R. H. Chen

Optical studies of ferroelastic phase transition and domain structures in K3Na(SeO4)2 crystals

Abstract Optical observation of domain structures of K 3 Na(SeO 4 ) 2 crystal was carried out at room temperature. When the crystal is warmed up through the phase transition points, there is an intermediate phase between 334 K and 346 K. Ferroelastic phase transition is observed at 346 K. The boundaries and domain walls are analysed in the room temperature (ferroelastic phase). The components of the spontaneous strain tensor are calculated from the lattice parameters of two phases. By applying the external forces, the movement of the domains was studied. The ability to switch of the domains under the directional external forces is explained by the retilting of SeO 4 −2 tetrahedra from its present orientation in one domain to other equivalent orientations which are associated with 3-fold symmetry along the c -axis of the high temperature phase. The final state is a minimum state of elastic potential energy.

Studies on the dielectric properties and structural phase transition of K2SO4 crystal

Abstract Electrical complex impedance measurements were carried out on K2SO4 single crystal. The temperature and frequency dependence of electrical conductivity, dielectric constant, and the electrical modulus were determined. The increase in conductivity with rise in temperature showed the superionic nature of the sample in the high-temperature phase. The results also show that the temperature dependence of AC conductivity and conduction relaxation time approximately follows the Arrhenius relation. The activation energies and characteristic relaxation times were calculated. The AC conductivity as a function of frequency is well described by σ(ω)=σ(0)+Aωs. Here σ(0) is the DC conductivity, and s is the fitting parameter. Non-Debye dynamic conduction was observed in the sample crystal. The obtained values of s decrease with increasing temperature, from 1.27 to 0.61 and 0.43 to 0.28 in the room-temperature phase and in the high-temperature phase, respectively. This effect is attributed to the interaction of the mobile ions with the SO4 groups in the K2SO4 single crystal.

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Abstract Electrical impedance measurements of Na 3 H(SO 4 ) 2 were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO 4 tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na + contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω s . The value of the exponent, s , lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.

Studies of ferroelastic domain structures in (NH4)3H(SO4)2, K3H(SO4)2 and Rb3H(SeO4)2 crystals

Abstract The domain structures of (NH 4 ) 3 H(SO 4 ) 2 , K 3 H(SO 4 ) 2 and Rb 3 H(SeO 4 ) 2 crystals grown from the aqueous solutions were studied by using an optical polarizing microscope. Three orientational domain states were observed in the sample crystals in their ferroelastic phase. The spontaneous strain tensors are used to describe the characteristic property of ferroelasticity. The observed number of domain states and domain walls can be well derived by the symmetry reduction, and by the compatibility condition of the spontaneous strain tensor on the boundary through the structural phase transition between the paraelastic and ferroelastic states. The twin operations found in these crystals are mirror plane, two-fold and three-fold rotations which are the missing symmetries during the structural phase transition.

Structural phase transitions and electrical conductivity studies of (NH4)3H(SeO4)2 crystal

Abstract The electrical conductivity measurements were performed on single crystals of (NH 4 ) 3 H(SeO 4 ) 2 in the temperature range of 25 °C to 80 °C along the c * axis. The crystal is found to be in a superionic state as it transforms to the high-temperature phase. The temperature dependence of the electrical conductivity of the sample crystal in the different phases follows the Arrhenius law. The increasing electrical conduction is due to the disorder of the oxygen atoms of the SeO 4 groups in the high-temperature phase. The proton ions can move among the sites which are the newly formed hydrogen bonds between the oxygen atoms of the nearest and next-nearest SeO 4 groups.

Studies of ionic conductivity and structural phase transitions of Na3H(SO4)2 crystal

Abstract The complex electrical impedance of Na3H(SO4)2 along the bm-axis has been measured from 25°C to 316°C in the frequency range 4 kHz–40 MHz. The temperature dependence of the electrical conductivity shows remarkable changes in the temperature range 160°C–260°C. The sample crystal becomes a fast ionic conductor above 260°C. The conduction mechanisms of proton and sodium ions in the different phases are analyzed in detail with respect to the structural features of the sample crystal.

Crystal structure and a new phase transition of (ND4)3D(SO4)2 in high temperature phase

Abstract Electric conductivity and X-ray diffraction measurements are performed on deuterated triammonium hydrogen disulphate (ND 4 ) 3 D(SO 4 ) 2 in high temperature phase. A new structural phase transition that corresponds to the transition at 463 K of (NH 4 ) 3 H(SO 4 ) 2 is found at 413 K. The space group symmetry (monoclinic C2) and the crystal structure of (ND 4 ) 3 D(SO 4 ) 2 are determined at 404 K in phase I. The electric conductivity due to the deuterium ion migration in phase I is discussed with respect to the results of the structure refinement.

Structural phase transition and crystal structure of Na3H(SO4)2 in the low temperature phase

Abstract Differential scanning calorimetry and X-ray diffraction measurements are performed on trisodium hydrogen disulphate, Na3H(SO4)2, crystals at low temperatures. A structural phase transition is found at 178(3) K. The space group symmetry (triclinic P1) and the crystal structure are determined at 139K in the new low-temperature phase. It is found that the H atoms on the O─H─O hydrogen bond are not located on the center of the bond. By the shift of the S atom from the center of the SO4 tetrahedron, it points out that Na3H(SO4)2 crystal in the new phase is possible to be ferroelectric with the dipole moment almost along the [1 2 0] direction.

Crystal structures in phases I and II of (ND4)D(SeO4)2 and its phase transitions

Abstract Differential scanning calorimetry, dielectric, electric conductivity and X-ray diffraction measurements were performed on deuterated triammonium hydrogen diselenate (ND4)3D(SeO4)2 crystals. It was found that (ND4)3D(SeO4)2 crystals undergo three phase transitions at 326, 280 and 234K, and that the transition corresponding to the II-III transition at 308K of (NH4)3H(SeO4)2 disappears at around its temperature. The space groups of (ND4)3D(SeO4)2 are trigonal R 3 m in phase I and trigonal R 3 in phase II. The crystal structures in both phases are determined by X-ray diffraction. The electric conduction due to deuterium ion migration is discussed with respect to the results of the observed structures.

X-ray diffraction studies of twinning in KNaSO4 and K3Na(SO4)2 crystals

Abstract X-ray diffraction was used as a probe to study the twinning of KNaSO 4 and K 3 Na(SO 4 ) 2 crystals. The results showed that twinning phenomena are frequently found in these compounds and the twinning is of the merohedral type.