Editing F-center (section)

==History==
Before the discovery of point defects it was already known that some crystals can be discolored using various methods. In 1830 T.J. Pearsall discovered that [[fluorspar]] could be discolored using violet light.<ref>Stephen R. Wilk (2013) ''How the Ray Gun Got Its Zap'', chapter 14: "Thomas J. Pearsall and the Ultraviolet", pp 80 to 84, [[Oxford University Press]]</ref> Thirty years later similar results were achieved by melting crystals together with a specific metal. In 1921 [[Wilhelm Röntgen]] extensively measured rock salts. One set of these tests measured a photoelectric conductivity 40,000 times larger, after the salt was radiated with x-rays. A similar result to x-rays was accomplished by coloring the crystals with metal vapors. The photoelectric effect mainly happened around specific wavelengths, which was later found to be non-colloidal in nature.

The discolorations were later named F centers, as in ''Farbe'', the German word for color. These defects were extensively studied by [[Robert Wichard Pohl]] and his institute at the [[University of Göttingen]] since 1920.<ref name="crystal maze">{{cite book |last1=Teichmann |first1=Jürgen |title=Out of the crystal maze |last2=Szymborski |first2=Krzysztof |date=1992 |publisher=Oxford University Press |isbn=0-19-505329-X |pages=238–291 |chapter=Chapter 4 ‘’ Point Defects and Ionic Crystals: Color Centers as the Key to Imperfections’’}}</ref> One of his assistants, {{Ill|Erich Mollwo|de}} concluded in 1933 that these F centers are atomic crystal defects.<ref name="crystal maze" /> Around this time people started to assert these defects were unpaired electrons. The vacancy model was first described by Pohl in 1937 but still was considered tentative.<ref name="crystal maze" /> It was formalized theoretically by [[Nevill Mott]] and [[Ronald Wilfred Gurney]] in 1940.<ref name="crystal maze" /> It took until 1957 to prove find conclusive experimental evidence using [[electron spin resonance]].{{Cn|date=April 2025}}