Nuclear Magnetic Resonance in (NH4)2(BeF4)x(SO4)1−x and Other Ferroelectric Systems

Abstract

The temperature dependence of fluorine and proton nuclear magnetic resonance (NMR) in polycrystalline samples of the solid solution ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2{\left(\mathrm{Be}{\mathrm{F}}_4\right)}_x{\left(\mathrm{S}{\mathrm{O}}_4\right)}_{1-x}$ has been measured for several $x$. This solid solution is ferroelectric for high and low $x$ and is paraelectric in between. A sharp transition in the second moment of the ${\mathrm{F}}^{19}$ resonance was observed but found to be independent of $x$, while the ferroelectric properties are dependent on $x$. The proton NMR showed the nonequivalence of the N${\mathrm{H}}_4$ groups, but again the temperature dependence could not be correlated with the ferroelectric properties. Thus, the ferroelectric behavior of this system cannot be associated with the appealing hypothesis of the freezing in of the vibrating N${\mathrm{H}}_4$ or Be${\mathrm{F}}_4$ groups. The temperature dependence of the proton NMR was also observed in the ferroelectric compounds ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2$ ${\mathrm{Cd}}_2$ ${\left(\mathrm{S}{\mathrm{O}}_4\right)}_3$ and N${\mathrm{H}}_4$HS${\mathrm{O}}_4$. Similar conclusions can be drawn from these measurements as those given above. In some of th alums, the crystallographic phase transition is again not accompanied by any change in the proton resonance line. However, in ${\mathrm{N}}_2$ ${\mathrm{H}}_5$Al alum and N${\mathrm{H}}_3$OHAl alum, there is a very abrupt change in the NMR line at the temperature of the phase transition.