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.

Dimethylammonium zinc formate ([(CH3)2NH2]­Zn­(HCOO)3 or DMZnF) is a model system for the study of hybrid perovskite-like dielectrics. It undergoes a phase transition from the paraelectric to ferroelectric phase at ∼166 K, as observed via NMR spectra. The mechanism of this phase transition has been shown to have contributions from ordering of the hydrogen bonds between [(CH3)2NH2]+ (DMA+) and the formate groups as well as buckling of the metal-formate framework, but the transition dynamics and atomistic mechanism are not fully clear. This work presents dielectric constant measurements as evidence of cluster formation of the low-temperature phase and the relaxor-like behavior of this metal–organic framework above the phase transition temperature. 13C CP-MAS is used to track the evolution of the chemical shift, T 1, and T 2 of the dimethylammonium cation and formate groups from room temperature to 120 K. 2D 13C–13C correlation measurements provide evidence of the formation of pretransitional clusters above the phase transition temperature. Density functional theory (DFT) calculations support the assignment of chemical shifts and the proposed model. The analysis of 13C CP-MAS spectra and DFT calculations is used to discuss the mechanism of the dielectric phase transition and the origin of relaxor-like behavior in DMZnF.

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.

LiH2PO4 (LDP) is a favored candidate for hydrogen fuel cells, but the mechanism of its high protonic conductivity remains unclear. A complicating factor has been the lack of resolution in the reported proton NMR spectra. We now report multinuclear magic angle spinning NMR in LDP at magnetic fields up to 21.2 T. Well-resolved 1H NMR spectra are observed that are assignable to protons in the short and long O–H···O hydrogen bonds and a peak to physisorbed H2O. The position and intensity for the H2O peak depend on the H 2 O content, implying fast exchange between the adsorbed H2O and the O–H···O protons. 31P and 7Li NMR spectra and spin–lattice relaxation measurements showed that the proton hopping/exchange processes involve concerted hindered rotational fluctuations of the phosphate groups. Conductivity data from adsorbed H2O-controlled samples clearly suggest that the mechanism of LDP’s protonic conductivity is dominantly the exchange (and hopping) of the adsorbed H2O protons with the short O–H···O hydrogen bonds, in contrast to an earlier model that ascribed it to intermolecular hopping of O–H···O protons. The new findings enable us to modulate LDP’s protonic conductivity by several orders of magnitude via controlling physisorbed water.

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NH4H2AsO4 (ADA) is a model compound for understanding the mechanism of phase transitions in the KH2PO4 (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at T N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (1H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, T 1, of the O–H···O and NH4 + protons through the T N. As T → T N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its T N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at T N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH4 + protons. Both sets of protons exhibited order–disorder characteristics in their T 1 data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into order–disorder and displacive ones, should rather be “order–disorder cum displacive” type.

Superprotonic conduction in the LiH2PO4 system has been studied by means of high-resolution nuclear magnetic resonance measurements, which enabled us to distinguish dynamics of the two different hydrogen bonds in the structure. The protonic motion, primarily associated with the longer hydrogen bond, rather than the Li ionic motion, was revealed to dictate the extraordinarily high electrical conductivity of the system.

We have studied the metal–insulator transition (MIT) taking place at 340 K in vanadium dioxide nanoparticles. A peculiar nanosize effect on the 10-nm-sized VO2 nanoparticles is reported. While the infrared transmittance at high wave numbers displayed a broad transition behavior, the magnetic susceptibility and infrared transmittance at low wave numbers showed a sharp first-order MIT. Our results suggest that the size effect on the MIT is due to the surface region while the core region undergoes the same MIT as that in the bulk.