80 research outputs found
Polymorph exploration of bismuth stannate using first-principles phonon mode mapping
Accurately modelling polymorphism in crystalline solids remains a key challenge in computational
chemistry. In this work, we apply a theoretically-rigorous phonon mode-mapping approach to
understand the polymorphism in the ternary metal oxide Bi2Sn2O7. Starting from the high-temperature
cubic pyrochlore aristotype, we systematically explore the structural potential-energy surface and
recover the two known low-temperature phases alongside three new metastable phases, together with
the transition pathways connecting them. This first-principles lattice-dynamics method is completely
general and provides a practical means to identify and characterise the stable polymorphs and phase
transitions in materials with complex crystal structures
Amorphous Mixtures of Ice and C₆₀ Fullerene
Carbon and ice make up a substantial proportion of our universe. Recent space exploration has shown that these two chemical species often coexist such as on comets and asteroids and in the interstellar medium. Here, we prepare mixtures of C60 fullerene and H2O by vapor codeposition at 90 K with molar C60/H2O ratios ranging from 1:1254 to 1:5. The C60 percolation threshold is found between the 1:132 and 1:48 samples, corresponding to a transition from matrix-isolated C60 molecules to percolating C60 domains that confine H2O. Below this threshold, the crystallization and thermal desorption properties of H2O are not significantly affected by C60, whereas the crystallization temperature of H2O is shifted toward higher temperatures for the C60-rich samples. These C60-rich samples also display exotherms corresponding to the crystallization of C60 as the two components undergo phase separation. More than 60 vol % C60 is required to significantly affect the desorption properties of H2O. A thick blanket of C60 on top of pure amorphous ice is found to display large cracks due to water desorption. These findings may help us to understand the recently observed unusual surface features and the H2O weather cycle on the 67P/Churyumov–Gerasimenko comet
Tunable thermal expansion in framework materials through redox intercalation
Thermal expansion properties of solids are of fundamental interest and control of thermal
expansion is important for practical applications but can be difficult to achieve. Many
framework-type materials show negative thermal expansion when internal cages are empty
but positive thermal expansion when additional atoms or molecules fill internal voids present.
Here we show that redox intercalation offers an effective method to control thermal
expansion from positive to zero to negative by insertion of Li ions into the simple negative
thermal expansion framework material ScF3, doped with 10% Fe to enable reduction. The
small concentration of intercalated Li ions has a strong influence through steric hindrance of
transverse fluoride ion vibrations, which directly controls the thermal expansion. Redox
intercalation of guest ions is thus likely to be a general and effective method for controlling
thermal expansion in the many known framework materials with phonon-driven negative
thermal expansion
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