Pressure-Induced Conductivity and Yellow-to-Black Piezochromism in a Layered Cu–Cl Hybrid Perovskite

Pressure-induced changes in the electronic structure of two-dimensional Cu-based materials have been a subject of intense study. In particular, the possibility of suppressing the Jahn–Teller distortion of d⁹ Cu centers with applied pressure has been debated over a number of decades. We studied the structural and electronic changes resulting from the application of pressures up to ca. 60 GPa on a two-dimensional copper­(II)–chloride perovskite using diamond anvil cells (DACs), through a combination of in situ powder X-ray diffraction, electronic absorption and vibrational spectroscopy, dc resistivity measurements, and optical observations. Our measurements show that compression of this charge-transfer insulator initially yields a first-order structural phase transition at ca. 4 GPa similar to previous reports on other Cu^II–Cl perovskites, during which the originally translucent yellow solid turns red. Further compression induces a previously unreported phase transition at ca. 8 GPa and dramatic piezochromism from translucent red-orange to opaque black. Two-probe dc resistivity measurements conducted within the DAC show the first instance of appreciable conductivity in Cu^II–Cl perovskites. The conductivity increases by 5 orders of magnitude between 7 and 50 GPa, with a maximum measured conductivity of 2.9 × 10^–4 S·cm^–1 at 51.4 GPa. Electronic absorption spectroscopy and variable-temperature conductivity measurements indicate that the perovskite behaves as a 1.0 eV band-gap semiconductor at 39.7 GPa and has an activation energy for electronic conduction of 0.232(1) eV at 40.2 GPa. Remarkably, all these changes are reversible: the material reverts to a translucent yellow solid upon decompression, and ambient pressure powder X-ray diffraction data taken before and after compression up to 60 GPa show that the original structure is maintained with minimal hysteresis

Pressure-Induced Metallization of the Halide Perovskite (CH₃NH₃)PbI₃

We report the metallization of the hybrid perovskite semiconductor (MA)­PbI₃ (MA = CH₃NH₃⁺) with no apparent structural transition. We tracked its bandgap evolution during compression in diamond-anvil cells using absorption spectroscopy and observed strong absorption over both visible and IR wavelengths at pressures above ca. 56 GPa, suggesting the imminent closure of its optical bandgap. The metallic character of (MA)­PbI₃ above 60 GPa was confirmed using both IR reflectivity and variable-temperature dc conductivity measurements. The impressive semiconductor properties of halide perovskites have recently been exploited in a multitude of optoelectronic applications. Meanwhile, the study of metallic properties in oxide perovskites has revealed diverse electronic phenomena. Importantly, the mild synthetic routes to halide perovskites and the templating effects of the organic cations allow for fine structural control of the inorganic lattice. Pressure-induced closure of the 1.6 eV bandgap in (MA)­PbI₃ demonstrates the promise of the continued study of halide perovskites under a range of thermodynamic conditions, toward realizing wholly new electronic properties

Hydrogen-Bond Symmetrization Breakdown and Dehydrogenation Mechanism of FeO₂H at High Pressure

The cycling of hydrogen plays an important role in the geochemical evolution of our planet. Under high-pressure conditions, asymmetric hydroxyl bonds tend to form a symmetric O–H–O configuration in which H is positioned at the center of two O atoms. The symmetrization of O–H bonds improves their thermal stability and as such, water-bearing minerals can be present deeper in the Earth’s lower mantle. However, how exactly H is recycled from the deep mantle remains unclear. Here, we employ first-principles free-energy landscape sampling methods together with high pressure-high temperature experiments to reveal the dehydrogenation mechanism of a water-bearing mineral, FeO₂H, at deep mantle conditions. Experimentally, we show that ∼50% H is released from symmetrically hydrogen-bonded ε-FeO₂H upon transforming to a pyrite-type phase (Py-phase). By resolving the lowest-energy transition pathway from ε-FeO₂H to the Py-phase, we demonstrate that half of the O–H bonds in the mineral rupture during the structural transition, leading toward the breakdown of symmetrized hydrogen bonds and eventual dehydrogenation. Our study sheds new light on the stability of symmetric hydrogen bonds during structural transitions and provides a dehydrogenation mechanism for hydrous minerals existing in the deep mantle

High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties

We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)­PbX₃ (MA = CH₃NH₃⁺, X = Br^– or I^–) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites’ precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)­PbI₃ obtained within a diamond-anvil cell show that the material’s resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, suggesting that the perovskite is approaching a metallic state. Furthermore, the pressure response of mixed-halide perovskites shows new luminescent states that emerge at elevated pressures. We recently reported that the perovskites (MA)­Pb­(Br_<i>x</i>I_1–<i>x</i>)₃ (0.2 < <i>x</i> < 1) reversibly form light-induced trap states, which pin their PL to a low energy. This may explain the low voltages obtained from solar cells employing these absorbers. Our high-pressure PL data indicate that compression can mitigate this PL redshift and may afford higher steady-state voltages from these absorbers. These studies show that pressure can significantly alter the transport and thermodynamic properties of these technologically important semiconductors

Effects of Molecular Geometry on the Properties of Compressed Diamondoid Crystals

