Micro-Raman Spectroscopy of Crystal Lattice Chemistry

Sep 01, 2012

Topochemistry is a discipline within solid-state chemistry in which the chemical reaction is localized to the crystal surface of one of the reactants. In general, this means that the crystalline reactant undergoes a chemical transformation induced either by exposure to another reactant or light of a suitable wavelength to cause a photochemical reaction. Recently, the term crystal engineering has been used more frequently than topochemistry. This class of chemistry has allowed chemists to produce reaction products that otherwise could not be generated under any other conditions. Furthermore, it has led to the engineering of thin-film structures and waveguides whose chemical and physical properties differ to some degree from those of the reactant crystal. Micro-Raman spectroscopy is ideally suited to characterize and spatially resolve the chemical and physical changes that occur in topochemical reactions, particularly in thin films and waveguides.

In November of 1974, J.M. Thomas published a review lecture titled "Topography and Topology in Solid-State Chemistry." To my knowledge, this was the first review of the emerging field of topological chemistry or topochemistry (1), although the terminology was first used by Kohlschütter in 1919 (2). Topological chemistry is not the type of chemistry that most of us are familiar with where reactions occur in a beaker, flask, or perhaps turbulent or laminar flowing streams. Topological chemistry is a branch of solid-state chemistry. This type of chemical reaction generally falls into the category of crystal chemistry, in particular crystal-to-crystal chemistry. We might say that in these types of reactions, the reactant crystal functions as our beaker.

In a topological chemical reaction the crystalline reactant undergoes a chemical transformation induced either by exposure to another chemical reactant or light at an absorbing wavelength suitable to cause a photochemical reaction. It is important to distinguish between reactions that are localized, such as in liquid–solid or gas–solid reactions in which the solid reactant is consumed (for example, rust formation), and topochemical reactions in which the solid reactant phase undergoes a chemical transition to a solid product phase within the host crystal.

Figure 1: Atomic structure of LiNbO3 single crystal in the ferroelectric C3v phase, in which the niobium-oxygen octahedral (NbO6) is distorted and the Li+ is displaced above the oxygen planes along the c-axis.
As you might imagine, the kinetics and thermodynamics of topochemical reactions are very much affected by the host crystalline environment. In some instances, compounds have been generated through crystal organic photochemistry that could not be produced in solution (3). The so-called reaction cavity must exist such that the intermolecular forces and structural arrangement of the atoms or molecules do not cause the crystal to simply break off or dissolve as the reaction occurs. At the same time, the crystal must allow for atomic or molecular movement sufficient for bond breaking and formation of the new compound as well as mass transport. One of the simpler topochemical reactions is an ion-exchange reaction in ferroelectric metal oxides.

One ferroelectric metal oxide that is of great technological importance is LiNbO3, which has a high optical nonlinear susceptibility and large electro-optic coefficient. The atomic structure of LiNbO3 is depicted in Figure 1. LiNbO3 has been used for decades to achieve frequency doubling and as an electro-optic switch. In general, individual crystals have been used in conjunction with other optical components to achieve the desired effect. However, in recent decades there has been extensive research on single crystals of LiNbO3 and other nonlinear optical single crystals to create integrated optical devices. This has largely been achieved through the fabrication of waveguides in single crystals through ion exchange. Waveguides fabricated in this fashion have made their way to the marketplace and are the active component in some commercially available optical modulators. In this installment, we discuss how micro-Raman spectroscopy has been used to depth profile a Li1-xHxNbO3 waveguide produced by an ion-exchange reaction in a single crystal of LiNbO3 and reveal the changes in chemical bonding and atomic structure that occur in this process (4).