Thermal expansion and phase transitions of sodium potassium tartrate tetrahydrate (RS)

Lattice parameters of Rochelle salt (RS) crystals, K.Na.C₄H₄O₆.4H₂O, have been accurately determined by the Bond method with a high-stability temperature attachment. The 2θ angles of the chosen Bragg reflections have been measured in a temperature range from 143 to 308 K. Thermal expansion coefficients have been computed from the fitting of the thermal variation of the lattice parameters to low-order polynomials. Temperature dependence of the thermal expansion coefficients has also been investigated; anomalies connected with the ferroelectric phase transitions are much stronger than those occurring in the low-temperature region but a low-temperature phase transition seems very probable.