Coordination Changes in Densified Aluminate Glass upon Compression up to 65 GPa: A View from Solid-State Nuclear Magnetic Resonance.

Deciphering the structural evolution in irreversibly densified oxide glasses is crucial for fabricating functional glasses with tunable properties and elucidating the nature of pressure-induced anomalous plastic deformation in glasses. High-resolution NMR spectroscopy quantifies atomic-level structural information on densified glasses; however, its application is limited to the low-pressure range due to technical challenges. Here, we report the first high-resolution NMR spectra of oxide glass compressed by diamond anvil cells at room temperature, extending the pressure record of such studies from 24 to 65 GPa. The results constrain the densification path through coordination transformation of Al cations. Based on a statistical thermodynamic model, the stepwise changes in the Al fractions of oxide glasses and the effects of network polymerization on the densification paths are quantified. These results extend the knowledge on densification of the previously unattainable pressure conditions and contribute to understanding the origin of mechanical strengthening of the glasses.

Probing Medium-Range Order in Oxide Glasses at High Pressure.

Densification in glassy networks has traditionally been described in terms of short-range structures, such as how atoms are coordinated and how the coordination polyhedron is linked in the second coordination environment. While changes in medium-range structures beyond the second coordination shells may play an important role, experimental verification of the densification beyond short-range structures is among the remaining challenges in the physical sciences. Here, a correlation NMR experiment for prototypical borate glasses under compression up to 9 GPa offers insights into the pressure-induced evolution of proximity among cations on a medium-range scale. Whereas amorphous networks at ambient pressure may favor the formation of medium-range clusters consisting primarily of similar coordination species, such segregation between distinct coordination environments tends to decrease with increasing pressure, promoting a more homogeneous distribution of dissimilar structural units. Together with an increase in the average coordination number, densification of glass accompanies a preferential rearrangement toward a random distribution, which may increase the configurational entropy. The results highlight the direct link between the pressure-induced increase in medium-range disorder and the densification of glasses under extreme compression.

Proton-transfer-induced 3D/2D hybrid perovskites suppress ion migration and reduce luminance overshoot.

Perovskite light-emitting diodes (PeLEDs) based on three-dimensional (3D) polycrystalline perovskites suffer from ion migration, which causes overshoot of luminance over time during operation and reduces its operational lifetime. Here, we demonstrate 3D/2D hybrid PeLEDs with extremely reduced luminance overshoot and 21 times longer operational lifetime than 3D PeLEDs. The luminance overshoot ratio of 3D/2D hybrid PeLED is only 7.4% which is greatly lower than that of 3D PeLED (150.4%). The 3D/2D hybrid perovskite is obtained by adding a small amount of neutral benzylamine to methylammonium lead bromide, which induces a proton transfer from methylammonium to benzylamine and enables crystallization of 2D perovskite without destroying the 3D phase. Benzylammonium in the perovskite lattice suppresses formation of deep-trap states and ion migration, thereby enhances both operating stability and luminous efficiency based on its retardation effect in reorientation.

Magic angle spinning NMR study of the ferroelectric transition of KH2PO4: definitive evidence of a displacive component.

Variable temperature magic angle spinning (MAS) NMR measurements are reported on 1H and 31P nuclei in KH2PO4 (KDP) in the vicinity of its paraelectric-ferroelectric phase transition temperature, T c, of 123 K, to examine the transition mechanism, in particular if this is a model order-disorder type or whether it also involves a displacive component. It has been well established that the temperature variation of the isotropic chemical shift, δ iso, in NMR measurements of the nuclei directly involved in the transition should remain constant or change smoothly through T c for an order-disorder type transition but it should show an anomalous change for a displacive one. Here we demonstrate that the δ iso for both 31P and 1H nuclei in KDP show clear anomalies as a function of temperature around KDP's T c, providing direct evidence of a displacive component for the phase transition of KDP in contrast to the generally accepted notion that it is a model order-disorder type.