ABSTRACT
Since the 1980s, the paddlewheel effect has been suggested as a mechanism to boost lithium-ion diffusion in inorganic materials via the rotation of rotor-like anion groups. However, it remains unclear whether the paddlewheel effect, defined as large-angle anion group rotations assisting Li hopping, indeed exists; furthermore, the physical mechanism by which the anion-group dynamics affect lithium-ion diffusion has not yet been established. In this work, we differentiate various types of rotational motions of anion groups and develop quaternion-based algorithms to detect, quantify, and relate them to lithium-ion motion in ab initio molecular dynamics simulations. Our analysis demonstrates that, in fact, the paddlewheel effect, where an anion group makes a large angle rotation to assist a lithium-ion hop, does not exist and thus is not responsible for the fast lithium-ion diffusion in superionic conductors, as historically claimed. Instead, we find that materials with topologically isolated anion groups can enhance lithium-ion diffusivity via a more classic nondynamic soft-cradle mechanism, where the anion groups tilt to provide optimal coordination to a lithium ion throughout the hopping process to lower the migration barrier. This anion-group disorder is static in nature, rather than dynamic and can explain most of the experimental observations. Our work substantiates the nonexistence of the long-debated paddlewheel effect and clarifies any correlation that may exist between anion-group rotations and fast ionic diffusion in inorganic materials.
ABSTRACT
The Li2S-P2S5 pseudo-binary system has been a valuable source of promising superionic conductors, with α-Li3PS4, ß-Li3PS4, HT-Li7PS6, and Li7P3S11 having excellent room-temperature Li-ion conductivity >0.1 mS/cm. The metastability of these phases at ambient temperature motivates a study to quantify their thermodynamic accessibility. Through calculating the electronic, configurational, and vibrational sources of free energy from first principles, a phase diagram of the crystalline Li2S-P2S5 space is constructed. New ground-state orderings are proposed for α-Li3PS4, HT-Li7PS6, LT-Li7PS6, and Li7P3S11. Well-established phase stability trends from experiments are recovered, such as polymorphic phase transitions in Li7PS6 and Li3PS4, and the instability of Li7P3S11 at high temperature. At ambient temperature, it is predicted that all superionic conductors in this space are indeed metastable but thermodynamically accessible. Vibrational and configurational sources of entropy are shown to be essential toward describing the stability of superionic conductors. New details of the Li sublattices are revealed and are found to be crucial toward accurately predicting configurational entropy. All superionic conductors contain significant configurational entropy, which suggests an inherent correlation between fast Li diffusion and thermodynamic stability arising from the configurational disorder.
ABSTRACT
BACKGROUND: A sudden, irreversible reduction in visual acuity ('wipe-out') is a feared complication of cataract surgery. Current literature on wipe-out is limited in quantity and quality, and largely predates modern cataract surgery and imaging techniques. The objectives of our study were to estimate the incidence of wipe-out and to identify potential risk factors. METHODS: We prospectively collated cases of wipe-out occurring in the UK during a 25-month study period using the British Ophthalmic Surveillance Unit reporting system. A total of 21 potential cases of wipe-out were reported, 5 of which met all inclusion and exclusion criteria. RESULTS: The estimated incidence of wipe-out during the study period was 0.00000298, or approximately 3 cases per million cataract operations. All cases of wipe-out occurred exclusively in patients with advanced glaucoma (mean deviation -21.0 decibels or worse in the operated eye), with an over-representation of black people (40%) in our case series. A prior diagnosis of retinal vein occlusion (60%) and elevated post-operative IOP (40%) were more common among individuals suffering from wipe-out compared to the general population, suggesting these factors may contribute to the pathogenesis of wipe-out. CONCLUSIONS: Our study shows that wipe-out is a rare complication, affecting approximately 3 per million undergoing cataract surgery. Patients with advanced glaucoma, black patients, and those with previous retinal vein occlusions may be at greater risk of wipe-out. We hope that the findings of our study will be used to help inform treatment decision-making and the cataract surgery consent process.
