The NMR spectra and the spin-lattice relaxation times, T1, of Rochelle salt crystals were studied as functions of temperature. Two groups of resonance lines were found in the 23Na and 39K NMR spectra of the paraelectric phases below TC1 and above TC2, whereas four groups of resonance lines were obtained in the 23Na and 39K NMR spectra of the ferroelectric phase between TC1 and TC2. To investigate the extra two groups of resonance lines in the ferroelectric phase, the stress-strain hysteresis loop of the Rochelle salt crystal was obtained, and the ferroelastic character of the Rochelle salt crystals suggested by Sapriel’s theory was established by the relationship between the 23Na and 39K NMR spectra and the stress-strain hysteresis loop results. Therefore, ferroelastic and ferroelectric properties coexist in the phase between TC1 and TC2.

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