Structure and dynamics in yttrium-based molten rare earth alkali fluorides.
The transport properties of molten LiF-YF3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y(3+) here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.
The effect of dispersion interactions on the properties of LiF in condensed phases
Classical molecular dynamics simulations are performed on LiF in the framework of the polarizable ion model. The overlap repulsion and polarization terms of the interaction potential are derived on a purely non-empirical, first-principles basis. For the dispersion, three cases are considered: a first one in which the dispersion parameters are set to zero and two others in which they are included, with different parametrizations. Various thermodynamic, structural and dynamic properties are calculated for the solid and liquid phases. The melting temperature is also obtained from direct coexistence simulations of the liquid and solid phases. Dispersion interactions appear to have an important effect on the densities of both phases and on the melting point, although the liquid properties are not affected when simulations are performed in the NVT ensemble at the experimental density.
Rethinking Health: Healthy or Healthier than?
Theorists of health have, to this point, focused exclusively on trying to define a state—health—that an organism might be in. I argue that they have overlooked the possibility of a comparativist theory of health, which would begin by defining a relation—healthier than—that holds between two organisms or two possible states of the same organism. I show that a comparativist approach to health has a number of attractive features, and has important implications for philosophers of medicine, bioethicists, health economists, and policy makers. 1 Introduction 2 Comparative and Non-comparative Concepts 3 Traditional Theories of Health 4 An Argument for Comparativism: Intergenerational Assessments of Health 5 Another Reason to be a Comparativist: Functionalist Theories of Health 6 A Non-comparative Alternative? 7 An Objection to Comparativism? 8 Why this Matters: The Consequences of Comparativism 1 Introduction 2 Comparative and Non-comparative Concepts 3 Traditional Theories of Health 4 An Argument for Comparativism: Intergenerational Assessments of Health 5 Another Reason to be a Comparativist: Functionalist Theories of Health 6 A Non-comparative Alternative? 7 An Objection to Comparativism? 8 Why this Matters: The Consequences of Comparativism