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Magnetic Structure of antiferromagnetic Mn₂P₂O₇ at 4.2º K has been established by neutron diffraction from a powder specimen. The magnetic unit cell is found to be the same size as the chemical unit cell. The spins are arranged in ferromagnetic sheets with the moments in each sheet aligned antiferromagnetically relative to its adjacent sheets. Neutron diffraction results in regard to the critical temperature (13±3)º K, and the spine being at an angle of 23º from the a-axis in the a-c plane have been found consistent with the ones obtained by Fowlis, Atkinson, Choh and Stager by susceptibility and NMR studies on the material. However, to confirm the angle with some precision, more work, probably with a single crystal, will be required. / Thesis / Master of Science (MSc)

<p> Neutron diffraction techniques have been used to determine the chemical and magnetic order in a single crystal of the Heusler alloy Ni2Mn0.8v0.2sn. This material orders in the Heusler L21 structure and is ferromagnetic. Nuclear Bragg scattering intensity ratios have been measured at 298 K and compared with nuclear structure factor calculations based on a model of the crystal structure. This comparison is used to determine chemical disorder. Magnetic Bragg scattering intensity ratios have been measured at 117 K. These ratios, along with bulk magnetization measurements, are used to determine the spatial distribution of the magnetic moment. The crystal is found to have the L21 structure with possibly 5% or so Ni-Sn disorder. Nearly all of the magnetic moment, which is 3.19 ± .03 μ8/mol, is found to exist on the Mn-V sites. The rest is present on the Sn atoms. The values 3.74 ± .10 μ8/Mn atom and .21 ± .08 μ8/Sn atom result from assuming the V atoms carry no moment. </p> / Thesis / Master of Science (MSc)

No description available.

Shape memory ceramics (SMCs) are burgeoning functional materials based on zirconia with a reversible, stress-inducible martensitic phase transformation. Compared to metallic shape memory alloys, SMCs have broader operating temperatures, higher critical stresses, and larger mechanical hysteresis loops. These advantages make SMCs attractive for high-output actuation and sensing in extreme environments or energy dissipation applications; however, the key phase transformation generates large stresses and strains that accumulate at grain boundaries and result in fracture of monolithic SMCs. This means that material forms with decreased mechanical constraint are necessary. Transformation without fracture has been previously demonstrated with SMC micropillars and individual microparticles, but these material forms lack useful applications. By utilizing easily scalable granular packings of discrete free particles, the transformation can be triggered in bulk without fracture in much the same way. The granular packing material form introduces significant complexity as the internal stress distributions responsible for the phase transformation are highly heterogeneous on the macro-, meso-, and micro-scales. Moreover, the unconstrained phase transformation behaves differently than the constrained transformation, which is more studied in zirconia. The interactions of these various factors are explored from a fundamental perspective in this work, notably including (1) a unique 'continuous mode' of both forward and reverse transformation in granular packings, (2) the dependence of transformation behavior on macro-, meso-, and microstructure, and (3) the evolution of the granular packings' structure and energy dissipation capacity over 10,000 loading cycles. Diverse experimental techniques are employed, ranging from mechanical testing and calorimetry to in situ neutron diffraction, to support theory based on the martensitic phase transformation in zirconia, the shape memory and superelastic effects, and granular material physics. / Doctor of Philosophy / Shape memory materials are capable of remembering their original shape even when they are deformed, and can return to that shape when they are heated. This unique property stems from a phenomenon called martensitic phase transformation which bridges the gap between microscopic structural changes and macroscopic shape changes as a response to specific environmental changes. Most of the common shape memory materials are metallic, like nitinol (NiTi), which has uses in orthodontic wires and cardiological stents, but there are also ceramic materials that can display the shape memory effect. These shape memory ceramics are based on zirconia (ZrO2), and are distinct from metallic shape memory materials because of their brittle behavior and high temperature stability owing to their chemical structure. The work presented in this thesis concerns the behavior of shape memory ceramics in granular form (i.e., loose powders) over a range of external conditions. Diverse experimental techniques are employed to investigate differences between granular and non-granular shape memory ceramics. This work shows how the unique structure of a granular material, which is dominated by highly uneven force distributions and microscopic effects, interacts with the martensitic phase transformation in shape memory ceramics to produce a 'continuous' mode of transformation that differs from non-granular shape memory materials. This continuous mode is itself dependent on the granular material's macro-, meso-, and micro-structure, and on the shape memory material's composition and history. In the future, shape memory ceramics might leverage the insights gained from this work for applications including energy dissipation or on-demand shape changes (i.e., actuation).

