In this thesis the Mossbauer effect in metallic iron has been studied as a function of the length of time of diffusion of the Co⁵⁷ source into metallic iron, of the source temperature over the range 156°K to 478°K, and of the absorber thickness. In each case, the line shape, width, intensity and shifts were measured. Measurements showed that the appearance of an appreciable Mossbauer effect, .122, arises within the first ten minutes of diffusion time at 900°C in a hydrogen atmosphere and that additional time is required to reduce the line width and to increase the intensity to .145. The observed temperature shift of the resonantly absorbed 14.4 kev radiation followed that which was predicted by the Josephson effect. However, corrections for an isomer and a hydrostatic compression shift were made to the data before comparing the measured shift with the theoretical shift. The results indicated a Debye temperature of Θ[subscript D] = (420±20)°K for both the source and absorber used. The measurements made over the temperature range indicated that the internal magnetic field H̰ followed the saturation magnetization curve (the Weiss Law) closely. For small temperature differences between the source and absorber, ΔΘ < 50°K, measurements were made indicating that the minimum line width occurred at ΔΘ = 24°K. These results indicated that the internal magnetic field at ΔΘ = 0°K for the source was .01x10⁵oe greater than that of the absorber. For ΔΘ ≥ 100°K, the Mossbauer line displayed a hyperfine structure arising from the temperature dependent difference in the internal magnetic field at the nuclei. The line profile, width, shift and intensity were measured for four absorber thicknesses - .0002", .00035", .00055", and .001". The detailed comparison of these line characteristics with the theoretical values required an extension of existing treatments, a discussion of which is given in chapter five. In each case it was observed that the Mossbauer line was accompanied by two small peaks, one on either side of the main line. The position of these peaks indicated that they were associated with a small or zero internal magnetic field at the site of some of the Fe⁵⁷ atoms. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Precision measurements of the Mössbauer fraction of the 14.4 kev Fe⁵⁷ transition were made using the black, wide absorber technique. The Fe⁵⁷ was diffused, as a dilute Mössbauer impurity, into 99.995% purity single crystal chips of Pt, Pd and Cu. By comparing the Debye form of -ln f, corrected for anharmonicity, with our data, we extracted a high and low temperature Debye theta, as well as an anharmonicity coefficient, є, for the three materials. We estimated the accuracy of our data to be ± 1%. From the data we concluded that the Fe⁵⁷ in Cu has stronger force constants and is more anharmonic than pure Cu, while Fe⁵⁷ in Pt and Pd has smaller force constants and about the same anharmonicity as the pure host materials.

No description available.

This dissertation is concerned with the incorporation of coincidence methods into standard Mössbauer techniques and the application of the Delayed Coincidence Mössbauer Effect (DCME) to the study of certain solid state phenomena. This technique involves the accumulation of a Mössbauer spectrum which reflects the environment of the decaying nuclei during some preset time interval following the formation of the Mössbauer state. Changing the preset time permits display of the time evolution of the nuclear environment. The theoretical analysis developed in this study involves a numerical integration of the exact expression for time dependent Mössbauer absorption over the experimental window in time and is shown here to be successful in matching experimental data for single and multi-lined spectra while previous analytical techniques have been unsuccessful. In systems where no time dependent solid state effects are present we have demonstrated experimentally and fit theoretically the expected line narrowing at long delay times and have applied this effect to enhance energy resolution in absorber experiments. Studies of time dependent solid state effects reported here involve aftereffects and various relaxation processes. The aftereffects problem pertains to the formation of highly charged ions following K-capture and an Auger shower, and also the local heating of the lattice associated with the relatively energetic events in the formation of the Mössbauer state. We investigated the time dependence of the ionic charge and the local heating, reflected in the recoilless fraction, as these excited configurations relaxed to equilibrium. Previous estimates of the characteristic time for the charge state decay to equilibrium in an insulator such as CoO were about 10^-8 sec. Our experiments in MgC show that this time must be greater than 2 x 10- 6 or that high charge states are stabilized in the lattice. There are no theoretical models or predictions concerning this characteristic time or that of the local heating decay. We show in this study that the decay to equilibrium in the latter case, using a single line source and absorber, takes place with a characteristic time of less than 10^-9 sec. The DCME is also applied to the study of ionic spin and electric-field-gradient (Jahn-Teller effect) relaxations. No relaxation effects were observed in sources of Co^57 in Ti0_2 whille the parallel absorber experiment has shown such an effect. Calculations done here show that the applicability of the DCHE method to study of dynamical-Jahn-Teller effect in MgO is doubtful due to the extremely small quadrupole splitting observed.

Precision measurements of the Mossbauer fraction of Fe57 in chromium were made using the black absorber technique over a range of temperatures from 78K to 792K. Because of uncertainty in corrections at high temperatures, only the data collected from 78K to 600K was used in analyzing the results. The f values were analyzed in terms of a Debye approximmation. This analysis of the data yielded a theoretical fit which was based on two adjustable parameters (a Debye temperature of 438K and an anharmonicity parameter of 2 x 10-4K-1). The f measurements were also analyzed using a theory proposed by Mannheim in 1968. Two different phonon density of states functions (Feldman, Muhlestein) were applied to Mannheim’s theory, and the resulting theoretical fits to experimental values showed a decrease in the force constant as a result of introducing an impurity into the chromium. Feldman’s phonon spectrum yielded a force constant ratio of 1.30±.07 and an anharmoncity parameter of ℇ=2.5(±.5) x 10-4K-1.

O ferro é conhecido como um causador de \"quenching\" de luminescência em vidros e cristais. Nos vidros boratos irradiados à baixa temperatura ocorre a formação de centros paramagnéticos que são conhecidos como o centro de elétron do boro (BEC) e o centro de lacuna do boro e oxigênio (BOHC), sendo os primeiros os doadores de elétrons e os últimos, centros de luminescência azulada que ocorre a temperaturas entre 77 e 300 K. Estudos recentes usando TL mostraram que a ocorrência de uma luminescência avermelhada em tomo de 200ºC é produzida pelas recombinações radiativas de lacunas temúcamente liberadas dos BOHCs com íons de Fe POT. +2 e que esta luminescência desaparece completamente quando se adiciona à estrutura do vidro concentrações de 0,35 atg% de Fe à composição do vidro. Investigamos o comportamento do BOHC em vidros de composições (x)MO (100-x) B IND. 2 O IND. 3 20 Al IND. 2 O IND. 3 + (y) Fe IND 2 O IND. 3, com 20 x 50 e 0 y 2,5 mol% para M = Ba e 10 x 50 e 0 y 1,75 mol% para M = Ca fundidos a 1300 c durante uma hora em forno elétrico e em atmosfera normal, utilizando RPE e TL e verificamos que a extinção da termoluminescência coincide com a completa extinção técnica do BOHC. O comportamento da valência e da coordenação dos íons de ferro determinado pela composição dos vidros foi efetuado utilizando Espectroscopia Mössbauer e absorção óptica. Observamos que em todas as composições analisadas o íon Fe POT. +3 aparece essencialmente na configuração tetraédrica, enquanto que o íon Fe POT. +2 apresenta-se tanto em sítios tetraédricos e octaédricos. A presença de algumas unidades estruturais específicas foi constatada utilizando espectroscopia Raman, verificando-se que para baixas concentrações de MO os vidros são formados principalmente por unidades de anéis boroxol. À medida que a concentração dos óxidos modificadores é aumentada, estas unidades são gradualmente substituídas por diversos tipos de grupos estruturais. / The iron is a well known quenching agent of luminescence in glasses and crystals. The irradiation of borate glasses at low temperatures produces paramagnetic centers known as boron electron centers (BEC) and boron-oxygen hole centers (BOHC), the former being electron donnors and the latter the recombination centers of the blue luminescence which occurs at temperatures in the range of ~77 K. Recent TL studies have show the occurrence of a red luminescence at temperatures about 470 K, produced by the radiative recombinations of boles thermally released from the BOHCs and Fe +l ions. This luminescence is completely quenched with the addition of 0,35 atg% of Fe to the glass composition. We have investigated the BOHC behavior in glasses of composition (x)M0(100-x) B IND 2 O IND 3 20Al IND. 2 O IND 3 + (y) Fe IND 2 O IND. 3, with 20 x 50 and 0 y 50 and 2.5 mol% for M=Ba and 10 x 50 and 0 y l.75 mol% for M=Ca, melted at the temperature of 1300 \'C for one hour in electric fumace at normal atmosphere, using electron paramagnetic resonance (EPR) and TL to observe that the \'IL vanisshes at the same temperature of the complete thermal bleaching of BOHC. The behavior of the oxidation state and coordination of the iron ions as determined by the glass composition was carried out by means of Mössbauer spectroscopy and optical absorption at the near infrared. We observed that in all the compositions analysed the Fe +2 ion appears essentially in tetrahedral configuration whereas the Fe+2 are found in both tetrahedral and octahedral sites. The presence of some specific structural units was detected by means of Raman spectroscopy with the observation that for low concentrations of MO the glasses are mainly formed by boroxol ring units. As the modifier cation concentration is increased, these units are gradually substituted by several types of specific structural units.

