A Study of Temperature Dependent Superprotonic Conductivity of Na + (1 mol%) doped [K 0.50 (NH 4 ) 0.50 ] 3 H(SO 4 ) 2 Mixed Crystal

Transcription

1 A Study of Temperature Dependent Superprotonic Conductivity of Na + (1 mol%) doped [K 0.50 (NH 4 ) 0.50 ] 3 H(SO 4 ) 2 Mixed Crystal Khin Kyu Kyu Han 1, Win Kyaw 2 and Win Win Thar 3 Abstract Na + (1 mol%) doped (50%) Tri-Potassium Hydrogen Disulphate and (50%) Tri-Ammonium Hydrogen Disulphate, [K 0.50 (NH 4 ) 0.50 ] 3 H(SO 4 ) 2 mixed crystal was grown by slow evaporation of aqueous solution at room temperature. Temperature dependent electrical conductivities of the crystal were observed by using FOTEK MT-20 Temperature Controller in the temperature range of 299 K 523 K to study the superprotonic conductivity, to examine the structural phase-transition temperature (T SPT ) and to evaluate the activation energy of the crystal. High temperature phases of the crystal were investigated by simultaneous Thermogravimetric Analysis and Differential Thermal Analysis (TG- DTA) method to conform the T SPT of the crystal. Introduction The conductivity phenomenon is connected with the dynamical disordering of the hydrogen-bond network, resulting in an increase of the number of possible positions for protons. This process includes the transfer of proton within the hydrogen-bond and breaking of the hydrogen-bond together with the reorientation of the ionic group involved in the hydrogenbond formation (Baranov, 1998). The crystals of M 3 H(XO 4 ) 2 type (where M = K, NH 4, Rb and X = S, Se) belong to the well-known family of superprotonic conductors (Lunden, 1996). All the crystals are monoclinic at room temperature. The crystals are isomorphic at room temperature and consist of dimmers formed by two XO 4 tetrahedra linked with a short hydrogen-bond. Hydrogen-bonded superprotonic crystals are well-known for their proton orderings at low temperatures as well as for high protonic conductivity which increases significantly in the high-temperature superionic phases (Lushnikov, 1999). 1. Demonstrator,Dr., Department of Physics, University of Yangon 2. Lecturer, Dr., Department of Physics, University of Yangon 3. Professor (Head), Dr., Department of Physics, University of Yangon

2 228 Materials and Methods Mixed crystals of Na + (1 mol%) doped [K 0.50 (NH 4 ) 0.50 ] 3 H(SO 4 ) 2 were grown by slow evaporation method at room temperature from the aqueous saturated solution containing 40% weight of [K 2 SO 4 ] [(NH 4 ) 2 SO 4 ] 0.50 with the addition of 1 mol% of Sodium Sulphate, Na 2 SO 4 salt powder and 24% weight of concentrated Sulphuric acid, H 2 SO 4. Temperature dependent resistances of the crystal were observed by PC-based temperature controller FOTEK MT-20 in the temperature range of 299 K-573 K. The area and thickness of the crystal were (1.13 x 10-2 ) cm 2 and 0.48 cm respectively. Temperature dependent resistances of the sample were measured by using MASTECH MS8216 DMM digital resistance meter. TG-DTA thermograms of the crystal were recorded on PC-based SHIMADZU (DTG-60H) thermal analyzer in the temperature range of 30 C-600 C to study the high temperature phases of the crystal. Results and Discussion Arrhenius plot of the variation of dc electrical conductivity of the crystal is shown in Figure 1. Slope of the curve is changed at 438 K in which structural phase transition occurs. According to the theory of ionic conductivity, slope of the electrical conductivity in Figure 1 corresponding to the activation energy for creating of defect states due to the structural phase transition (T SPT ) of the crystal from ambient monoclinic to high temperature rhombohedral structure. The activation energy (E i ) can be obtained by using the slope of the ln(σ) versus 10 3 /T graph as shown in Figure 2. The activation energy of the crystal is 1.13 ev. As shown in Figure 2, temperature dependent electrical conductivities of the crystal are increased with increasing temperatures. Slopes of the conductivity curves are clearly found with two steps; (1) before T SPT region and (2) after T SPT region. In the first region of before T SPT (438 K), the electrical conductivities of the crystal are slowly increased with thermal energy. In the region of after T SPT, the electrical conductivities of the crystal are abruptly increased by ionic motion coupled with structural phase transition from ambient monoclinic to high temperature rhombohedral structure. The crystal is exhibited as the superprotonic (or superionic) conductor at high temperature due to the protonic motion. The

