Optical absorption in dielectric crystals

When a defect is introduced into a lattice it modifies the lattice vibrations in two ways. First there is a modification of the amplitude of the vibrations as a function of frequency. This is greatest in the immediate vicinity of the defect and negligible at large distances from it. The second is that new frequencies of vibration may appear. These are the localised modes. If the impurity has a much lighter mass than any of the constituent atoms of the lattice the frequency of this vibration is considerably higher than any of the lattice frequencies and the amplitude of the mode falls off rapidly away from the defect. The resultant mode is well localised and is almost solely confined to the defect atom itself.
Localised modes of this type are a subject of investigation in this thesis. The particular system studied was that of substitutional hydride and deuteride ions in the alkaline earth fluorides. The localised vibrations of these defects were detected as absorption lines in the infrared spectra of these crystals. The fundamental, second harmonic and a third harmonic doublet were measured for both the hydrogen and deuterium impurities. The lines display a marked temperature dependence and this was measured in the temperature range 4°K to 300°K.
Mr. C. T. Sennett of this laboratory has developed a theory appropriate for localised vibrations of light impurity atoms in lattices and this was used to interpret the observations. The existence of harmonic lines indicated that the light atom is present as a negative ion at a tetrahedral fluorine site. The deviation of the harmonic lines from exact harmonicity and the temperature shift of the peak position and width are attributed to anharmonic effects. The experimental data was sufficient to determine the magnitude of these anharmonic forces to terms quartic in the light ions displacement.
Satellite lines are present around the main vibration lines and these are ascribed to perturbations of the light impurity ion by nearby lattice defects and, in the case of the more distant lines, to light impurity ions present in lattice sites of different symmetry. The light impurity ion thus can be used as a probe to investigate other defects in the fluorite lattice.
Broad sidebands occur equally spaced in frequency about the main line. These bands are interpreted as multiphonon combination bands arising from the simultaneous excitation or decay of lattice phonons with excitation of the local mode phonon. The frequencies of these bands were used to determine the frequencies of peaks in the density of states of various branches of the dispersion curves of the three alkaline earth fluorides. These impurity induced lattice bands are experimentally more accessible than the pure lattice combination bands and thus offer a simpler method of determining dispersion data for these crystals.
Nickel and cobalt ions can be introduced into lithium fluoride crystals in sufficient concentration for their optical absorption to to be detectable. These ions substitute for the lithium and are thus octahedrally surrounded by six fluorine ligands. The measured spectra were interpreted on the basis of the crystal field theory of Tanabe and Sugano in the cubic field approximation. Spin-orbit coupling effects were included in the analysis.
Room temperature X-irradiation of pure lithium fluoride crystals results in the formation of intrinsic lattice defects of which the F and M centres are the most well established. The F centre consists of an electron bound to a negative ion vacancy while the M centre is believed to be a pair of F centres. Interesting phenomena are observed after irradiation of lithium fluoride crystals containing nickel and cobalt. The nickel impurity strongly suppresses the formation of both the F and M centres while the cobalt impurity has little effect. The specific effect of the nickel impurity is related to the strong electron trapping properties of this ion. This interpretation is consistent with earlier electron spin resonance measurements on the irradiated crystals.
While investigating the irradiation behaviour of pure and iron group impurity doped lithium and sodium fluoride crystals an absorption due to a V centre of the molecular F2− ion type was observed. This centre has been shown by Känzig (by electron spin resonance methods) to consist of an isolated hole located on two neighbouring fluorine ions. The optical absorption of this centre in the two lattices differ in some respects. The frequencies are similar (28700 cm−1 for LiF and 27300 cm−1 for NaF), but the disorientation temperatures (at which the centres are just mobile) differ, being 114°K for LiF and 155°K for NaF.
Several transition metal compounds exhibit antiferromagnetic ordering phenomena at low temperatures. These affect the frequency of optical absorption lines slightly and the onset of the ordering may be studied t in suitable cases, by the temperature dependence of the frequency of sharp optical absorption lines. Experiments of this type performed on MnF2, KMnF3, and KNiF3 revealed a marked frequency shift of sharp absorption lines in the spectra of these substances near the Néel temperatures. The magnitude of such shifts is discussed.
The optical absorption of trivalent rare earth ions are characterised by the presence of groups of sharp weak lines which arise from transitions within the 4fn configuration. In a suitable host lattice the rare earth ions may be present in cubic symmetry sites and the structure of the line groups is then readily amenable to analysis.
The optical absorption of holmium in calcium fluoride was investigated as a typical example of such systems. The line group positions were interpreted using the electrostatic and spin-orbit matrices for the f10 configuration. The structure of the line groups indicated that the holmium ions were not in cubic symmetry sites, but were perturbed by the presence of adjacent charge compensating defects, and an analysis of this structure was not attempted.
The occurrence of the holmium in noncubic sites is consistent with the electron spin resonance observations which show that only 2% of trivalent holmium ions, namely those on cubic sites, undergo conversion into the divalent form.
An experimental study of various ferrous iron compounds revealed a large (approximately 1500 cm−1) splitting of the broad absorption band near 10000 cm−1 in all the compounds. This band arises from the 5T2(5D) → 5E(5D) transition of the 3d6 configuration. This splitting cannot be due to spin-orbit coupling effects or, in the case of the cubic crystals, to low symmetry fields. It was attributed to a dynamic Jahn Teller distortion of the upper Eg level. The magnitude of such splitting is discussed on this model.
In analogy to the molecular F2− ion V centre of the alkali halides, a centre of similar type occurs in the alkaline earth fluorides. This was detected by J. W. Twidell of this laboratory using electron spin resonance methods. This centre possesses <100> symmetry in contrast to the <110> symmetry shown by the alkali halide centres and this is due to the different symmetries of the two lattices. An optical absorption band present at 28000 cm−1 in irradiated crystals of calcium fluoride doped with thulium is tentatively assigned to these centres.