In broad terms, hydrothermal synthesis is a technology for crystallising materials (chemical compounds) directly from aqueous solution by adept control of thermodynamic variables (temperature, pressure and composition). The objective of this review is to introduce the field of hydrothermal materials synthesis and show how understanding solution thermodynamics of the aqueous medium can be used for engineering hydrothermal crystallisation processes. In this review, powder synthesis, and their applications are introduced. In Section ‘Introduction’, we will focus on the hydrothermal synthesis as a materials synthesis technology by providing history, process definitions, technological merits and comments on its current implementation in the laboratory. In Section ‘Scope of hydrothermal synthesis in future’, we will describe the development of hydrothermal technology for materials synthesis, their results and comparison with other methods.

In this paper we present the impedance spectroscopy of ternary solid solutions of BiFeO₃, TbFeO₃ and PbTiO₃, prepared by solid-state reaction method. The preliminary structural studies were carried out by x-ray diffraction technique, showing the formation of polycrystalline sample with ABO₃ type of perovskite structure with hexagonal symmetry for Bi_0.8Tb_0.1Pb_0.1Fe_0.9Ti_0.1O₃system at room temperature. Dielectric and impedance study of this ceramic has been characterized in the temperature range 175 - 325 ⁰C and frequency range 100 Hz - 1 MHz. The maximum ferroelectric transition temperature (T_c) of this system was in the range 210 - 225 ⁰C with the dielectric constant having maximum value ~2480 at 1 kHz. The complex impedance graph exhibited one impedance semicircle arc at all reported temperatures, which indicates that the impedance response is a Cole-Cole type relaxation. Single semicircle indicate that the grain effect of the bulk in ceramic. The bulk resistance of the material decreases with increasing temperature showing negative temperature showing a typical semiconducting property, i.e. negative temperature coefficient of resistance (NTCR) behavior.