If no, which ones do not? If yes, is there a theoretical proof that a counterexample is impossible? Most substances have a fluid phase which is a single phase at pressures above critical point pressure - but two separate phases at lower pressures. The solid sublimates into a gas at lower pressure, and at higher pressures it melts into a liquid. Some substances have wide pressure range of boiling. For example quicksilver has a triple point pressure of just 0,165 mPa, at about -39 degrees, yet its critical point pressure is reputed 172 MPa, at +1477 degrees. Yet other substances have relatively narrow boiling pressure range. Carbon dioxide has triple point pressure of 0,52 MPa, at -57 degrees, but its critical point pressure is 7,3 MPa, at +31 degrees. Is it fundamentally possible for a substance to not boil at all, under any pressure or temperature, because the solid crystallizes from a single fluid phase under any pressures, such that triple and critical point are nonexistent because "degenerate"? If yes, what substances are such? If no, what is the theoretical proof if the impossibility of such a substance?