Elastic Constants and Related Mechanical Properties of the Monoclinic Polymorph of the Carbamazepine Molecular Crystal

Elastic anisotropy of mechanically responsive molecular solids

Unique mechanical properties in molecular solids arise from a specific combination of structure and interaction anisotropy. Powder Brillouin light scattering offers new insight into the latter contribution to test current models for mechanical design.

Theoretical and experimental investigation of Xenotime-type rare earth phosphate REPO4, (RE = Lu, Yb, Er, Y and Sc) for potential environmental barrier coating applications

The mechanical and thermophysical properties of Xenotime-type REPO₄ (RE = Lu, Yb, Er, Y and Sc) have been theoretically and experimentally investigated for a potential environmental barrier coating (EBC) topcoat application. The results show that the current studied REPO₄ exhibits a quasi-ductile property, suggesting a potential long life expectancy of its made coatings. Further, from the study of underlying parameters governing thermophysical properties of a ceramic, low thermal expansion coefficients (TECs) and low thermal conductivities cannot be achieved simultaneously, due to mutual exclusive nature of above two parameters. REPO₄ has been unveiled to have rather small TECs, attributing partly to its weak lattice anharmonicity, and is thus well-matched with silicon carbide based ceramic matrix composites. Last, the current investigated REPO₄ exhibits very good high-temperature water vapor corrosion resistance, excellent calcium-magnesium aluminosilicates (CMAS) resistance as well as excellent chemical compatibility with silicon bond coats at elevated temperatures. Therefore, the Xenotime-type rare earth phosphates are a promising EBC topcoat material.

Brillouin Light Scattering: Development of a Near Century-Old Technique for Characterizing the Mechanical Properties of Materials

Brillouin light scattering (BLS), a technique theoretically described nearly a century back by the French physicist Léon Brillouin in 1922, is a light-scattering method for determining the mechanical properties of materials. This inelastic scattering method is described by the Bragg diffraction of light from a propagating fluctuation in the local dielectric. These fluctuations arise spontaneously from thermally populated sound waves intrinsic to all materials, and thus BLS may be broadly applied to transparent samples of any phase. This review begins with a brief historical overview of the development of BLS, from its theoretical prediction to the current state of the art, and notes specific technological advancements that enabled the development of BLS. Despite the broad utility of BLS, no commercial spectrometer is currently available for purchase, but rather individual components are assembled to suit a specific application. Central to any BLS spectrometer is the interferometer, and its performance characteristics-scanning or non-scanning, multi-passing, and stabilization-are critical considerations for spectrometer design. Consistent with any light-scattering method, the frequency shift is a key observable in BLS, and we summarize the connection of this measurement to evaluate the mechanical properties of materials. With emphasis toward pharmaceutical materials analysis, we introduce the traditional BLS approach for single-crystal elasticity, and this is followed by a discussion of more recent developments in powder BLS. We conclude our review with a perspective on future developments in BLS that may enable BLS as a novel addition to the current catalog of process analytical technologies.

Aggregate Elasticity and Tabletability of Molecular Solids: a Validation and Application of Powder Brillouin Light Scattering

Describing the elastic deformation of single-crystal molecular solids under stress requires a comprehensive determination of the fourth-rank stiffness tensor (C_ijkl). Single crystals are, however, rarely utilized in industrial applications, and thus averaging techniques (e.g., the Voigt or Reuss approach) are employed to reduce the C_ijkl (or its inverse S_ijkl) to polycrystalline aggregate mechanical moduli. With increasing elastic anisotropy, the Voigt and Reuss-averaged aggregate moduli can diverge dramatically and, provided that drug molecules almost exclusively crystallize into low-symmetry space groups, warrants a significant need for accurate aggregate mechanical moduli. This elasticity data, which currently is largely absent for pharmaceutical materials, is expected to aid understanding how materials respond to direct compression and tablet formation. Powder Brillouin light scattering (p-BLS) has recently demonstrated facile access to porosity-independent, aggregate mechanical moduli. In this study, we extend our previous p-BLS model for obtaining mechanical properties and validate our approach against a broad library of molecular solids with diverse intermolecular interaction topologies and with previously determined C_ijkl which permits benchmarking our results. Our Young's and shear moduli determined with p-BLS strongly correlate, with limited bias (i.e., a near 1:1 relation), with the Voigt-averaged Young's and shear moduli determined using the C_ijkl. Through follow-on tabletability studies, we introduce initial classifications of tabletability behavior based on the results of our p-BLS studies and the apparent elastic anisotropy. With further development, this approach represents a robust and novel method to potentially identify materials for optimum tabletability at early developmental stages.

Exceptionally low shear modulus in a prototypical imidazole-based metal-organic framework

Using Brillouin scattering, we measured the single-crystal elastic constants (C(ij)'s) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn(2-methylimidazolate)(2)], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C(ij)'s under ambient conditions are (in GPa) C(11)=9.522(7), C(12)=6.865(14), and C(44)=0.967(4). Tensorial analysis of the C(ij)'s reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G(min) < or approximately 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C(ij)'s can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN(4) tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications.