Sodium niobite $({\mathrm{NaNbO}}_3, \mathrm{NNO})$ is a technologically important material featuring a complex phase diagram with multiple, intricately coupled structural phases. While the structural complexity of NNO has inspired extensive experimental and theoretical investigations, the finite temperature behavior of lattice excitations and their role in temperature-induced phase transitions remain poorly understood. Here, we bridge this gap by studying the spectra of collective lattice vibrations using a first-principle-based effective Hamiltonian model. Our results reveal that the two highest-in-temperature structural transitions are driven by condensation of softening phonon modes. Such maximum and next-maximum temperature transitions give rise to an in-phase and antiphase oxygen octahedral tilting, respectively. Moreover, we find that in the vicinity of the highest transition temperature, paraelectric NNO features dispersionless transverse optic (TO) modes at the $\text{R}-\text{M}$ edges of the Brillouin zone, with the corresponding flat bands rigidly softening upon cooling. Our results thereby reveal a highly degenerate manifold of competing antipolar states and antiferrodistortive instabilities.

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• Antiferroelectricity
• Dynamical phase transitions
• Ferroelectricity
• Lattice dynamics
• Order parameters
• Phase transitions
• Phase transitions by order
• Phonons
• Specific phase transitions
• Temperature