Phase transitions and ferroelectricity in NaSb$_{3}$F$_{10}$

The structural prediction that NaSb$_3$F$_{10}$ is a new ferroelectric has been confirmed experimentally. The mean phase transition temperature $T_{\mbox{c}}$ $\approx$ 461 K with an associated entropy change $\sim$6 J mol$^{-1}$ K$^{-1}$. The colorless crystals melt at $T_{\mbox{m}}$ $\sim$515 K with decomposition starting at $\sim$600 K. A thermal hysteresis in $T_{\mbox{c}}$ of $\sim$35 K between heating and cooling at 25 K min$^{-1}$ is typical of a first order phase transition. The space group in ferroelectric phase III is $P6_3$, that in the predicted antiferroelectric phase II is $P6_3$22, a supergroup of $P6_3$. The space group of prototypic nonferroic phase I is supergroup $P6_3/mmc$, of which the space group of phase III is not a subgroup. The dielectric permittivity at 100 Hz increases more than an order of magnitude from 350 K before undergoing a major inflection at $T_{\mbox{c}}$ = 460(10) K; it increases thereafter to $T_{\mbox{m}}$. The dielectric loss at 100 Hz is low but increases an order of magnitude from its value at $\sim$350 K before undergoing an inflection at $\sim$460 K, also rising steadily thereafter to $T_{\mbox{m}}$. The reproducible dielectric hysteresis loop, with $P_{\mbox{s}}$ $\approx$ 20$\mu$ C m$^{-2}$ at room temperature under the application of 0.3 MV m$^{-1}$ a.c. or greater, unambiguously verifies the predicted ferroelectric property. The pyroelectric coefficient $\langle{p}\rangle$ = 17(5)$\mu$ C m$^{-2}$ K$^{-1}$ at 298 K.