We have investigated several topics in the area of manganites including oxygen disorder, grain boundaries, low field magnetoresistance, magnetoresistive anisotropy and magnetic properties. Studied materials were in the form of polycrystalline samples and epitaxial thin films. The studied compounds were Sm(1-x)Sr(x)MnO3 (SSMO) and La(1-x)Ca(x)MnO3 (LCMO). 1-We have studied the effects of oxygen disorder and grain boundary disorder in the SSMO system close to half hole doping level. The temperature dependencies of resistivity and magnetoresistance were measured as a function of the vacuum annealing time. We observed a logarithmic increase of the resistivity as a function of vacuum annealing time. We have shown that an increasing grain boundary disorder softens the magnetic phase transition from a first order phase transition into a second order transition. Furthermore, the peaks in the resistivity and specific heat are broadened and there is an increase in the charge-carrier scattering rates in the metallic state. On the other hand, the polaronic hopping activation energies in the insulating state changed slightly as a function of grain boundary disorder. The origin of these phenomena is discussed. Magnetoresistive anisotropy has been studied as a function of the grain size. Results showed a strong grain size dependence of anisotropic electrical transport in granular samples of manganites. 2-We investigated the anisotropic magnetoresistance (AMR) in ultrathin LCMO films grown on various substrates. It was found that depending on the strain state, the AMR in some of these systems exceeds 100% and can even change sign. These changes are dramatic when compared to the few percent change in AMR in conventional ferromagnets. The mechanism behind these changes in the AMR is discussed. We have also studied the effects of strain on resistive peak broadening with a simple percolation model. We have shown that strain associated with a lattice mismatched substrate in thin films can cause new electronic behavior, not found in bulk materials or thicker films of the same chemical composition. Resistivity of the ultra thin films exhibit strong relaxation effects when measured as a function of time in a constant magnetic field.

In the practice of solid state chemistry, structural phase transitions are fairly common events. Nonetheless, their understanding, in terms of both: A rationalization of the observed changes in symmetry pattern and; An understanding of the mechanisms allowing for a particular transformation, are outstanding problems. The thermodynamic classification of phase transitions distinguishes between first and second order transitions, on the basis of the discontinuous behavior of quantities related to first or second derivatives of the free energy, respectively. Small atomic displacements are typically associated with second order phase transitions, and latent heat changes amount to a few calories per gram only. Additionally, the symmetries of the phases surrounding the transition are typically in the relation of a group and a subgroup. Reconstructive phase transitions, on the contrary, involve breaking of (large) parts of the bond scaffolding of the initial structure, and exhibit drastic changes at the transition point, with large latent heat and hysteresis effects. The corresponding atomic displacements can be in the order of the lattice parameters, and no group-subgroup is found, between the symmetry of the phases. These type of transitions have generally a strong first-order character. Landau theory accounts for continuous, second-order phase transitions. As a phenomenological theory, it does not establish the existence of a phase transition, which remains an experimental fact. It only bridges microscopic characteristics, like space-group symmetries and structural changes, or phonon softening effects, with measurable macroscopic quantities. Therein, distortions are carried by an order parameter, which fully specifies the form of the analytical variational free energy. The latter is continuous and derivable with respect to temperature, pressure and atomic displacement, at the transition point. First order, non-continuous phase transitions are still within the scope of Landau theory in the mentioned special case of the existence of a group-to-(isotropic) subgroup relationship. In the majority of cases, however, and for the most interesting phase transitions (for basic and applied research), such a relationship is missing, making the choice of an order parameter less straightforward. Most of the allotropic transformations of the elements, many intermetallic systems, and numerous insulating systems belongs to this class. This class also includes most interesting and fundamental electronic effects, like metallization in perovskites or spinel oxides for example. This very simple fact of a missing symmetry condition has helped reinforcing the opinion of first-order phase transitions being a world apart, and possibly contributed to discouraging a firm