Nanocomposites and nanomaterials

Impedance spectroscopy of opal–KH2PO4 nanocomposite 
Bilotserkovskyi M.1, 2, Derhachov M.2, Pliaka S.2, Jankowska-Sumara I.1
1 Faculty of Mathematics, Physics, and Technical Sciences, Pedagogical University of Cracow. Ul. Podchorążych, 2, 30-084 Kraków, Polska
2 Faculty of Physics, Electronics and Computer Systems, Oles Honchar Dnipro National University. Prospect Haharyna, 72, Dnipro-49010, Ukraine. 
E-mail: derhachov_mp@dnu.dp.ua 

Potassium dihydrogen phosphate (KH2PO4, KDP) crystals are still of a great interest owing to the high proton mobility and uncertain phase transition at temperatures higher than 390 K. The crystals can be applied in electro-optical units and elements for laser radiation frequency multiplying. This work deals with investigation of dielectric parameters and charge transfer processes in multi-grained nanoscale opal–KDP composite. 
The composite can be considered as a skeleton, formed by amorphous 250 nm SiO2 spheres, with the pores filled with KDP nanocrystals grown by spontaneous nucleation in aqueous solution. XRD measurements proved mostly tetragonal phase of the nanocrystals with estimating their linear sizes as no more than 50 nm. Conductivity and other dielectric parameters of KDP nanocrystals embedded in opal matrix were investigated by employing standard impedance technique over a 153 K – 473 K and 20 Hz – 2 MHz ranges. The direct-current (dc) measurements were also performed over a 290 K – 473 K range. 
The temperature dependences of capacity and conductivity at different frequencies show a distinct hysteresis within a 413 K – 463 K interval. A clear maximum of dielecric permittivity at about 438 K is observed in heating while there is no anomaly in the cooling. Similar behavior was observed in temperature range 153 K – 313 K. Maxima of the Cp and G values are observed at about 300 K under heating sample in this case. These anomalies are most probably caused by thermal activation of protons with their further capturing by defects in the composite structure. Besides, the Cole-Cole diagram cannot be simply described by Debay’s theory and is considered in the framework of heterogeneous dielectric model.