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I found two space groups of $\ce{LaCrO3}$ material Pnma and Pm-3m. If someone is familiar with this material, please help to understand which phase is more stable. what are the factors that determine which phase is more stable?

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First of all make sure both phases are stable or not, to check stability of any phase, find mechanical stability and dynamical stability. From mechanical stability, I am referring to stiffness matrix should be positive definite, and dynamical stability means no imaginary vibration frequency exists.
Compare stability of two phases
Basically you need to find Gibbs energy of both phases at particular temperature and at zero kelvin, Gibbs energy will come down to E0 with zero point energy correction (assuming pv term to be small) .In short, phase with lower Gibbs energy is more stable.

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  • $\begingroup$ +10 for using Gibbs energy in an answer $\endgroup$
    – B. Kelly
    Dec 25, 2021 at 4:14

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