theory to develop, able to account for their transformation mechanisms and the change of physical properties across phase transition. The thermodynamic distinction between first and second order phase transitions is too narrow, as, in case of first order phase transitions, it embraces both weakly discontinuous transition and reconstructive ones, where bonds are being strongly modified. Especially, a mean to qualify the distance between two structures (geometric, with respect to symmetry, a.s.o.), is missing. Clearly, a group-subgroup relationship may, and typically does imply shortest shifting distances, as a tiny atomic displacement can already do for a symmetry lowering. Naively, missing such a relation means no constraints, and apparently no means to conclude at a connection of two structures in general, let alone a full mechanistic analysis. First order phase transitions proceed by nucleation and subsequent growth of the new phase from the initial one. Different from (second-order) continuous phase transitions, they do imply coexistence of the transforming motifs. The discontinuity in some order parameter between the two phases is driven by lowering of the free energy as the new phase forms. However, close to the transition, the original phase remains metastable, and a fluctuation is needed to cause the formation of the new phase to set in. Such a process responds to thermal changes, and depending on the height of the nucleation barrier, its rate may be slower or faster. In the former case, large deviations from equilibrium may be required to achieve transformation to the stable phase, which means that large hysteresis effects will be observed in the course of transformation. The intent of this work consists in giving a face to the intermediate configurations appearing in first order phase transitions, in solid-solid reconstructive processes. Apart of a mechanistic elucidation, consisting in answering the question “Which atomic displacements bring structural motif A into structural motif B ?”, another purpose of this work is a rather pedagogical one, that is, showing that first-order phase transitions can be understood in detail, not only in principle but in fact. The core of the examples illustrated in this work is concerned with phase transformations where pressure represents the thermodynamic controlling parameter. Pressure is extensively used in chemical synthesis, as a mean to achieve novel properties, optical or mechanical just to mention a few. Additionally, reports on novel high-pressure polymorphs are regularly appearing. In this sense, pressure is a relevant parameter for approaching fundamental questions in solid state chemistry.

The magnetocaloric and thermomagnetic properties of the Ni1-xCrxMnGe1.05 (for x = 0, 0.035, 0.070, 0.105, 0.110, 0.115, and 0.120) system have been studied by X-ray diffraction, differential scanning calorimetry (DSC), resistivity and magnetization measurements. A change in crystal structure from orthorhombic to hexagonal was observed in the XRD data with an increase in chromium concentrations. The values of the cell parameters and volume of the unit cell for hexagonal phase were determined. It was found that the partial substitution of Cr for Ni in Ni1-xCrxMnGe1.05 results in a first order magnetostructural transition from antiferromagnetic to ferromagnetic (FM) at TM of about132 K, 100 K, and 110 K for x= 0.105, 0.115, and 0.120, respectively. A FM to paramagnetic second order transition has been observed at TC around 200K. A magnetic entropy change of = 4.5 J/kg K, 5.6 J/Kg K, and 5.06 J/Kg K was observed in the vicinity of TC for x = 0.105, 0.115, and 0.120 respectively at ΔH = 5T. The values of the latent heat and corresponding total entropy changes have been determined from Differential Scanning Calorimetry (DSC) measurements. Magnetoresistance values of about -5% were measured near TC for x =0.105. The maximum value of refrigeration capacity (RC) and relative cooling power (RCP) was found to be 155 J/Kg and 175 J/Kg respectively for x = 0.120. A concentration-dependent (T-x) phase diagram of transition temperatures has been constructed using the magnetic and DSC data. The structural, magnetic and magnetocaloric properties of GdNi2Mnx system (for x = 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5) have been studied by x-ray diffraction and magnetization measurements. A mixture of the Laves phase C15 and a phase with rhombohedral structure PuNi3- type (space group R m) was observed in the XRD data. A second order magnetic phase transition from ferromagnetic (FM) to paramagnetic (PM) was found, characterized by a long-range exchange interaction as predicted by mean field theory. The maximum value of magnetic entropy changes, -∆SM, near TC for ∆H = 5T, was found to be 3.1 J/KgK, 2.8 J/KgK, 2.9 J/KgK, and 2.5 J/Kg K for x = 0.8, 1.2, 1.4, and 1.5 respectively. In spite of the low values of ΔSM, the RC and RCP value was found to be 176 J/Kg and 220 J/Kg for the GdNi2Mn0.8 compound, respectively.

