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The reason for zirconia's cubic and tetragonal phases not being stable is that the cation-anion ratio is too small for proper contact between ions.

One way is to use other oxides that have a bigger cation, such as yttria and calcia, to increase the average cation size, increasing the ratio. However, it is not the only way the ratio can be reduced. Reduction of the anion size can also increase the ratio.

However, the only anion smaller than the oxide anion is the fluoride anion. So, would fluorides stabilize cubic and/or tetragonal zirconia?

The article, "Formation of stabilized zirconia with rare earth fluoride", is related to my question.

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    $\begingroup$ Check this out: chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/… It talks about how an anionic exchange between commercial zirconium oxide/hydroxide with a high surface area and trifluoracetic acid solutions can be used to create fluorinated zirconia. The novel fluoride-based catalysts maintain high surface areas and contain significant amounts of fluoride, with superficial Zr/F ratios of 0.9 and 1.2 after calcination at 300°C and 400°C, respectively $\endgroup$
    – Ronith
    Commented Apr 27 at 6:20
  • $\begingroup$ Is it still useful for its original purpose, if fluoride ions happen to lower its melting point? $\endgroup$
    – Paul Kolk
    Commented Apr 27 at 19:37
  • $\begingroup$ for non-high-temperature applications such as ceramic knives, it may even help with sintering. But for high-temperature applications, one would have to monitor the decrease in melting point $\endgroup$
    – user145034
    Commented Apr 27 at 19:48
  • $\begingroup$ Wikipedia's article on yttria-stabilized zirconia acually quotes that cubic zirconia is sometimes stabilized by magnesia or alumina despite wrong cation sizes. Introducing oxide ion vacancies, thus diminishing the coordination demand, also figures into the stabilization. $\endgroup$
    – Oscar Lanzi
    Commented Apr 27 at 22:42
  • $\begingroup$ so by missing a few oxygen anions, the "average size" of oxide anion decreases $\endgroup$
    – user145034
    Commented Apr 27 at 23:06

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