German term (edited): die in weiten Bereichen gegebene L�slichkeit

The text is talking about solid solutions - i.e. if you have ZrO2 and Y2O3 (yttria), the yttria can "dissolve" by replacing Zr in the ZrO2 lattice and still retain the tetragonal crystal structure. These oxides have a wide range of solubility in ZrO2. In some cases this destabilises the structure and in others, here for example, it has a stabilising effect and suppresses the critical t/m (tetragonal/monoclinic) transformation thus toughening the ceramic.

Take a look at the ref. it explains phase transformation toughening and the solid solutions very nicely.





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Note added at 6 hrs 55 mins (2004-02-23 12:04:25 GMT)
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Extra info (waiting for lunch to cook)!:
If you have 2 solids of similar structure (A and B), a wide range of solubilty would be e.g. 10%A/90%B to 25%A/75%B, whereby mixtures of 0-10%A/90-100% may not be mutually soluble. This is not like chucking salt into water where the salt will dissolve in any amount up to the limit. In solid solutions only certain ranges are compatible and these can change, depending on the temperature/pressure. If the temperature drops (e.g. the material cools after sintering), one component may no longer be soluble at the given concentration and separates out into a separate phase (see your other question). This is known as segregation.