Diamondoids are an intriguing group of carbon-based nanomaterials, which combine desired properties of inorganic nanomaterials and small hydrocarbon molecules with atomic-level uniformity. In this Letter, we report the first comparative study on the effect of pressure on a series of diamondoid crystals with systematically varying molecular geometries and shapes, including zero-dimensional (0D) adamantane; one-dimensional (1D) diamantane, [121]­tetramantane, [123]­tetramantane, and [1212]­pentamantane; two-dimensional (2D) [12312]­hexamantane; and three-dimensional (3D) triamantane and [1­(2,3)­4]­pentamantane. We find the bulk moduli of these diamondoid crystals are strongly dependent on the diamondoids’ molecular geometry with 3D [1­(2,3)­4]­pentamantane being the least compressible and 0D adamantane being the most compressible. These diamondoid crystals possess excellent structural rigidity and are able to sustain large volume deformation without structural failure even after repetitive pressure loading cycles. These properties are desirable for constructing cushioning devices. We also demonstrate that lower diamondoids outperform the conventional cushioning materials in both the working pressure range and energy absorption density

Strain-Induced Modification of Optical Selection Rules in Lanthanide-Based Upconverting Nanoparticles

NaYF₄:Yb³⁺,Er³⁺ nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter

Strain-Induced Modification of Optical Selection Rules in Lanthanide-Based Upconverting Nanoparticles

NaYF₄:Yb³⁺,Er³⁺ nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter

Bright, Mechanosensitive Upconversion with Cubic-Phase Heteroepitaxial Core–Shell Nanoparticles

Lanthanide-doped nanoparticles are an emerging class of optical sensors, exhibiting sharp emission peaks, high signal-to-noise ratio, photostability, and a ratiometric color response to stress. The same centrosymmetric crystal field environment that allows for high mechanosensitivity in the cubic-phase (α), however, contributes to low upconversion quantum yield (UCQY). In this work, we engineer brighter mechanosensitive upconverters using a core–shell geometry. Sub-25 nm α-NaYF₄:Yb,Er cores are shelled with an optically inert surface passivation layer of ∼4.5 nm thickness. Using different shell materials, including NaGdF₄, NaYF₄, and NaLuF₄, we study how compressive to tensile strain influences the nanoparticles’ imaging and sensing properties. All core–shell nanoparticles exhibit enhanced UCQY, up to 0.14% at 150 W/cm², which rivals the efficiency of unshelled hexagonal-phase (β) nanoparticles. Additionally, strain at the core–shell interface can tune mechanosensitivity. In particular, the compressive Gd shell results in the largest color response from yellow-green to orange or, quantitatively, a change in the red to green ratio of 12.2 ± 1.2% per GPa. For all samples, the ratiometric readouts are consistent over three pressure cycles from ambient to 5 GPa. Therefore, heteroepitaxial shelling significantly improves signal brightness without compromising the core’s mechano-sensing capabilities and further, promotes core–shell cubic-phase nanoparticles as upcoming in vivo and in situ optical sensors

Upconverting Nanoparticles as Optical Sensors of Nano- to Micro-Newton Forces

Mechanical forces affect a myriad of processes, from bone growth to material fracture to touch-responsive robotics. While nano- to micro-Newton forces are prevalent at the microscopic scale, few methods have the nanoscopic size and signal stability to measure them in vivo or in situ. Here, we develop an optical force-sensing platform based on sub-25 nm NaYF₄ nanoparticles (NPs) doped with Yb³⁺, Er³⁺, and Mn²⁺. The lanthanides Yb³⁺ and Er³⁺ enable both photoluminescence and upconversion, while the energetically coupled <i>d</i>-metal Mn²⁺ adds force tunability through its crystal field sensitivity. Using a diamond anvil cell to exert up to 3.5 GPa pressure or ∼10 μN force per particle, we track stress-induced spectral responses. The red (660 nm) to green (520, 540 nm) emission ratio varies linearly with pressure, yielding an observed color change from orange to red for α-NaYF₄ and from yellow–green to green for <i>d</i>-metal optimized β-NaYF₄ when illuminated in the near infrared. Consistent readouts are recorded over multiple pressure cycles and hours of illumination. With the nanoscopic size, a dynamic range of 100 nN to 10 μN, and photostability, these nanoparticles lay the foundation for visualizing dynamic mechanical processes, such as stress propagation in materials and force signaling in organisms

A₂TiO₅ (A = Dy, Gd, Er, Yb) at High Pressure

The structural evolution of lanthanide A₂TiO₅ (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (<i>Pnma</i>) phase for a certain pressure range. Orthorhombic Dy₂TiO₅ and Gd₂TiO₅ form <i>P</i>2₁<i>am</i> at pressures below 9 GPa and <i>Pnma</i> above 13 GPa. Pyrochlore-type Dy₂TiO₅ and Er₂TiO₅ as well as defect-fluorite-type Yb₂TiO₅ form <i>Pnma</i> at ∼21 GPa, followed by <i>Im</i>3<i>̅m</i>. Hexagonal Dy₂TiO₅ forms <i>Pnma</i> directly, although a small amount of remnants of hexagonal Dy₂TiO₅ is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy₂TiO₅ phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (<i>Im</i>3<i>̅m</i>) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er₂TiO₅. The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A₂BO₅ composition at high pressure