Phase transitions and ferroelectricity in NaSb3F10

Abstract

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 NaSb3F10is ferroelectric is confirmed herein without ambiguity. Its spontaneous polarizationPsis found to exhibit reproducible dielectric hysteresis at room temperature, withPs≃ 60 µC m−2, under the application of a field of 0.3 MV m−1or greater. The pyroelectric coefficient 〈p〉 = 17 (5) µC m−2 K−1at 298 K. NaSb3F10undergoes a phase transition atTC≃ 461 K, on correction for thermal hysteresis, with entropy change ΔS= 5.7 (3) J mol−1 K−1. The colorless crystals melt atTm ≃ 515 K and decompose above ∼600 K. The thermal hysteresis of ∼35 K inTC, on heating and cooling at 5–25 K min−1, is typical of first-order phase transitions. The space group in ferroelectric phase III isP63, and that in phase II is predicted to beP6322, a nonpolar supergroup ofP63; the supergroup expected in the prototypic nonferroic phase I isP63/mmc. The space group of phase III isnota direct subgroup of phase I. The dielectric permittivity ∊′ at 1 kHz increases over an order of magnitude between 300 K and a major inflection atTC, continuing to increase steadily thereafter toTm.