xiii, 110 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation investigates the interplay between, and the possible coexistence of, magnetic and superconducting order in metals. We start with studying the electromagnetic properties of s-wave superconductors near a ferromagnetic instability. By using a generalized Ginzburg-Landau theory and scaling arguments, we show that competition between magnetic order and superconducting order can change the scaling of observables. For instance, the exponent for the temperature dependence of the critical current can deviate from the Ginzburg-Landau value of 3/2. These results may be relevant to understanding the observed behavior of MgCNi 3 . We then study the nature of the superconductor-to-normal-metal transition in p-wave superconductors. Although the phase transition is continuous at a mean- field level, a more careful renormalization-group analysis in conjunction with large-n expansion techniques strongly suggest that the transition is first order. This conclusion is the same as for s-wave superconductors, where these techniques also predict a first-order transition. In p-wave superconductors, topological excitations known as skyrmions are known to exist in addition to the more common vortices. In the third part of this dissertation, we study the properties of skyrmion lattices in an external magnetic field. We propose iv experiments to distinguish vortex lattices from skyrmion lattices by means of their melting curves and their μSR signatures. / Adviser: Dietrich Belitz

ARAÚJO, Bruno Sousa. Transições de fase induzidas por pressão e tamanho de partícula no ferroelástico Pb8O5(VO4)2. 2014. 79 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-10-31T19:40:08Z No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-10-31T19:41:19Z (GMT) No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) / Made available in DSpace on 2014-10-31T19:41:19Z (GMT). No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) Previous issue date: 2014 / Ferroelastic compounds are characterized by the possibility to present two or more states of spontaneous strain which could be permuted by application of mechanical stress. Pb8O5(VO4)2 can be classified as belonging to this group. Its crystal structure is not well defined in the literature due to certain adversities found during the data analysis. However, making use of X-ray diffraction in several crystals of this compound we will discuss their probable point groups. It is also known that there is a direct relation between the states of strain and the domains pattern at ambient conditions of temperature and pressure. Several spectroscopic techniques were employed in order to analyze how the crystal structure of these compounds varies according to the dimensions of each crystal. Therefore, we observed the existence of three phases under ambient conditions as well as the possibility of a spontaneous phase transition in crystals of order to units of square micrometers. The structural behavior of Pb8O5(VO4)2 with increasing temperature has been studied in detail by different authors. They reported the existence of two phase transitions on heating, one second-order transition at about 440 K and another first-order around 520 K. The first-order phase transition leads the Pb8O5(VO4)2 crystals of ferroelastic phase to paraelastic phase. However, since ferroelastics crystals show changes of state of spontaneous strain through the application of mechanical stress, we made use of Raman scattering measures with increasing of hydrostatic pressure on samples of Pb8O5(VO4)2 to accompany the spectral behavior of the same during these pressure variations. This way we observed a phenomenon of amorphization of the sample around 11 GPa and obtained strong evidences from three phase transitions at approximately 1, 3.5 and 6 GPa. / Compostos ferroelásticos são caracterizados pela possibilidade de apresentarem dois ou mais estados de deformação ou strain espontâneos devendo haver a possibilidade de permutação entre estes estados através da aplicação de um stress mecânico. O Pb8O5(VO4)2 pode ser classificado como pertencente a este grupo. Sua estrutura