Molecular crystal
A molecular solid is a solid composed of molecules held together by the Van der Waals forces. Because these dipole forces are weaker than covalent or ionic bonds, molecular solids are soft and have relatively low melting temperature. Molecular solids can be amorphous solids or crystals. Examples of molecular solids include hydrocarbons, ice, sugar, fullerenes, sulfur, and solid carbon dioxide. Pure molecular solids are electrical insulators but they can be made conductive by doping.
Other classes of solid include ionic solids, held together by ionic bonding; glasses, ceramics, and polymeric solids, held together by covalent bonds; and metals, held together by metallic bonding.
The term "molecular solid" may refer not to a certain chemical composition, but to a specific form of a material. For example, solid phosphorus can crystallize in different allotropes called "white", "red", and "black" phosphorus. White phosphorus forms molecular crystals composed of tetrahedral P4 molecules. Heating at ambient pressure to 250 °C or exposing to sunlight converts white phosphorus to red phosphorus where the P4 tetrahedra are no longer isolated, but are connected by covalent bonds into polymer-like chains. Heating white phosphorus under high (GPa) pressures converts it to black phosphorus which has a layered, graphite-like structure.
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