[en] The ferroelastic and antiferroelectric transition (between phases called P and R) in NaNbO₃ samples, pure and doped with Mn (0-3.3 mol %), was investigated. The phase diagram was obtained from DTA tests. The doping resulted in lowering of the P-R phase transition temperature. The shift of the P-R phase transition temperature was observed also in dielectric permittivity ε(T) measured under compression. Changes in enthalpies obtained from DTA were compared to the latent heat values calculated independently from the Clapeyron - Clausius equation. Values of thermodynamic parameters of the P-R phase transition were evaluated. The EPR test showed that the Mn dopant causes disorder and local strains. Athermal martensitic behaviour of the P-R phase transition in NaNbO₃ was enhanced by applied compression and the dopant subsystem. (author)