Chemists usually distinguish inorganic materials by their structure and composition, and in some cases, properties. The dominance of x-ray crystallography in the study of the solid state commonly leads to delineation between materials based on their diffraction properties, along with analysis of their composition by methods such as atomic emission spectroscopy or mass spectrometry. Pure molecular materials take their composition from their molecular formula, but may still form different crystal structures, or polymorphs, which have different properties and are often considered to be distinct materials. Non-molecular materials, such as those we are concerned with here, can also show polymorphism, the most famous example being diamond and graphite as polymorphs of carbon, but in addition to this, non-molecular materials do not have a chemical composition restricted by molecular formulae. Their composition is not quantized, but instead can be incrementally changed. One primary example of this is solid solutions, for instance, a solution of KCl and NaCl, which could be written Na 1-x K x Cl, where x can take any value between 0 and 1. Doping is a related concept where some percentage of an impurity is incorporated into a material; doped silicon is the basis of modern electronics due to the large effect on electron transport imparted by a small concentration of impurity. This ability to incrementally alter composition challenges concepts of what constitutes a new material.