Novel Temperature Dependence of the Dielectric Constant in Rb2ZnI4 at Low Temperatures

Abstract

Halide ferroelectrics of the general formula A2BX4 type are classified into two groups from the viewpoint of crystal structures. One is the group of crystals having an orthorhombic -K2SO4 structure (space group of Pmcn ), such as ferroelectric Rb2ZnCl4. 1) Another is that having a monoclinic Sr2GeS4 structure (space group of P21=m ), such as ferroelectric Tl2ZnI4. 2) In the present paper, we shall report on a possible quantum-paraelectric nature observed in Rb2ZnI4, which is a crystal with the above monoclinic Sr2GeS4-type structure at room temperature. Concerning phase transitional properties of Rb2ZnI4, Geshi 5) first reported the temperature dependence of the dielectric constant along the b-direction, where a -type peak at 62.4K and a break at around 7.5K are observed. Later, our research group also reported a broad peak at around 5K accompanied by a frequency dispersion in the temperature dependence of the dielectric constant. However, thus far, neither ferroelectricity nor pyroelectricity has been detected in this crystal and, especially in the temperature region below 5K, the nature of the temperature dependence of the dielectric constant has not been well studied in detail. Single crystals of Rb2ZnI4 were prepared by evaporation from the aqueous solution and the mixed system (Rb1 xKx)2ZnI4 (x 1⁄4 0:0{0:8) was prepared by the Czochralski method from the melt, both in nitrogen gas atmosphere. For b-plate samples, the ac-dielectric constants (real part) and b (imaginary part), the dc-dielectric constant ð0Þ and residual polarization were measured using a precision LCR meter (HP 4284A) and a vibrating reed electrometer (Advantest TR8411), respectively. The temperature of the samples was controlled in the temperature region between 380 and 1.4K, using a liquid helium continuous flow cryostat (Oxford Instruments SpectrostatCF) with a helium gas pumping system. We measured dielectric constants along the band c-directions and perpendicular to the a-plane (cleavage plane). Only the b-direction dielectric constant shows an anomalous temperature dependence as indicated in the first report by Geshi. Figure 1 shows the temperature and frequency dependences of and b in Rb2ZnI4, measured for an aqueous solution crystal. To show simply the overall feature, and b at 1 kHz are plotted on the linear temperature scale. The feature of the curve including the -type peak at 62.4 K agrees well with the already reported data in the temperature region at least above 4K, whereas the result below 4K is newly obtained in this work, where a considerably sharp peak exists on this scale at around 4K. However, it is not a simple peak, as shown by the inset with a logarithmic temperature scale, where dielectric constants depend on measuring frequencies. The bold curve shows the static dielectric constant ð0Þ (dc-dielectric constant) measured using the electrometer by the application of a dc field. For b, the measuring frequencies are 1 kHz, 10 kHz, 10 kHz, and 1MHz from the top, and for 00 b , they are inverse. It is strange that both ‘‘the flat temperature dependence’’ of the dc-dielectric constant such as that in the quantum paraelectric state and ‘‘the glasslike’’ ac-dielectric dispersion are seen. How should we explain this phenomenon? The most important point to determine is whether the state below 4K is spontaneously polarized. To elucidate such points, we measured the residual polarization Pr of the above b-plate sample, cooled from 110K under the application of a dc field of 200 kV/m. After switching off the field at 1.4K, the observed Pr is less than 10 5 C/m, which is negligibly small in our apparatus, indicating a nonpolar state below 4K. As for the Curie point TC of A2BX4-type halide ferroelectrics, there is an empirical relationship indicating that TC is increased by substituting potassium for rubidium on an A-site. In the present case, if the dielectric anomaly below 4K is related to a ferroelectric phase transition, it should shift toward a high temperature side in a mixed system, such as (Rb1 xKx)2ZnI4. The phase diagram of such a system has already been known by our previous study using crystals prepared by the Bridgman method but detailed features for Pr are lacking, particularly for low x values and low temperatures. For this reason, we prepared a mixed system by the Czochralski method and carried out the same Pr measurement. The obtained results are what we expected, as shown in Fig. 2. The dielectric constant at 1 kHz is also shown. 0 50 100 150 200 250 300 0 20 40 60 80 100 120