K 2 (NbO) 2 Si 4 O 12 : a new ferroelectric

The atomic coordinates of K 2 (NbO) 2 Si 4 O 12 reported in space group P 4 bm satisfy the structural criteria for ferroelectricity. The estimated phase‐transition temperature, T c = 2230 (250) K, is substantially in excess of T melting = 1476 (5) K; the hypothetical paraelectric phase in space group P 4/ mbm is hence experimentally inaccessible. A strong indication that the phase transition does not occur at T < T melting is provided by the absence of both calorimetric and dielectric permittivity anomalies below T melting , the linear thermal dependence of unit‐cell parameters with expansion coefficients α 11 = –0.53 (12) × 10 –6 and α 33 = 24.66 (11) × 10 –6  K –1 between 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and alternating current, with spontaneous polarization P s ≃ 0.2 C m –2 , provides unambiguous verification of the ferroelectric property.