Effect of pore size on phase transitions in rubidium tetrachlorozincate nanoparticles in porous glass matrices

It is well known that below a certain temperature (Ti), local displacements of individual atoms from their original positions occur in ferroelectric crystals with incommensurate phases. They form a spatial wave with a length of l, which is incommensurate with the lattice period a, i.e. the l/a ratio is irrational. The wavelength increases as the temperature decreases. Near the phase transition temperature TC, it reaches a length comparable to the size of the ferroelectric domains, as in the model rubidium tetrachlorozincate crystal (Rb2ZnCl4). In ultrafine Rb2ZnCl4 crystals, the increase in l is hindered by the size of the crystallite. Therefore, the physical properties of nanocrystalline rubidium tetrachlorozincate are expected to be considerably different from those of the bulk sample. One of the methods for producing nanosized ferroelectric materials is a method based on embedding the material from a solution into porous matrices with nanometre-sized through-pores. We applied this method to study the effect of the size of ultrafine rubidium tetrachlorozincate crystallites on its dielectric properties and the phases occurring in the nanocrystallites. For the experiment, we used samples of polycrystalline Rb2ZnCl4 and composites obtained by incorporation of Rb2ZnCl4 salt from aqueous solution into porous silicon oxide matrices with an average through-pore diameter of 46 and 5 nm (RS‑46 and RS-5, respectively). The temperature dependencies of their dielectric permittivity were studied within the range of 100 to 350 K. We determined the temperatures of transition to the incommensurate (Ti) and ferroelectric (TC) phases, as well as the mobility deceleration temperatures of ferroelectric domain boundaries in rubidium tetrachlorozincate nanocrystallites in the RS-46 composite. In Rb2ZnCl4 particles in the RS-5 composite, only the transition to the incommensurate phase occurs. In contrast to the bulk material, it shows features of the first-order phase transition