High Temperature Phase Transition in Cs3D(SO4)2

Abstract

The M3XðYO4Þ2 type crystals have so-called zerodimensional hydrogen bonds consisting of isolated YO4{X{YO4 dimers, where M 1⁄4 NH4, K, Rb, Cs; X 1⁄4 H, D; Y 1⁄4 Se, S. It is well known that some of the member crystals undergo successive structural phase transitions, and show marked isotope effects on the antiferroelectric (ferroelectric) phase transitions below room temperature. On the other hand, above room temperature, most of the members exhibit the ferroelastic-superionic phase transitions, and show a high electric conductivity above the transition points. The high temperature phase transitions are due to the destruction and the reconstruction of hydrogen bonds. However, for most cases, the phase transition points do not exhibit the normal type of isotope effect on deuteration. These show the negative isotope effect in which lowering of the transition temperature is seen by the deuteration, for example, Cs3H(SeO4)2 ! Cs3D(SeO4)2. Tricesium hydrogen disulfate, Cs3H(SO4)2, (abbreviated as TCHS) also belongs to M3XðYO4Þ2 type crystals. So far no phase transition point has been reported in TCHS above and below room temperature. Then the deuterated compound TCDS may exhibit the ferroelastic-superionic phase transition at a some point above room temperature, if this compound follows the negative isotope effect on ferroelastic-superionic transition temperature (Ti). In order to confirm the effect on Ti in TCDS, the measurements of the dielectric constant, the electric conductivity and the differential scanning calorimetry (DSC) are performed. Single crystals of TCDS were prepared by successive recrystallizations from D2O solution containing Cs2SO4 and H2SO4 with molar ratio 3 : 1. Both (001) and (00 1) planes of the sample crystal were coated with Ag paste electrodes. The electric capacity and the complex impedance were measured in the direction perpendicular to the (001) plane, in the temperature region from 350K to 450K, with an automatic LCR-meter, HIOKI-3531, using 100KHz electromagnetic wave. The dielectric constant and the electric conductivity of TCDS were calculated from the data of electric capacity and the complex impedance, respectively. The temperature fluctuation of the sample is suppressed within 0:1K by a precise temperature controller. The measurement by DSC was performed in the temperature region from 350K to 470K in the heating rate of 3 C/min. It was also comfirmed by DSC signals that no protonated compounds TCHS exhibited the phase transition in the temperature region from 350K to 490K. In Fig. 1, the temperature dependence of the dielectric constant of TCDS is shown; the dielectric constant increases with increasing temperature and jumps discontinuously at about 422K. On further heating, it decreases rapidly. when temperature is increased further, the dielectric constant increases exponentially. The temperature dependence of the dielectric constant around 422K seen in Fig. 1 is similar to those found in the cases of the ferroelastic-superionic phase transitions of Cs3D(SeO4)2 and K3H(SeO4)2. 5,6) The temperature dependence of the electric conductivity of TCDS is plotted in Arrhenius coordinates as shown in Fig. 2. The conductivity has a value of about 5:0 10 8 cm 1 at 350.1K (1000=T 2:86) and increases sharply with increasing temperature up to about 422K (1000=T 2:37). The electric conductivity jumps suddenly from 7:3 10 7 cm 1 to 9:7 10 4 cm 1 at 422K, and drops just above this temperature as seen also in the case of the dielectric constant in Fig. 1. On further heating, it increases exponentially. The temperature dependence of the electric conductivity shown in Fig. 2 is very similar to that of AgI which undergoes the superionic phase transition. High values of dielectric constant above 422K in Fig. 1 may be considered as due to high electric conductivities in this temperature region. However, the reason for the dropping of just above 422K is unknown. According to Fig. 1. Temperature dependence of the real part of dielectric constant 0 of Cs3D(SO4)2 along the direction perpendicular to the (001) plane at 100KHz.