K-2(NbO)(2)Si4O12: a new ferroelectric

The atomic coordinates of K-2(NbO)(2)Si4O12 reported in space group P4bm sa tisfy the structural criteria for ferroelectricity. The estimated phase-tra nsition temperature, T-c = 2230 (250) K, is substantially in excess of T-me lting = 1476(5) K; the hypothetical paraelectric phase in space group P4/mb m is hence experimentally inaccessible. A strong indication that the phase transition does not occur at T < T-melting is provided by the absence of bo th calorimetric and dielectric permittivity anomalies below T-melting, the linear thermal dependence of unit-cell parameters with expansion coefficien ts alpha(11) = -0.53 (12) x 10(-6) and alpha(33) = 24.66(11) x 10(-6) K-1 b etween 5 and 843 K, and the generation of second harmonics between 300 and 1353 K. Demonstration of dielectric hysteresis under both direct and altern ating current, with spontaneous polarization P-s similar or equal to 0.2 C m(-2), provides unambiguous verification of the ferroelectric property.