It is well known from growing binary semiconductors that at least two different AB semiconductors can be produced d with A-excess or AB with B-excess have distinctly different properties: one possibly being n-type and the other could be p-type. IFor the discovery of new functional semiconductors, these multiple, distinct states of the same nominal composition expand the space of materials to investigate. In thermoelectrics for example, researchers have been examining hundreds of nominally single phase materials for decades in search of, for example, n-type Zintl compounds with predicted high thermoelectric efficiency. The discovery of high performance n-type Mg3Sb2,4 only recently, highlights the importance of examining all the distinct thermodynamic states by identifying the phase boundaries (Mg-excess as well as Sb-excess in this case) we call phase boundary mapping. Futher examples in CoSb3 skutterudites and complex Zintl phase Ca9Zn4+xSb9 will be given.