A calorimeter was constructed for the determination of heat capacities of solids from 1.5 to 84 K. Results from this and other calorimeters are discussed, on bis(adiponitrile)copper(I) nitrate and tetramethylammonium trichloromanganate(II) ("TMMC") from 1.5 to 300 K, and on β-modification metal-free, copper(II), and nickel(II) phthalocyanines from under 5 to 80 K. Bis(adiponitrile)copper(I) nitrate has not-very-pronounced heat capacity maxima at 51 and 63 K. The total molar anomalous entropy change is estimated as between 1/2 R1n2 and R1n2. A previous assertion by X-ray crystallographers (Bull.Chem.Soc. Japan, <strong>32</strong>, 1221 (1959)) that the nitrate ions are disordered among four orientations at room temperature is not supported by these results or by consideration of structure factors or of the potential field on the nitrate ions. The TMMC heat capacity results are consistent with those given in Solid State Comm., <strong>15</strong>, 1185 (1974) and Phys.Rev. B, <strong>12</strong>, 5858 (1975). The anomalous molar entropy change associated with the monoclinic-hexagonal transition at 126 K is estimated as R1n2.8, of which 1/3 occurs between 124 and 127 K; it is suggested that the NC₄ tetrahedra undergo twofold disordering and that the H atoms also undergo some disordering. The heat capacities of the copper and nickel phthalocyanines are surprisingly different from each other, especially below 25K, where the former is markedly lower except for two first-order transitions at about 9-13K with a total molar entropy change of not less than approximately R1n4. Magnetic and Schottky anomalies are, apparently, not involved in either material; perhaps the transitions in copper phthalocyanine involve disordering of the copper ions among four positions in the plane of each molecule. Below 8 K, the heat capacities of copper and metal-free (but not nickel) phthalocyanine apparently obey the T³ law.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by James David Hodge. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Includes bibliographical references. / by Gretchen Lynn Kalonji. / Ph.D.

In order to understand better the defect structure and dynamics associated with lower valent dopants complexed with native defects in group IV oxides, In/Cd perturbed angular correlation spectroscopy was performed in ceria and zirconia. Examining the orientation symmetry axis of defects in ceria single crystals at low temperature has allowed the identification of a cadmium with a bound near-neighbor oxygen-vacancy complex as well as a complex involving a cadmium with two opposing, near-neighbor oxygen vacancies. The orientation of the symmetry axis of a third complex is reported; however, this information is not sufficient to identify it. Complementing these low temperature studies, the dynamics of the cadmium/oxygen-vacancy interaction in zirconia at high temperatures was studied. The motion of the oxygen vacancy at high temperatures results in a damping of the PAC signal. This damping is not well characterized by the heuristic Marshall-Meares PAC fitting function, and a model is proposed to fit the data in terms of three physical parameters associated with the vacancy's motion. These parameters are the rate at which a bound oxygen vacancy hops among equivalent sites about the probe, the rate at which a bound vacancy detraps, and the rate at which a vacancy is trapped by cadmium. Fits of individual spectra using this model give respective activation energies of 0.3-0.6 eV, 0.9-1.6 eV, and 0.4-0.6 eV. The uncertainty in these energies can most likely be reduced by fitting spectra from multiple temperatures simultaneously. Despite the large uncertainty in the fitted energies, the values are physically reasonable and indicate that the model adequately describes the motion of the oxygen vacancy about cadmium. / Graduation date: 1997

published_or_final_version / abstract / toc / Physics / Doctoral / Doctor of Philosophy

We examine the problem of the dynamics of a hexagonal close packed crystal with a single substitutional impurity. The effects of the mass and the force constant changes due to the introduction of the impurity atom are taken into account assuming nearest neighbor interactions under the harmonic approximation. Using Green’s function and group theoretical methods, the equations of motion for the perturbed normal modes are obtained in an exact form. The calculations are performed by allowing for very general force constant changes which can have noncentral as well as central contributions. The analytical expressions obtained expressions obtained by Mannheim and Cohen for cubic systems.

No description available.

