京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18073号 / 理博第3951号 / 新制||理||1569(附属図書館) / 30931 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 鶴 剛, 教授 谷森 達, 教授 柴田 大 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Cholesterol is known to affect biophysical quantities in one and two component membranes, overall increasing membrane thickness and orientational order and decreasing membrane fluidity. Although these effects are useful in the plasma membrane of cells where strength is a desired property, there is evidence suggesting that the appearance of cholesterol in other cellular membranes has adverse effects. Indeed, cholesterol is found at elevated levels in the mitochondria membranes of cancer cells and is associated with chemotherapy resistance. At the molecular level, the link between cancer and cholesterol seems to be that cholesterol interferes with apoptosis, or programmed cell death, specifically by inhibiting the insertion of the pro-apoptotic protein Bax in the mitochondria outer membrane. We studied the effects of cholesterol on a five component mitochondria-like membrane, in order to determine which of these are relevant for Bax membrane insertion. As expected we found, using x-ray and neutron scattering, that upon cholesterol addition: (1) the thickness of the mitochondria-like membrane increases, (2) the area per phospholipid decreases, and (3) the orientational order of the membrane increases. Interestingly, our data indicate that the ordering effect of cholesterol is less efficient for the five component mitochondria-like membrane than for a single component membrane. Finally, we determined that in spite of the relatively high degree of unsaturation of the lipids in the mitochondria-like membrane, cholesterol adopts a canonical orientation. At higher cholesterol concentrations, cholesterol's polar hydroxyl group moves outwards and comes in proximity with the phospholipid's carbonyl group, allowing hydrogen bonding between the two types of molecules. Any of the above effects could in principle be responsible for cholesterol's inhibition of Bax insertion, thus follow up studies are required to confirm which, or what combination of them are relevant for apoptosis. / Thesis / Master of Science (MSc)

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The lead-free tin telluride (SnTe) is considered as a potential candidate to substitute lead telluride (PbTe) for thermoelectric power generation based on their similar crystal and electronic structures. However, the relatively high lattice thermal conductivity and low Seebeck coefficient of pristine SnTe are detrimental for real-life applications. This dissertation explored elements-doping/substituting of SnTe to overcome those shortcomings and improve SnTe thermoelectric performance. A series of the Sn1-xGexTe phases were synthesized and studied. When the Ge amount reaches 50% or higher, Sn1-xGexTe undergoes a phase transition from the rock-salt structure (Fm3̅m) to the rhombohedral one (R3m). The Sn0.5Ge0.5Te phase was explored in more details because it delivers the best thermoelectric performance with the Sn1-xGexTe series. The electron-richer Sb and Bi were substituted on the Sn/Ge site to optimize the charge transport properties, and Cu2Te was added into the matrix to improve the thermoelectric performance further. The In/Sb and In/Bi co-doping on the Sn/Ge sites was employed for Seebeck coefficient optimization. A comparative study of the electronic structure of the Sn0.5Ge0.5Te-based samples was performed. The calculations indicated a band convergence and changes in the valence band, thus providing insight into the co-doping effects. Suppression of the lattice thermal conductivity of SnTe was performed via alloying with AgSnSe2 and PbTe, which introduced strong atomic disorder. Additionally, AgSnSe2 showed a hole donor behavior in SnTe, and the increased carrier concentration compensated for the reduction in the carrier mobility, thus rendering a decent electrical conductivity in alloyed samples. As a result, the alloying effectively improved the samples' thermoelectric performance. / Thesis / Doctor of Philosophy (PhD) / In recent decades, renewable energy has attracted a lot of attention due to an increase in the global energy use and depletion of fossil fuel reserves. Thermoelectric materials are expected to play a vital role as green energy generators to overcome the upcoming energy crisis as they can directly convert waste heat into electricity through the Seebeck effect. In this dissertation, the main goal is optimizing the thermoelectric performance of SnTe for the above room temperature applications. Different doping/ substituting/alloying strategies were applied to improve the performance. The obtained thermoelectric properties of the SnTe-based materials were rationalized in terms of the charge carrier behavior, changes in the electronic structure, and phonon propagation.

