Contrasting temperature dependences of isostructural one-dimensional ferroelectric crystals NH₄HSO₄and RbHSO₄in terms of thermal conductivities

Temperature-dependent thermal conductivities are reported for one-dimensional (1D) hydrogen-bonding ferroelectric crystals of isostructural compounds NH4HSO4 and RbHSO4. As the temperature was decreased from 300 K, at which point they were paraelectric in the P21/n space group, their thermal conductivities decreased, similar to those of glassy materials. At the ferroelectric transition points (T1A = 270 K for NH4HSO4 and T1R = 264 K for RbHSO4), a change from P21/n to Pn space groups was observed, and the thermal conductivity of the NH4HSO4 crystal decreased without any anomalies, whereas that of RbHSO4 increased, similar to that of crystalline materials. At the second ferroelectric-to-paraelectric transition point of NH4HSO4 (T2A = 154 K), the thermal conductivity increased from 1.00 W m-1 K to 1.32 W m-1 K and increased with a subsequent decrease in temperature, similar to that of crystalline materials. Single-crystal x-ray structure analyses revealed that the thermal conductivity transition of RbHSO4 at T1R = 264 K corresponds to the rotational motion excitation of the HSO4- chains. The abrupt thermal conductivity jump of NH4HSO4 was likely related to the order-disorder type transition in NH4+ ions, accompanied by lattice vibration excitation, coupled with internal rotation. At the T2A ferroelectric-to-paraelectric phase transition of NH4HSO4, 21 crystal symmetry recovery was observed, similar to the Rochelle salt, and the space group at low temperatures was P21/n. For the RbHSO4 crystals, the thermal conductivity parallel to the 1D chains was 1.5-times higher than the corresponding perpendicular orientation.