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The lattice parameters of NaNbO3 have been measured to a precision between 1 part in 105 and 1 part in 104 at temperatures up to 800°C. Six distinct phases are found: P (room temperature to 373°C), R(373-480°C), S(480-520°C), T1(520-575°C), T2(575-641°C), cubic (> 641°C). Coexistence of phases S and T1 between 520 and 535 °C has been found, together with twinning effects, and a possible explanation for this is given.
Glazer et al. (Sat,) studied this question.