Transformation of amorphous calcium phosphate (ACP) and brushite into hydroxylapatite, an important biomineral, has been documented. The relationships between synthesis conditions and the formation and transformation of these phases are not comprehensively understood. The metastable nature of ACP has made it historically challenging to investigate, and many analyses attempt to stabilize the phase through drying or including additional ions or proteins in the reaction. In situ investigations provide an incisive approach to examining the structure and transformation of ACP and brushite as a function of synthesis conditions. The first project develops a new method for in situ analyses of the structure of ACP and brushite shortly after reagent mixing, without chemical stabilization. This method was used in the second project to examine how the initial Ca/P affects ACP structure and transformation. Our results identify the first structural differences in types of ACP, controlled by the initial Ca/P. At ratio 0.2 the Ca – P bonding geometry is primarily monodentate, ratio 5.0 produces a coordination that is primarily bidentate, and there is a mix of monodentate and bidentate coordinates at intermediate ratios between the two. These results are independent of system pH between the examined range of 6-11. Further ex situ transformation experiments identified that at ratio 0.2, ACP transformed directly into hydroxylapatite, but at higher ratios the transformation product is brushite. This is a promising mechanism for direct ACP to hydroxylapatite conversion at a biologically relevant pH. In the final project, the statistically significant synthesis parameters (age, pH, temperature, supersaturation, and initial ion ratio) for ACP, brushite, and hydroxylapatite formation are evaluated. Binary logistic regression analysis and nonlinear continuous logistic regression analysis are applied to a dataset compiled from the literature. Equations were developed that predict the percentage of ACP and brushite that will form. The equations and significant variables seem to depend on the transformation pathway of brushite and ACP. The current analysis did not comprehensively describe hydroxylapatite formation when trying to concurrently evaluate the ACP to hydroxylapatite and brushite to hydroxylapatite pathways. Taken together, these studies provide new ways to study and interpret calcium phosphate phases as they form and transform. Experiments identified new relationships between the chemistry and structure of ACP. The new in situ experimental method and the equations we developed can be used to improve future experimental designs towards a comprehensive understanding of the calcium phosphate system. / Doctor of Philosophy / Hydroxylapatite is a mineral made of calcium and phosphate, that is similar to the mineral components of bones and teeth in humans and other mammals and fish. Hydroxylapatite and other calcium phosphate phases can form when solutions, rich in calcium and phosphate, are mixed. Phases without long-range crystal structure, are amorphous calcium phosphates (ACP). Additional calcium phosphate minerals, like brushite can also form. If brushite and ACP are left in solution, they will transform into hydroxylapatite over time. Major questions include the need to learn the short-range atomic structure of ACP and how ACP and brushite transform into hydroxylapatite In this dissertation, I investigate how chemistry and other variables such as age and time impact the calcium phosphate phase to form and how it transforms with aging. The first project develops a new method to study the structure of ACP and brushite without drying the study materials or adding additional chemicals or proteins to prevent them from transforming. The sample forms in a solution and flows directly through an X-ray beam for structural analysis. This method is used in project two to examine how the ratio of calcium and phosphate in the beginning of the reaction affected the structure of ACP and how it transformed. The results identify the first structural differences in types of ACP, controlled by the initial Ca/P. At a ratio of 0.2 a calcium and phosphorus atom mostly share only one oxygen between them, but at Ca/P = 5.0, they mostly share two oxygens. At ratios in between 0.2 and 5.0 they share a mix of one and two oxygens. The results are independent of pH. Additionally, at ratio 0.2, ACP transformed directly into hydroxylapatite, but at all other ratios it transformed to brushite. Investigations of direct ACP to hydroxylapatite transformation are usually performed at a pH above that found in humans, but the transformation at low ratio occurred at a biologically relevant pH. In the final project statistical analysis was used to identify what synthesis conditions (out of age, pH, temperature, supersaturation, and initial ion ratio) had a significant impact on the formation of ACP, brushite, and hydroxylapatite. Equations were developed that can be used to predict the percentage of ACP and brushite that form based on the statistically significant variables. The current analysis did not fully describe hydroxylapatite formation. Results suggest separate equations are needed for hydroxylapatite forming directly from ACP and directly from brushite. Combined, these studies have created new ways to study calcium phosphate phases as they form and transform. This work experimentally identified new relationships between the chemistry and structure of ACP. Both the method and equations will help researchers improve their future experimental designs so investigations can be more directly compared to create a comprehensive understanding of calcium phosphates.