cristalina ainda não é bem definida na literatura por conta de certas adversidades encontradas durante as análises das mesmas, no entanto, fazendo uso de difração de raios-X em diversos cristais deste composto discutiremos seus prováveis grupos pontuais. Sabe-se também que há uma relação direta entre os estados de strain e as estruturas de domínios sob condições ambientes de temperatura e pressão. Diversas técnicas espectroscópicas foram empregadas afim de analisar como a estrutura cristalina destes compostos pode variar de acordo com as dimensões de cada cristal. Dessa forma, verificamos a existência de três fases sob condições ambiente, bem como a possibilidade de uma transição de fase espontânea para cristais da ordem de unidades de micrometros quadrados. O comportamento estrutural do Pb8O5(VO4)2 com o aumento de temperatura foi estudado detalhadamente por diversos autores. Eles reportaram a existência de duas transições de fase durante o aquecimento, uma transição de segunda ordem por volta de 440 K e outra de primeira ordem por volta de 520 K. A transição de primeira ordem leva os cristais de Pb8O5(VO4)2 da fase ferroelástica para a fase paraelastica. Contudo, uma vez que cristais ferroelásticos apresentam mudanças de estados de strain espontâneos através da aplicação de stress mecânico, fizemos uso de medidas de espalhamento Raman com aumento de pressão hidrostática em amostras de Pb8O5(VO4)2 para acompanhar o comportamento espectral dos mesmos durante estas variações de pressão. Assim, observamos um fenômeno de amorfização da amostra por volta de 11 GPa e obtivemos fortes indícios de três transições de fase em aproximadamente 1, 3.5 e 6 GPa.

ARAÚJO, Bruno Sousa. Transições de fase induzidas por pressão e tamanho de partícula no ferroelástico Pb8O5(VO4)2. 2014. 79 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-04-09T19:07:17Z No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-04-10T20:49:59Z (GMT) No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) / Made available in DSpace on 2015-04-10T20:49:59Z (GMT). No. of bitstreams: 1 2014_dis_bsaraujo.pdf: 4719840 bytes, checksum: 34b47793096a5251b04f1f4121ba3485 (MD5) Previous issue date: 2014 / Ferroelastic compounds are characterized by the possibility to present two or more states of spontaneous strain which could be permuted by application of mechanical stress. Pb8O5(VO4)2 can be classified as belonging to this group. Its crystal structure is not well defined in the literature due to certain adversities found during the data analysis. However, making use of X-ray diffraction in several crystals of this compound we will discuss their probable point groups. It is also known that there is a direct relation between the states of strain and the domains pattern at ambient conditions of temperature and pressure. Several spectroscopic techniques were employed in order to analyze how the crystal structure of these compounds varies according to the dimensions of each crystal. Therefore, we observed the existence of three phases under ambient conditions as well as the possibility of a spontaneous phase transition in crystals of order to units of square micrometers. The structural behavior of Pb8O5(VO4)2 with increasing temperature has been studied in detail by different authors. They reported the existence of two phase transitions on heating, one second-order transition at about 440 K and another first-order around 520 K. The first-order phase transition leads the Pb8O5(VO4)2 crystals of ferroelastic phase to paraelastic phase. However, since ferroelastics crystals show changes of state of spontaneous strain through the application of mechanical stress, we made use of Raman scattering measures with increasing of hydrostatic pressure on samples of Pb8O5(VO4)2 to accompany the spectral behavior of the same during these pressure variations. This way we observed a phenomenon of amorphization of the sample around 11 GPa and obtained strong evidences from three phase transitions at approximately 1, 3.5 and 6 GPa. / Compostos ferroelásticos são caracterizados pela possibilidade de apresentarem dois ou mais estados de deformação ou strain espontâneos devendo