What is the definition of a component in a phase diagram?

I have been trying to understand the concept of a component in relation to phase diagrams in my materials class. I have tried searching online and found that the definition is supposed to be that a component is a singular element, however I have also found a water (a 2 element compound) phase diagram is listed as a singular component phase diagram, which contradicts the previous definition of a component.


So is a component all relative to the temperatures/concentrations it operates under? Like, if a liquid is made of two elements and you're looking at it between 20-100 degrees Celsius and it only separates into its individual elements after 100 degrees Celsius, then would it be classified as a singular component phase diagram under 100 degrees Celsius?


So in short I need help understanding what a component is in relation to phase diagrams.




Since you are coming from a materials background, likely the first binary phase diagram you encountered was a diagram with two pure elements on either end of it. This might look something like:


[![enter image description here](https://i.stack.imgur.com/69wTk.png)](https://i.stack.imgur.com/69wTk.png)


And you probably understand how to parse out the x-axis - one goes from pure silver on the left to pure tin on the right. At any point you can determine the phase(s) stable at a given atomic ratio and temperature.


Now one comes to binary diagrams that are more properly called pseudo-binary diagrams. Here the components are indeed not elements, but some variety of compound(s) that can interact like you are used to on a classic binary diagram. For example:


[![enter image description here](https://i.stack.imgur.com/fDOT6.png)](https://i.stack.imgur.com/fDOT6.png)
Here one has pure GdF$\_{3}$ on the left, and pure CsF on the right. The composition axis is no longer in atomic fraction (my bad on labeling the axis), but is instead a weighted average of the pure compounds. So, for instance, the purple line representing Cs$\_{3}$GdF$\_{6}$, is found at a 'composition' of 0.75. This is because it is an amalgamation of 3 CsF units and 1 GdF$\_{3}$ unit, so it is (3/4) units towards CsF. Similarly, the CsGd$\_{2}$F$\_{7}$ is one CsF and two GdF$\_{3}$ units, so is (1/3) units towards pure CsF.


The math works out just as for the true binary element case, but you have to think in terms of ratios of the compounds, not specific elements in the compounds.


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