The silver (Ag)-platinum (Pt) phase diagram shown below introduces a few more concepts that the phase diagrams discussed previously did not contain. Similar to the eutectic system, there is a distinct phase on each side of the diagram relating to a high purity of each species. In this diagram, there is a phase that has a high concentration of silver and another phase related to a high concentration of platinum.
One of the new features of this diagram, however; is the Ag15Pt17 intermetallic phase at 53 at% platinum. This silver-platinum intermetallic is referred to as a "line compound," a phase whose composition range is very narrow. There is very low solubility of excess silver or platinum in the silver-platinum intermetallic phase, so it is listed on the phase diagram as a vertical line, hence the term "line compound."
Another feature introduced on this diagram is a peritectic point. Peritectic points are spots on the phase diagram at which, upon heating, a one phase solid transforms into a different solid phase mixed with a liquid. On this diagram, the peritectic is at about 38 at% platinum and 1188°C. At this point, the pure silver phase, upon heating, will decompose into a liquid mixed with the pure platinum phase.
The third feature we will discuss on this page is a peritectoid point. As the name suggests, a peritectoid is similar to a peritectic but just a little bit different. At a peritectoid point, upon heating, a single phase solid will decompose into two different solid phases, unlike a peritectic, where the solid phase decomposes into a different solid and a liquid phase. In the Ag-Pt phase diagram, this occurs at 53 at% and 803°C where the silver-platinum intermetallic decomposes into a mixture of the pure silver and pure platinum phases, both solids.