3 229 temperature of 438 K can be taken as the structural phase-transition temperature of the crystal. It can be discussed as the electrical conductivity of the hydrogen-bonded crystal is more than a hundred times greater than that of most ionic crystals, but many million times less than that of a metal. The chemical formula of the salt is, e.g., (NH 4 ) 3 H(SO 4 ) 2 ; it is composed of positive ammonium ions (NH + 4 ), negative sulphate ions (SO 2-4 ), and proton (H 2 ). Each SO 2-4 ion consists of four oxygen atoms covalently bonded to a 2- sulphur atom. A SO 4 ion carries the extra negative charge of three electrons, one of which it obtains from the NH + 4 part of the structure and the two other hydrogen atoms that furnish in this way two conducting protons /T (K -1 ) ln(σ) (S cm -1 ) Figure 1 Arrhenius plot of the temperature dependent electrical conductivity of the crystal TG-DTA thermograms of the crystal are shown in Figure 3. As shown in Figure, three endothermic reaction peaks are observed at C, C and C respectively. The first peak is indicated by the structural phase-transition T SPT of the crystal. The enthalpy change of the second endothermic reaction peak is the largest one in which dehydration of water and decomposition ammonium from the sample during the crystal growth condition.

4 /T (K -1 ) ln(σ) (S cm -1 ) y = x Figure 2 Temperature dependent electrical conductivity of the crystal (T T SPT ) Figure 3 TG-DTA thermograms of the crystal

5 231 The third endothermic reaction peak is rather due to the onset of partial dehydration at particular localities on the surface. When the crystal is heated above C, the constitutional water on the surface that is produced during growth is evaporated. Then the crystal melted. TGA thermogram shows the mass variation of the sample with two steps of baseline changes. The first step occurs in the temperature range of C C; in this step, water and ammonium are escaped from the sample and then the sample is melted with the mass variation of 6.508%. Thus, the first step is the solid phase to liquid phase of the sample. The second step is neglected due to the sample melted. Conclusion Na + (1 mol%) doped [K 0.50 (NH 4 ) 0.50 ] 3 H(SO 4 ) 2 mixed crystal was grown by the slow evaporation of aqueous solution. The hydrogen related sulphate (H---SO 4 ) will be utilized as a sensitive probe for hydrogen motion process because it is clearly observed by temperature dependent electrical conductivity measurement. It is said concerning the M 3 H(XO 4 ) 2 family, i.e., the "simplest" one with zero dimensional hydrogen bond network, that the high conductivity is due to a complex interaction of hydrogen and oxygen displacements combined with breaking and formation of hydrogen bridges. Acknowledgements We are indebted to Professor Dr Pho Kaung, Pro-Rector and Director of Asia Research Centre (ARC), University of Yangon for his stimulating suggestions and comments. References Baranov, A. I Influence of the NH 4 -Rb Substitution on the Phase-Transitions with different kinds of Proton Disorder in mixed [(NH 4 ) 1-x Rb x ] 3 H(SO 4 ) 2 Crystals: Ferroelectrics, 217, 285. Lunden, A Phase Transitions and Protonic Conduction in Cesium Hydrogen Sulphate and Related Compounds: Solid State Ionics New Developments, 245. Lushnikov, S. G Isotope Effect in Cs 5 H 3 (SO 4 ) 4.0.5H 2 O Crystals: Solid State Ionics, 125, 119.