haver a possibilidade de permutação entre estes estados através da aplicação de um stress mecânico. O Pb8O5(VO4)2 pode ser classificado como pertencente a este grupo. Sua estrutura cristalina ainda não é bem definida na literatura por conta de certas adversidades encontradas durante as análises das mesmas, no entanto, fazendo uso de difração de raios-X em diversos cristais deste composto discutiremos seus prováveis grupos pontuais. Sabe-se também que há uma relação direta entre os estados de strain e as estruturas de domínios sob condições ambientes de temperatura e pressão. Diversas técnicas espectroscópicas foram empregadas afim de analisar como a estrutura cristalina destes compostos pode variar de acordo com as dimensões de cada cristal. Dessa forma, verificamos a existência de três fases sob condições ambiente, bem como a possibilidade de uma transição de fase espontânea para cristais da ordem de unidades de micrometros quadrados. O comportamento estrutural do Pb8O5(VO4)2 com o aumento de temperatura foi estudado detalhadamente por diversos autores. Eles reportaram a existência de duas transições de fase durante o aquecimento, uma transição de segunda ordem por volta de 440 K e outra de primeira ordem por volta de 520 K. A transição de primeira ordem leva os cristais de Pb8O5(VO4)2 da fase ferroelástica para a fase paraelastica. Contudo, uma vez que cristais ferroelásticos apresentam mudanças de estados de strain espontâneos através da aplicação de stress mecânico, fizemos uso de medidas de espalhamento Raman com aumento de pressão hidrostática em amostras de Pb8O5(VO4)2 para acompanhar o comportamento espectral dos mesmos durante estas variações de pressão. Assim, observamos um fenômeno de amorfização da amostra por volta de 11 GPa e obtivemos fortes indícios de três transições de fase em aproximadamente 1, 3.5 e 6 GPa.

GONÇALVES, Ricardo Oliveira. Espalhamento Raman polarizado em cristais de L-arginina.HCl:H2O e estudo da L-arginina.2H2O sob altas pressões e sob altas temperaturas. 2012. 127 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2012. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-08T19:50:33Z No. of bitstreams: 1 2012_tese_rogoncalves.pdf: 5273230 bytes, checksum: e2ecec57843458258d1b0a7a56cfc839 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-08T19:51:32Z (GMT) No. of bitstreams: 1 2012_tese_rogoncalves.pdf: 5273230 bytes, checksum: e2ecec57843458258d1b0a7a56cfc839 (MD5) / Made available in DSpace on 2015-06-08T19:51:32Z (GMT). No. of bitstreams: 1 2012_tese_rogoncalves.pdf: 5273230 bytes, checksum: e2ecec57843458258d1b0a7a56cfc839 (MD5) Previous issue date: 2012 / The objective of this Thesis is the study through Raman spectroscopy of two crystals containing amino acid L-arginine: L-arginine monohydrated hydrochloride (LARHCL) and L-arginine dihidrated (LARDH). In the first part of the work it was presented structural and vibrational aspects of LARHCL and LARDH, particularly the distribution of normal modes in terms of the irreducible representations of factor groups C2 and D2, respectively. The room temperature normal modes of LARHCL observed through polarized Raman spectroscopy using nine different scattering geometries was presented. Using ab initio calculations it was possible to tentatively assign the Raman-active normal modes of LARHCL. In another part of the work it is presented the analysis of Raman spectra of LARDH under several temperature and pressure conditions. It was studied the normal modes of LARDH crystal through Raman spectroscopy in the range between 30 oC and 200 oC. From this study it was observed the occurrence of anomalies in the Raman spectra, such as change in the curves dw/dT related to modes of low frequency (at about 45 oC), as well as the fact that, starting from 50 oC, in a heating process, some bands decrease intensity. Such a picture suggested that LARDH crystal undergoes a phase transition between 45o and 50oC. Also, it was possible to note that the spectra recorded between 45o and 60oC were different from those recorded for T < 45oC and for T > 60oC suggesting the coexistence of phases in this temperature interval. Another hypothesis was that