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I was reading around about the different solid states of materials and I got to a point where I have reached the multi-component solids. I was not able to find a major difference between the cocrystal and the eutectic mixture from their definitions. Do you know of any fundamental difference?

The question in another way: if you had a mixture of 2 solids that bind to each other somehow, how can you know whether this is a cocrystal or a eutectic mixture?

Many thanks,

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That explains a eutectic but does not answer the question, which is a good one for solid-state chemists. A cocrystal is a crystalline, multiple component solid with a distinct stoichiometry (molar ratio) between components where a single component is neutral overall(can exist alone without a counter ion, typically non-ionic but a zwitterion counts as a single neutral component) and the components interact by non-covalent interactions that can be partially ionic and has distinctive crystalline characterization data, i.e. to be crystalline a solid the mixed solid phase needs to have X-Ray or electron diffraction data that distinguishes it from its starting components. So, the eutectic phase of 2 solids, or the solid mixture with the lowest melting pt, can be a cocrystal, but a cocrystal is not necessarily a eutectic. A solid mixture with no regular repeating crystal lattice and no consistent molar ratio (stoichiometry) is called a solid solution. So a eutectic phase could be a cocrystal, or a salt, or a mix of those, or physical mix (solid solution).

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Simple: consider a system whose components are fully miscible in solution and upon solidifying separates into two phases. At the eutectic point both solid phases melt at the same temperature. At other compositions you'd see liquid phase and one solid phase.

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