Published by AMSS Press, Wuhan, China. Acta Mechanica Solida Sinica, Vol. 22, No. 6, December, 2009 ISSN 0894-9166 THERMODYNAMIC MODELING OF NANOSCALE FERROELECTRIC SYSTEMS ⋆⋆ Yue Zheng Biao Wang ⋆ (State Key Laboratory of Optoelectronic Materials and Technologies/Institute of Optoelectronic and Functional Composite Materials, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China) Chung-Ho Woo (Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong SAR, China) Received 1 July 2009; revision received 23 December 2009 ABSTRACT Thermodynamic models formulated based on the Landau free-energy expansion are popular and well suited to studies involving properties of the ferro/para-electric transition, or near it. Indeed, the general nature of thermodynamics, from which the strength of the model is derived, allows a valid model to be constructed based on simple functional forms with parameters fitted to experiments, by passing the mechanistic complexity. Despite inaccuracy due to the neglect of fluctuations, this approach has been proven effective and powerful for recent research develop- ment of ferroelectrics in nanoscale. Efforts in some important works have recently faced much challenge, when free-energy contributions have to be incorporated to account for the presence of depolarization fields, surfaces and other defects. To minimize the problems with mechanistic obscurity, it is of paramount importance that the electromagnetics, mechanics and thermodynam- ics involved are accounted for explicitly and with full self-consistency. It is important that the free-energy functional of nanoscale ferroelectric systems, such as ferroelectric thin films (FTF), bilayers (FB), superlattices (FS), nanowires (FNW), nanotubes (FNT) and tunneling junctions (FTJ) etc., must be derived thermodynamically from first principles. KEY WORDS ferroelectrics, nanoscale, thin film, superlattice, nanowire, nanotube, tunneling junction, mechanical load I. INTRODUCTION Ferroelectricity is a collective phenomenon, the characteristics of which depend on the combined effects of many factors, such as the ambient temperature, boundary conditions, sample dimensions, misfit epitaxial stresses, surface effects and electrostatic interactions, etc. [1–24] This is particularly true for the state of ferroelectricity itself. Studies of the effects of nanoscale dimensions on the polarization, dielectric constants, Curie temperature and critical sizes of ferroelectrics have been conducted for the last several decades. Recently, research in this area has found new relevance due to the surge of technological ⋆ Corresponding author. E-mail: wangbiao@mail.sysu.edu.cn ⋆⋆ Project supported by the National Natural Science Foundation of China (Nos.10732100, 10831160504, 10972239 and 10902128), and the Research Grants Council of the Hong Kong Special Administrative Region (G-YX0T, 5322/04E, N53408).