A series of new germanates was prepared and their structures were characterized with X-ray diffraction (XRD). We employed solid-state synthesis, flux growth, and crystallization from melts to obtain crystals of the new materials. The crystals were studied by means of single crystal XRD, providing the information on the structure and composition of the new materials. Germanates suitable to accommodate Mn4+ – a well-known activator ion for the preparation of the rare-earth-free red phosphors – were of a particular interest. The Ge4+ substitution for Mn4+ is possible if the crystal structure features octahedrally coordinated germanium atoms, and we indeed were able to prepare such germanates. The crystal structures of the following phases were characterized: Mg3Ge1-O4(1-)F2(1+2) ( ≈ 0.1), Mg14Ge4O20F4, Mg2Pb2Ge2O7F2, Sr3GeO4Cl2, Ba3GeO4Br2, Sr6Ge2O7Cl6, Ba5GeO4Br6, Na2BaGe8O18, Rb2BaGe8O18, Na0.36Sr0.82Ge4O9, Na2SrGe6O14, and K2SrGe8O18. Two phases, Mg3Ge1-O4(1-)F2(1+2) and Na0.36Sr0.82Ge4O9 demonstrate deficiency on certain crystallographic sites. We analyzed the connectivity of the GeO4 and GeO6 units in the new and reported tetra- and octagermanates. Despite the similar stoichiometry, the connectivity of GeOn polyhedra is different in RI2Ge4O9, RIIGe4O9 and RIRIIGe8O18 germanates (RI = alkali, RII = alkaline earth metal). The Ge4+ substitution for Mn4+ was successfully done for the Na2BaGe8O18, Rb2BaGe8O18, Na2SrGe6O14, and K2SrGe8O18 phases yielding new red phosphors. Based on the powder XRD data, the optimal synthetic strategies were developed yielding high purity (≈ 99 wt. %) samples. The photoluminescent excitation and emission spectra were collected for the new phosphors. Strong absorption of the UV light and emission in the far-red region of the visible spectra were confirmed, which is in a good agreement with the literature. The Mn4+ doping level was optimized to achieve the highest luminescence in the studied phases. Temperature-dependent luminescence spectra were collected for the Na2SrGe6O14 : Mn4+ and K2SrGe8O18: Mn4+, and the K2SrGe8O18: Mn4+ showed the highest resistance to temperature quenching. / Thesis / Doctor of Philosophy (PhD)

<p>This thesis is comprised of three original works presented in Chapters 4,5, and 6. The three projects are comprised of three different problems in condensed matter physics, the commonality between the topics is the technique used to study the materials, diffraction. Two of which are in the form of journal articles that have been peer-reviewed and published in Physical Review B, while the third is that of an unpublished work. The unpublished work is presented first and uses neutron scattering techniques, while two published papers were performed using x-ray diffraction.</p> <p>The neutron scattering experiments were performed by myself with the assistance of William J .L. Buyers and B.D. Gaulin. These experiments were performed at Chalk River Laboratories in northern Ontario. The x-ray scattering experiments were carried out on the rotating anode x-ray laboratory at McMaster University and the Advanced Photon Source in Argonne II. All data analysis was performed by myself under the guidance of B.D. Gaulin.</p> / Thesis / Doctor of Philosophy (PhD)

<p> A coarse grained amphibolite from the metamorphosed rim of the Whitestone Anorthosite was prepared in several ways for the purpose of determining the modal abundance of the constituent minerals by calibrated X-ray diffraction. A simple two component amphibolite consisting of plagioclase and amphibole was chosen and five major methods of mounting the specimens for X-ray diffraction were employed.</p> <p> It was hoped that a method could be found which would produce randomly oriented, homogeneous samples. A series of such samples each of a different component ratio would then provide a calibration curve from which the mode of a whole rock specimen could be estimated.</p> <p> The calibrated X-ray charts were produced on Philips scanning X-ray diffractometers.</p> <p> The inconsistencies in the results indicate that better technical procedures are required.</p> / Thesis / Bachelor of Science (BSc)

<p> The crystal structure of dioxotetrapyridine-rhenium(V) chloride dihydrate has been determined by single crystal x-ray diffraction methods. The structure has been found to belong to the trans-dioxo- group of compounds. The Re=O bond in the structure has been shown to have a bond-order of about 2, with the average Re=O bond length of 1.76(J) A. The probable structures of the monohydrate and anhydrous dioxotetrapyridine-rhenium(V) chloride have been discussed on the basis of the observed pseudo-symmetric structure for the dihydrate with a non-centrosymmetric space group. The nature and strength of the hydrogen bonds in the structure have also been discussed.</p> / Thesis / Master of Science (MSc)

<p> The crystal structure of k2SnBr6 has been reinvestigated using single crystal x-ray diffraction techniques. Three dimensional intensity data obtained photographically have been used to refine the structure, by the least square analysis. The structure is found to be slightly distorted from the regular cubic k2PtCl6 structure in a manner similar to K2TeBr6. The Sn-Br bond is found to be 2.601 A. </p> <p> The structure of k2SnBr6 is found to be monoclinic with space group P21/n and a = 7.435 ± 0.017 A, b= 7.437 ± 0.017 A, and c = 10.568 ± 0.006 A. </p> <p> A review of other crystals with similar structure is included in this thesis included the theory of x-ray diffraction and crystal structure as applicable to the present problem is discussed briefly. </p> / Thesis / Master of Science (MSc)