The fluorescence of compounds containing manganese

Abstract

The objects of the present paper are (1) to study the fluorescence of some pure compounds of the transition element manganese, and (2) to investigate the fluorescence of a wide variety of solids to which manganese has been added in small traces. The term “added” implies heat treatments over a range of temperatures varying from 500 to 1300° C according to the nature of the solid. Luminescence in solids, it is now known, may arise from a variety of causes. As the subject is in a rather complicated state at the present time it seems necessary to give a brief introduction to some of the more important features. Broadly speaking two chief classes of luminescent solids are recognized. The more restricted of these is concerned with pure substances such as platinocyanides, the uranyl salts, and certain rare earth compounds. Most luminescent solids, however, owe their properties to the deliberate addition of some impurity. These impurities (or activators as they are commonly called) are frequently “ metallic” in the following sense. A trace (say 1 in 10 3 ) of Mn may be added to an oxide such as MgO in the form of (say) chloride solution. The dried mixture is then subjected to heat treatment at, say, 1000-1200° C, as a result of which it is assumed that the manganese chloride becomes completely oxidized, and thus the only element effectively added to the oxide is manganese. Deliberate metallic impurities of this nature may be detected in solids by the use of spectroscopic tests or the use of sensitive organic reagents for metals. In some cases, however, luminescence is met with in solids containing no foreign metal atoms, and it is difficult to decide to what extent such luminescence is a property of an essentially perfect lattice or to what extent it is to be linked up with a “self” impurity. There are reasons which strongly suggest, for example, that the luminescence of “pure” zinc oxide (Ewles 1938; Randall 1939) is a function of the loss of a certain amount of oxygen. Intermediate cases of this nature lie between the more sharply defined classes to which attention has already been drawn. They are also of interest because it is frequently possible to modify their luminescence by the addition of metallic impurities. Examples of "pure” solids, the luminescence of which may be modified by the addition of manganese will be discussed later. Some activators of luminescence in solids, such as chromium or samarium, can easily be recognized in the fluorescence emission on account of their sharp line spectra. Many of the more interesting luminescent solids are, however, characterized by diffuse emission spectra, and it is of considerable importance to determine to what degree the luminescence is a function of the added impurity, and to what extent the luminescence depends on the parent lattice. In this paper we deal from this point of view with manganese which is a specially suitable impurity, as it induces fluorescence in a wide variety of compounds; this may be seen from the list given in Table I, which is not exhaustive. It is of interest to note that four of these compounds, viz. zinc orthosilicate, zinc beryllium silicate, zinc mesodisilicate, and cadmium chloro-phosphate are amongst the technically most important fluorescent solids at the present time.