The bond fluctuation model of superionic conductors predicts that the polarizability of solids that exhibit high ionic conduction is large. Based on this background, a study on the nonlinear optical constants in superionic glasses has been started. As a first step, the relationship between the third-order susceptibility χ (3) and the linear susceptibility χ (1) of various kinds of glasses has been studied. It is found that the values of χ (3) of superionic conducting glasses exceed considerably the values predicted by the usual Miller rule. The deviation arises from the increase of the ionic coordinate dependent electronic polarizability, which plays also an important role in the ion transport processes.

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According to the bond fluctuation model of superionic conductors, the polarizability of a material that exhibit high ionic conductivity must be large. That is, the model predicts that ion conducting materials must have large values of optical constants. This prediction has been confirmed recently, through a comparative study of the nonlinear optical constants of ion conducting and non-conducting glasses. In the present report, the values of the nonlinear optical constants (3) of R 2 O-B 2 O 3 (R=Rb, Cs, Ag) glasses have been estimated by using the bond orbital theory. For the Rb 2 O-B 2 O 3 and Cs 2 O-B 2 O 3 glasses, a good agreement between the calculated and the measured values has been found. On the other hand, for the Ag 2 O-B 2 O 3 system, a deviation was observed. Such a disagreement has been explained by taking into account the contribution of d-electrons of the Ag ions, which plays also a fundamental role in the ion transport mechanism.

The aim of the present work is to understand the melting phenomena from a chemical bond point of view. We have studied the chemical trend of the melting temperature of AB type compounds by using the bond orbital model. It has been shown that the dominant interaction parameter determining the melting temperature depends on the types of materials. The peculiar behavior of Cu and Ag halides among the AB type compounds have been pointed out.KEYWORDS: melting, bond orbital model IntroductionMelting is one of the most popular physical phenomena. However, concerning the fundamental aspect of melting phenomena, many questions remains unsolved and not well understood. In order to understand the complicated mechanism of melting, many models have been proposed. For example, the famous empirical model proposed by Lindemann 1) states that a material melts when the amplitude of the thermal oscillation of the atoms becomes approximately 10% of the equilibrium interatomic separation. Most of the modern approaches to melting are based on lattice dynamical considerations.2) In the present report, the melting phenomena are investigated from a chemical bond point of view.3) We have studied the chemical trend of the melting temperature by using the bond orbital model. 4) By using this model, we can consider in a relatively simple way the essence of the nature of the chemical bond. In the past, this model has been used to study the structural stability of many AB type compounds such as NaCl and GaAs.

Guided by the prediction of the bond fluctuation model of superionic conductors, the relation between the nonlinear optical constants and the ion transport properties in ionic conductors has been studied. Since the measured values of nonlinear optical constants in ionic conductors are very limited, they have been evaluated through the Sheik‐Bahae equation. Using such values, it is shown that the activation energy of ion transport and the superionic transition temperature decrease with the increase of the nonlinear refractive index. It is also pointed out that the band gap energy and the linear refractive index in superionic conductors are relatively weakly correlated when compared with non‐superionic materials. The development of a new field of study that could be called photoionics is suggested (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)