Table 2 Calculated and experimental mode softening temperatures for the phonon mode instability at the M, R, and Γ points listed for KNbO3 and NaNbO3, respectively.

From: Exploring anharmonic lattice dynamics and dielectric relations in niobate perovskites from first-principles self-consistent phonon calculations

Oxide

Method

M point instability

R point instability

Γ point instability

KNbO3

Calc.

–

–

751 K

 

Exp.

–

–

708 K (C → T)

NaNbO3

Calc.

1045 K

902 K

655 K

 

Exp.

912 K (U → T1)

848 K (T1 → T2)

632 K (R → P)

  1. The calculated mode softening temperatures are predicted from the SCP calculations with the bubble self-energy correction (SCP+bubble) while considering volumetric changes (VSCP). The experimental temperatures are taken from References 34,35 for KNbO3 and NaNbO3, respectively. These correspond to the phase transition temperatures where the soft modes with relevant distortion patterns would appear dominantly.
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