the crystal undergoes a phase transition at 45oC, remaining in this phase up to the temperature of 60oC. Additionally, it was possible to perform thermal analysis measurements, differential thermal analysis (DTA) and thermogravimetric analysis (TG). From the DTA it was observed that between 45o and 50oC begins an endothermic event that is observed up to 84oC. A second endothermic event begins and remains up to 109oC. By using TG data it was possible to understand that these two events are related to water that left the crystal, although a small quantity remains in the crystal, as pointed out by Raman spectroscopy. In the final part of the work LARDH crystal under high pressure conditions was studied up to 8 GPa. From this study, in particular the analysis of external modes, it was revealed that the crystal undergoes a phase transition at ~ 2,5 GPa. However, for pressures between 2,5 and 8,0 GPa no additional phase transition was observed, pointing to a great stability of the crystal when compared with other amino acid hydrated crystals. Additionally, there is no great change in the internal mode region of the Raman spectrum, suggesting that the phase transition undergone by LARDH under high pressure conditions can be classified as a conformational one. / Esta Tese teve como objetivo o estudo por meio de espectroscopia Rama de dois cristais contendo o aminoácido L-arginina em sua constituição: a L-arginina monohidratada clorohidrato (LARHCL) e a L-arginina dihidratada (LARDH). Na primeira parte do trabalho foram discutidos aspectos estruturais gerais da LARHCL e da LARDH, em particular a distribuição dos modos normais do material em termos das representações irredutíveis do grupos fatores C2 e D2, respectivamente. Foi realizado um estudo detalhado dos modos normais de vibração da LARHCL à temperatura ambiente através de espesctroscopia Raman polarizada observando-se os modos normais de vibração do material em nove diferentes geometrias de espalhamento. Com o auxílio de cálculo ”ab initio” foi feita a classificação tentativa dos modos normais de vibração da LARHCL ativos no Raman. Numa outra etapa do trabalho analisou-se os espectros Raman da LARDH sob diversas condições de temperatura e de pressão. Realizou-se um estudo dos modos normais do cristal de LARDH através da espectroscopia Raman no intervalo entre 300 e 2000C. Deste estudo observou-se a existência de algumas anomalias nos espectros Raman, como mudança de inclinação de curvas dw/dT para modos de baixa frequência em torno de 45oC, além do fato de que a partir de 50oC algumas bandas vão perdendo intensidade. Estes fatos foram interpretados como uma transição de fase sofrida pelo cristal entre 45o e 50oC. Observou-se que os espectros Raman entre 450 e 60oC eram diferentes dos espectros Raman para T < 45oC e para T > 60oC sugerindo a coexistência de fases neste intervalo de temperatura. Uma outra hipótese foi que o cristal sofreria uma transição de fase em 45oC, ficando nesta fase até a temperatura de 60oC. Foram realizadas medidas térmicas do tipo análise térmica diferencial (DTA) e análise termogravimétrica (TG). Da análise de DTA observou-se que entre 45o e 50oC começa um evento endotérmico que se prolonga até cerca de 84oC. A seguir, um segundo evento endotérmico se inicia, prolongando-se até 109oC. Com o auxílio das medidas de TG compreendeu-se que estes dois eventos endotérmicos estão associados à expulsão das moléculas de água, embora uma pequena quantidade permaneça no cristal como demonstrado pela espectroscopia Raman. Como parte final do trabalho estudou-se cristais de LARDH sob condições de altas pressões até cerca de 8 GPa. Deste estudo inferiu-se pela análise da região dos modos externos, que o cristal sofre uma transição de fase em torno de 2,5 GPa. Entretanto, para pressões no intervalo entre 2,5 e 8,0 GPa nenhuma outra transição de fase foi percebida, o que aponta para uma grande estabilidade do cristal quando comparado a outros cristais hidratados de aminoácidos. Além disso, nas regiões dos modos internos não foram observadas grandes mudanças nos espectros Raman, sugerindo que a transição de fase com a pressão seja do tipo conformacional.

