Rediscovering Polarized Light Microscopy

Rediscovering Polarized Light Microscopy by Robert Weaver A microscope able to provide simultaneous physical, chemical, and crystallographic data on particles down to 500 nm would surely be hailed as an incredible instrument. This microscope would likely find itself rapidly integrated into a variety of materials analysis procedures. One may be surprised to learn that a microscope with these capabilities has existed for more than 170 years. The polarized light microscope (PLM) has demonstrated its value as an indispensable analytical instrument with continuous usage and the development of new applications since its earliest reported use circa 1834.1 However, although the modern PLM is more useful than ever, there has been a general decrease in the awareness of its operation and capabilities in materials analysis. Several factors have contributed to this decrease, including 1) a sharp decline in optical crystallography course offerings with the retirement of key professors; 2) changes in academic and research funding trends; and 3) the popularity of other microscopes, Table 1 namely the scanning electron microscope with energy dispersive spectroscopy (SEM-EDS); and the scanning probe microscope (SPM).2 Microscopists seeking to understand and apply the PLM must now rely on texts,3,4 on-thejob training, or training at the McCrone Research Institute (Chicago, IL). However, those microscopists who invest the time to become proficient with the PLM will enjoy the many advantages this technique offers in routine and specialized analyses. Those who choose to remain unaware of its capabilities are not fully appreciating the foundation on which so much of our current analytical knowledge is based. Optical crystallography Depending on the crystal system and the history (processing, growth, etc.), a material may possess one refractive index (i.e., n), two (nε, nω), or three (nα, nβ, nγ) (see Table 1). The six crystal systems are defined by the length (a, b, c) of each crystallographic axis (X, Y, Z) and their interaxial angles (α, β, γ). These crystallographic constants govern the symmetry of a material’s properties (Neumann’s principle). If the magnitude and orientation of the refractive index (or any other vector property) is expressed as vectors emanating from a common point, then the locus of all points will map a threedimensional surface called an indicatrix (Figure 1). The shape of the indicatrix for the six crystal systems is therefore a sphere (isometric), a biaxial ellipsoid (hexagonal or tetragonal), or a triaxial ellipsoid (orthorhombic, monoclinic, or triclinic). By virtue of the lengths of the indicatrix axes, restrictions are placed on the possible crosssectional shapes. Figure 1 shows the principal sections (i.e., an ellipse parallel to two of the principal refractive index values [nε, nω, nα, nβ, nγ]), circular sections, and random sections for the three indicatrix types. For the random uniaxial sections, either the major (uniaxial negative) or minor (uniaxial Definitions, abbreviations, and conventions for optical and crystallographic properties Crystal system Isometric Hexagonal Axial constants a=b=c a=b≠c Angular Optical constants symmetry α=β=γ =90° Isotropic γ=β=90°, α = 120° Uniaxial T