The mineral prehnite, Ca2(Al,Fe,Mn)(AlSi3O10)(OH)2, is a layered structure consisting of double-sheets of (Al,Si)O4 and SiO4 tetrahedra alternating with single sheets of AlO4(OH)2 octahedra. To understand the ordering in the structure and differences between various samples of prehnite, single-crystal X-ray diffraction data at ambient conditions were collected on four single crystals of prehnite from different localities. The positions of the H atoms have been determined for the first time, from a combination of X-ray and neutron diffraction data. The equation of state and high-pressure behavior of prehnite have been investigated using single-crystal X-ray diffraction up to 9.75(3) GPa. A second-order Birch–Murnaghan equation of state fit to the isothermal P-V data to 8.7 GPa yields a bulk modulus, K = 109.29(18) GPa. Structural data collected at high pressures indicate that the structure compresses uniformly. Above 8.7 GPa there is additional softening of the volume and the b-axis related to polyhedral tilting. However, the average structure is maintained across the transition. Ambient and high-pressure Raman and synchrotron infrared spectra were collected from 1 bar to 20 GPa. Raman spectra measured at ambient conditions of four prehnite crystals with different compositions confirmed that there are no structural changes with different compositions. High-pressure results showed the majority of modes shift to higher frequencies (in a smooth, linear fashion) with increasing pressure. The greatest change in the spectra is the softening of the modes in the OH-stretching region above 9 GPa, thought to be related to the polyhedral tilting around the H environment. / Master of Science

In this work, X-ray and neutron powder diffraction experiments and complementary solid-state NMR spectroscopy are used to characterise NaNbO₃-based perovskite phases. Samples of NaNbO₃, KₓNa₁₋ₓNbO₃ and LiₓNa₁₋ₓNbO₃ are synthesised using a variety of techniques and subsequently characterised. For NaNbO₃, it is observed that at least two room temperature perovskite phases can co-exist, P and Q, and that each phase can be formed exclusively by manipulating the synthetic approach utilised. Phase Q can also be formed by the substitution of a small amount of K⁺ or Li⁺ for Na⁺. The room temperature phases of these materials are also analysed using NMR spectroscopy and X-ray diffraction. It is found that, for KₓNa₁₋ₓNbO₃, preferential A-site substitution of K⁺ for Na⁺ may occur, and this observation is supported using a range of NMR techniques and density functional theory calculations. The high-temperature phase behaviour of NaNbO₃ and KₓNa₁₋ₓNbO₃ (x = 0.03 to 0.08) is analysed using high-resolution neutron and X-ray powder diffraction to determine when phase changes occur and to characterise each phase. Characterisation of these materials is supported used complementary symmetry mode analysis. For the LiₓNa₁₋ₓNbO₃ perovskite system, complex phase behaviour is observed at room temperature. High-resolution neutron powder diffraction data shows that, over the range 0.08 < x < 0.20, phase Q may co-exist with a rhombohedral phase, with the proportions of the two highly dependent upon the synthetic conditions used. Furthermore, using X-ray diffraction and NMR spectroscopy, phase Q is shown to undergo a crystal-to-crystal transition to the rhombohedral phase. For higher values of x, two compositionally-distinct rhombohedral phases are formed, termed Na-R3c and Li-R3c, as determined from neutron powder diffraction data.