The generalization that ionic substances have higher melting and boiling points than covalent substances is a fundamental principle of chemistry based on strong electrostatic forces versus weaker intermolecular forces, requiring no special citation.
The claim is a well-established foundational chemical fact taught in basic science, comparing the strong electrostatic forces in ionic lattices to intermolecular forces in covalent compounds. None of the retrieved papers specifically test or evaluate this broad comparative rule as their primary empirical focus; they are specialized studies on batteries, polymers, and drug delivery. Therefore, this is classified as common knowledge.