Investigamos o fenômeno da segregação de fases em ligas ferro-níquel de estrutura cúbica de faces centradas, na região do invar (~35 a 50% Ni ). As amostras foram preparadas pela decomposição térmica de oxalatos mistos Fe1-x Ni xC2O4.2H20, a temperaturas selecionadas; este método de preparação assegura que o equilíbrio termodinâmico é atingido muito mais rapidamente que pelos métodos convencionais. As amostras foram examinadas por espectroscopia Mössbauer; evidências anteriores mostraram que as duas f ases resultantes da segregação diferem notavelmente em suas propriedades magnéticas, mas são idênticas do ponto de vista estrutural, o que impossibilita a sua distinção pelas técnicas usuais de raios X. Amostras foram preparadas com concentrações entre 36 e 50% Ni e a temperaturas entre 350 e 600°C. O aparecimento de uma linha central no espectro Mössbauer foi usado como critério para a ocorrência de segregação. Dessa forma, mapeamos a fronteira do \"gap\" de miscibilidade nessa região do diagrama de fases do Fe-Ni. / We have investigated the phenomenon of phase segregation in iron-nickel alloys with a face-centered cubic structure, in the invar region (~35 to 50% Ni ). The samples were prepared by thermal decomposition of mixed oxalates, Fe1-x Ni xC2O4.2H20, at selected temperatures. This method ensures that thermodynamic equilibrium is reached much more rapidly than by conventional methods. The samples were examined by means of Mössbauer spectroscopy; previous evidence has shown that the two phases resulting from segregation differ markedly in their magnetic properties but are structurally identical, which renders their distinction impossible by usual X-ray techniques. Samples were prepared at concentrations between 36 and 50% Ni and at temperatures ranging from 350 to 600°C. The appearance of a central line in the Mössbauer spectrum was taken as the criterion for segregation occurrence. We thus mapped the miscibility gap boundary in this region of the Fe-Ni phase diagram.

Investigamos o fenômeno da segregação de fases em ligas ferro-níquel de estrutura cúbica de faces centradas, na região do invar (~35 a 50% Ni ). As amostras foram preparadas pela decomposição térmica de oxalatos mistos Fe1-x Ni xC2O4.2H20, a temperaturas selecionadas; este método de preparação assegura que o equilíbrio termodinâmico é atingido muito mais rapidamente que pelos métodos convencionais. As amostras foram examinadas por espectroscopia Mössbauer; evidências anteriores mostraram que as duas f ases resultantes da segregação diferem notavelmente em suas propriedades magnéticas, mas são idênticas do ponto de vista estrutural, o que impossibilita a sua distinção pelas técnicas usuais de raios X. Amostras foram preparadas com concentrações entre 36 e 50% Ni e a temperaturas entre 350 e 600°C. O aparecimento de uma linha central no espectro Mössbauer foi usado como critério para a ocorrência de segregação. Dessa forma, mapeamos a fronteira do \"gap\" de miscibilidade nessa região do diagrama de fases do Fe-Ni. / We have investigated the phenomenon of phase segregation in iron-nickel alloys with a face-centered cubic structure, in the invar region (~35 to 50% Ni ). The samples were prepared by thermal decomposition of mixed oxalates, Fe1-x Ni xC2O4.2H20, at selected temperatures. This method ensures that thermodynamic equilibrium is reached much more rapidly than by conventional methods. The samples were examined by means of Mössbauer spectroscopy; previous evidence has shown that the two phases resulting from segregation differ markedly in their magnetic properties but are structurally identical, which renders their distinction impossible by usual X-ray techniques. Samples were prepared at concentrations between 36 and 50% Ni and at temperatures ranging from 350 to 600°C. The appearance of a central line in the Mössbauer spectrum was taken as the criterion for segregation occurrence. We thus mapped the miscibility gap boundary in this region of the Fe-Ni phase diagram.

