Phase transitions and ferroelectricity in NaSb₃F₁₀

Atomic coordinate analysis allows materials with appropriate but previously unrecognized dielectric properties to be predicted as new ferroelectrics if their crystal structure is known. An earlier such prediction that NaSb₃F₁₀ is ferroelectric is confirmed herein without ambiguity. Its spontaneous polarization P_s is found to exhibit reproducible dielectric hysteresis at room temperature, with P_s ≃ 60 µC m⁻², under the application of a field of 0.3 MV m⁻¹ or greater. The pyroelectric coefficient 〈p〉 = 17 (5) µC m⁻² K⁻¹ at 298 K. NaSb₃F₁₀ undergoes a phase transition at T_C ≃ 461 K, on correction for thermal hysteresis, with entropy change ΔS = 5.7 (3) J mol⁻¹ K⁻¹. The colorless crystals melt at T_m ≃ 515 K and decompose above ∼600 K. The thermal hysteresis of ∼35 K in T_C, on heating and cooling at 5–25 K min⁻¹, is typical of first-order phase transitions. The space group in ferroelectric phase III is P6₃, and that in phase II is predicted to be P6₃22, a nonpolar supergroup of P6₃; the supergroup expected in the prototypic nonferroic phase I is P6₃/mmc. The space group of phase III is not a direct subgroup of phase I. The dielectric permittivity [epsilon]′ at 1 kHz increases over an order of magnitude between 300 K and a major inflection at T_C, continuing to increase steadily thereafter to T_m.