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I see this trend from enthalpy change of decomposition data, though I struggle to find a decent explanation for why.

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closed as off-topic by Todd Minehardt, user55119, A.K., Tyberius, Jan Dec 5 '18 at 15:24

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For more detailed info, search "cation-anion radius ratio" or "Pauling's rules". Below is my review of this idea.

This ratio is ${R_{C}/R_{A}}$ (C = cation, A=anion).

Small cation (I mean, too small) attracts anions in such a way that they come too close to each other and repulsive force comes into play. This happens when ratio is <0,155. So for larger anion, larger cation suits better. Ratio can be 0.155-1 for stable compounds, and this ratio also determines coordination number and type of void! (Check the picture below, recall that since <0.155 ratio is for unstable compounds, you have empty "example" line there).

enter image description here

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