O ferro é conhecido como um causador de \"quenching\" de luminescência em vidros e cristais. Nos vidros boratos irradiados à baixa temperatura ocorre a formação de centros paramagnéticos que são conhecidos como o centro de elétron do boro (BEC) e o centro de lacuna do boro e oxigênio (BOHC), sendo os primeiros os doadores de elétrons e os últimos, centros de luminescência azulada que ocorre a temperaturas entre 77 e 300 K. Estudos recentes usando TL mostraram que a ocorrência de uma luminescência avermelhada em tomo de 200ºC é produzida pelas recombinações radiativas de lacunas temúcamente liberadas dos BOHCs com íons de Fe POT. +2 e que esta luminescência desaparece completamente quando se adiciona à estrutura do vidro concentrações de 0,35 atg% de Fe à composição do vidro. Investigamos o comportamento do BOHC em vidros de composições (x)MO (100-x) B IND. 2 O IND. 3 20 Al IND. 2 O IND. 3 + (y) Fe IND 2 O IND. 3, com 20 x 50 e 0 y 2,5 mol% para M = Ba e 10 x 50 e 0 y 1,75 mol% para M = Ca fundidos a 1300 c durante uma hora em forno elétrico e em atmosfera normal, utilizando RPE e TL e verificamos que a extinção da termoluminescência coincide com a completa extinção técnica do BOHC. O comportamento da valência e da coordenação dos íons de ferro determinado pela composição dos vidros foi efetuado utilizando Espectroscopia Mössbauer e absorção óptica. Observamos que em todas as composições analisadas o íon Fe POT. +3 aparece essencialmente na configuração tetraédrica, enquanto que o íon Fe POT. +2 apresenta-se tanto em sítios tetraédricos e octaédricos. A presença de algumas unidades estruturais específicas foi constatada utilizando espectroscopia Raman, verificando-se que para baixas concentrações de MO os vidros são formados principalmente por unidades de anéis boroxol. À medida que a concentração dos óxidos modificadores é aumentada, estas unidades são gradualmente substituídas por diversos tipos de grupos estruturais. / The iron is a well known quenching agent of luminescence in glasses and crystals. The irradiation of borate glasses at low temperatures produces paramagnetic centers known as boron electron centers (BEC) and boron-oxygen hole centers (BOHC), the former being electron donnors and the latter the recombination centers of the blue luminescence which occurs at temperatures in the range of ~77 K. Recent TL studies have show the occurrence of a red luminescence at temperatures about 470 K, produced by the radiative recombinations of boles thermally released from the BOHCs and Fe +l ions. This luminescence is completely quenched with the addition of 0,35 atg% of Fe to the glass composition. We have investigated the BOHC behavior in glasses of composition (x)M0(100-x) B IND 2 O IND 3 20Al IND. 2 O IND 3 + (y) Fe IND 2 O IND. 3, with 20 x 50 and 0 y 50 and 2.5 mol% for M=Ba and 10 x 50 and 0 y l.75 mol% for M=Ca, melted at the temperature of 1300 \'C for one hour in electric fumace at normal atmosphere, using electron paramagnetic resonance (EPR) and TL to observe that the \'IL vanisshes at the same temperature of the complete thermal bleaching of BOHC. The behavior of the oxidation state and coordination of the iron ions as determined by the glass composition was carried out by means of Mössbauer spectroscopy and optical absorption at the near infrared. We observed that in all the compositions analysed the Fe +2 ion appears essentially in tetrahedral configuration whereas the Fe+2 are found in both tetrahedral and octahedral sites. The presence of some specific structural units was detected by means of Raman spectroscopy with the observation that for low concentrations of MO the glasses are mainly formed by boroxol ring units. As the modifier cation concentration is increased, these units are gradually substituted by several types of specific structural units.

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Bibliography: leaves 80-84. / M.S.