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Orientadores: Silvio Antonio Sachetto Vitiello, Maurice de Koning / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-16T14:29:42Z (GMT). No. of bitstreams: 1 Vale_RenatoPessoa_D.pdf: 5594892 bytes, checksum: 6cd9dc97441dc729a416210efb11ec2d (MD5) Previous issue date: 2010 / Resumo: Neste trabalho apresentamos um estudo feito sobre defeitos em um sólido quântico formado por átomos de 4He. Embora ainda não exista um mecanismo para explicar a fase supersólida deste sistema, ela tem sido frequentemente associada com a presença de desordem no cristal, que pode ocorrer com a existência de algum tipo de defeito. O método do trabalho reversível é aplicado para calcular a concentração de vacâncias e a energia livre de ligação entre estes defeitos pontuais. Inicialmente, esta metodologia é aplicada a um sólido quântico descrito por uma função de onda tipo Jastrow, para em seguida ser aplicada no nosso sistema de interesse. A função de onda sombra é utilizada para modelar o 4He sólido hcp, cujas configurações são amostradas através do método de Monte Carlo utilizando o algoritmo de Metropolis. Além da determinação das concentrações de monovacâncias e divacâncias, nossos resultados indicam que não existe uma tendência de se formar aglomerados deste defeito, que poderia levar a uma separação de fases. Posteriormente, utilizamos o método de Peierls-Nabarro para estudar defeitos lineares do tipo discordâncias. Para isto, determinamos as constantes elásticas do material, sendo esta a primeira estimativa teórica destas propriedades para o hélio sólido. Nosso modelo indica que estes defeitos lineares possuem uma tendência em se separar em pares de discordâncias parciais. Além disso, a resistência intrínseca da rede cristalina desempenha um papel importante na mobilidade destes defeitos. Portanto, um mecanismo para explicar a fase supersólida do 4He, que envolva a presença de discordâncias, deve levar em conta esta resistência / Abstract: In this work we present a study about defects in a quantum solid formed by atoms of 4He. Although there is no mechanism to explain the supersolid phase of this system, it has often been associated with the presence of disorder in the crystal, which can occur with the presence of some type of defect. The reversible work method is applied to calculate the concentration of the vacancies and the binding free energy between these point defects. Initially, this methodology is applied to a quantum solid described by a Jastrow wave function, to then be applied in our system of interest. A shadow wave function is used to model the hcp solid helium, whose con gurations are sampled by the Monte Carlo method using the Metropolis algorithm. Besides the determination of monovacancy and divacancy concentrations, our results indicate that there is not a tendency to form clusters of these defects, which could lead to a phase separation. Subsequently, we used the Peierls-Nabarro method to study linear defects like dislocations. With this purpose, we determined the elastic constants of material, this being the rst theoretical estimates of these properties for solid helium. Our model indicates that these linear defects have a tendency to separate into pairs of partial dislocations. Furthermore, the intrinsic resistance of the crystal lattice plays an important role in the mobility of these defects. Therefore, a mechanism to explain the supersolid phase of helium, which involves the presence of dislocations, should take account of this resistance / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Nematic liquid crystals are mesogenic materials that are popular working materials for optical displays. There has been an increased interest in bistable liquid crystal devices which support two optically distinct stable equilibria. These devices typically exploit a complex geometry or anchoring conditions, which often induces defects in the equilibria. There remains a great deal to be understood about the structure of the defects and how they stabilize multiple equilibria in modern devices. This thesis focuses on four problems: the first three explore the effect of confinement and defects on nematic equilibria in simple geometries, with the aim of exploring multistability in these geometries; the fourth problem concerns the fine structure of point defects, essential for future modelling of nematic equilibria in more complex geometries. Firstly, we study nematic liquid crystals confined to two-dimensional rectangular wells using the Oseen-Frank theory. Secondly, we study equilibria within a semi-infinite rectangular domain with weak tangential anchoring on the surfaces. Thirdly, we study nematic equilibria within two-dimensional annuli. We derive explicit expressions for the director fields and free energies of equilibria within these geometries and discuss the stability of the predicted states. These three problems are motivated by the experimental work on colloidal nematic liquid crystals, which we interpret in the context of our results. Finally, we study the fine structure and stability of the radial hedgehog defect in the Landau-de Gennes theory with a sixth order bulk potential, relevant to the observability of global biaxial phases in a model with higher order potential terms.