As transformações de fase em cristais de cianeto de potássio (KCN) a 168&#176K e 83&#176K e suas ligas com KC1, foram estudadas através da absorção ótica no infravermelho do íon de impureza OCN&#8254 e dos íons CN&#8254 da rede. Por aplicação de tensão uniaxial foram obtidos pela primeira vez cristais únicos, da KCN-KCl, nas duas fases de mais baixa temperatura. A absorção ótica dicróica devida à vibração interna do íon CN&#8254 nesses monocristais, permitiu a determinação de seu arranjo estrutural direcional, de forma inequívoca, como também correções a modelo recentemente proposto para esse arranjamento, baseado em experiências com cristais de muitos domínios. O estudo do modo de flexão da impureza OCN&#8254 permitiu o seguimento das transformações de fase anteriormente observadas por meio de medidas de calor específico em cristais puros de KCN. A evolução dessas transformações em função da concentração do íon esférico de Cl&#8254 em substituição do íon elipsoidal CN&#8254 foi estudada numa larga faixa de concentrações. Um modelo para a fase de mais baixa temperatura e proposto com arranjo antiferroelétrico dos dipolos CN&#8254 e com distinção na sub-rede dos íons positivos. Em vista do modelo proposto, duas alternativas para as duas transições de fase poderiam ocorrer. Na transição de mais alta temperatura (168&#176K), ocorreria o alinhamento ferroelástico e antiferroelétrico, dos dipolos CN&#8254, e a transição de mais baixa temperatura (83&#176K) corresponderia somente à distorção na sub-rede dos íons positivos (e possivelmente dos negativos). Na outra possibilidade, a 168&#176K ocorreria somente um arranjo ferroelástico e a 83&#176K o arranjo antiferroelétrico e a distorção na rede. / The phase transformations in potassium cyanide crystals (KCN) at 168&#176K and 83&#176K and KCN-KCl mixed crystals, were studied through the I-r optical absorption of the CN&#8254 stretching mode and the OCN&#8254 impurity bending mode and Fermi resonance application of uniaxial stress in KCN-KCl mixed crystals gives, by the first time, oriented single crystals in the lower temperature phases. The dichroic optical absorption of the CN&#8254 stretching mode in these oriented single crystals allowed the explicit determination of the structural arrangement of the C\'N POT.-\' dipole. The results of our measurements are not in agreement recently proposed by Julian and Luty based on electrical experiments with polidomain crystals. The bending mode and Fermi resonances of the substitutional OC\'N POT.\'impurity give further information in agreement with previous specif heat measurements. The evolution of the phase transformation was studied as a function of the spheroidal Cl&#8254 íon concentration replacing the CN&#8254 ellipsoidal íon. A model for the lower temperature phase is proposed with antiferroelectric ordering of the CN&#8254 dipoles and additional distortion on positive and negative sub lattices. As a consequence of the proposed model the phase transitions, may occur in two different ways that can not be distinguished with the used techniques. In the first possibility during the higher temperature phase transition (168&#176K) would occur the ferroelastic and antiferroelectric alignement of the CN&#8254 dipoles and the lower phase transition (83&#176K) would be due to a lattice distortion as necessary to explain our results. In the other possibility the 168&#176K phase transition would occur only the ferroelastic transition;and at 83&#176K the antiferroelectric ordering of the CN&#8254 ions and the lattice distortion would occur

A multivariate generalization of mutual information, multi-information, is defined in the thermodynamic limit. The definition takes phase coexistence into account by taking the infimum over the translation-invariant Gibbs measures of an interaction potential. It is shown that this infimum is attained in a pure state. An explicit formula can be found for the Ising square lattice, where the quantity is proved to be maximized at the phase-transition point. By this, phase coexis-tence is linked to high model complexity in a rigorous way.

No description available.