It is shown that ferroelectric KNb${\mathrm{O}}_3$ undergoes another phase transition at -10°C in addition to two phase transitions previously reported at 225° and 435°C. At this lowest phase transition KNb${\mathrm{O}}_3$, on cooling, changes its orthorhombic structure to a rhombohedral one, and this change is accompanied by a sharp change of the dielectric constant. The specific heat anomalies at three transitions were measured, and the results are compared with the case of BaTi${\mathrm{O}}_3$.

• Received 30 October 1953

DOI:https://doi.org/10.1103/PhysRev.93.672