a solid solution

You can dissolve sugar into water and it vanishes into a single clear liquid. The surprising thing is that a solid can dissolve another substance the very same way, without melting. A solid solution is a single crystalline phase in which foreign atoms are stirred into the host lattice — either taking the place of host atoms (substitutional) or squeezing into the gaps between them (interstitial) — while the crystal keeps its one continuous structure. Ruby is the classic beauty: a few percent of Cr atoms sit in aluminium sites of Al2O3, and the sapphire-clear crystal turns red.

How much will dissolve depends on how well the guest fits, much like the old rules for metals: similar ion size (within roughly 15 percent), the same crystal structure, similar charge, and similar chemical character all push toward high solubility. When everything matches, you can get complete solid solution — the two end members mix in every proportion, as MgO and NiO do (both rock-salt, similar sizes), giving a single phase all the way across the diagram with no eutectic at all, just a lens of liquidus over solidus. When the fit is poorer you get partial (limited) solid solution: the host swallows only so much guest before a second phase must appear, and the diagram shows a solubility limit that usually widens as temperature rises.

In ceramics solid solutions are the quiet engine behind whole classes of materials. Stabilized zirconia is a solid solution of Y2O3 in ZrO2 that locks in the cubic or tetragonal form. Spinel MgAl2O4 takes extra alumina into solid solution at high temperature. The colours of glazes and gems, the tuning of a ferrite's magnetism, the deliberate aliovalent doping that seeds vacancies for ionic conduction — all are solid solutions read straight off a phase diagram. Where a diagram shows a shaded single-phase field with a slanted edge rather than a sharp vertical line, you are looking at a solid solution.