[Physics] What compounds or elements only have one phase or two phases

Question

Wood appears to be one. I think gases like helium and hydrogen cannot exist in the solid state under normal pressures, correct? And why do those "phase cheaters"– those elements/compounds which sublimate directly, skipping a phase, or "procrastinators"– elements/compounds which just never reach the phase– why do they do that?

Answer 1

Most substances can perform a large number of phase transitions. There are even different kinds of phase transitions and sometimes two phases can be connected by more than one process.

The quantities governing what phase transition occurs are so-called state variables; temperature and pressure are the best known representatives, but e.g. magnetic fields can also be a factor.

Depending on the state variables, there usually is one configuration that is energetically favorable. The substance will take that configuration. If the state variables change, such that a different configuration is more favorable, the substance will perform a phase transition.

This phase diagram (from Wikipedia's Phase Diagram page), shows this quite well. Using the dashed green line instead of the solid green line we have the phase diagram for water. At high temperatures and low pressures, vapour is energetically favored. If we reduce the temperature the vapour will either turn into liquid water (if the the pressure is between $p_{tp}$ oand $p_{cr}$) or directly to solid ice (if the pressure is below $p_{tp}$). You see that depending on the state variables different transitions are possible.

Now there are many more phases than the classic states solid, liquid, gaseous. Just regular ice has eleven phases and magnetic materials might change their magnetic behavior in addition to their stiffness.

Moreover, phase transitions take some time to happen and these phase diagrams assume that you change the state variables slowly enough for the transition to happen completely. It is standard practice in material science to cool down a material quickly, barring phase transitions. This can result in a material with advantageous properties, see e.g. heat treating.

What phases are available to what material is studied in materials science and there are books of tables on the topic. As you might guess by now, one can get arbitrarily detailed...

By the way, Helium also has more than two phases, as there is also a superfluid phase. All other materials can be made solid at appropriately low temperatures with potentially higher-than average pressure.

Finally, compound materials such as wood usually are not considered to make phase transitions as a whole, since the different materials that wood is made of might behave very differently at different temperatures/pressures.