was cancelled.

Probably the long term behavior of this alloy is still a "state secret" and apparently the speaker got his talk quashed by security, even as late as 2024.

It's esoteric, but we were both amused when he told me about it.