Pressure-induced Lattice Research Articles

Anisotropic optical response of Nb2SiTe4 under pressure

The optical response of a layered anisotropic crystal Nb2SiTe4 was investigated under varying pressure using synchrotron infrared spectroscopy, Raman spectroscopy, and first-principles calculation. This study revealed diverse trends in optical response and bandgap at different pressure levels. Below 5 GPa, the bandgap decreases rapidly due to the reduction of interlayer distance. The Raman and infrared optical response show significant changes at 11 and 21 GPa, suggesting structural and electronic structure transformation at these pressure points. Meanwhile, the optical responses exhibited distinct pressure effects along different crystal axes. Overall, these results provide valuable insights into the pressure-induced lattice deformation and optical transitions in Nb2SiTe4 and similar layered anisotropic materials, contributing to an insightful understanding of layered materials under pressure.

Coevolution of superconductivity and Hall coefficient with anisotropic lattice shrinkage in compressed KCa2Fe4As4F2

Abstract The stability of superconductivity in superconductors is widely recognized to be determined by various factors, including charge, spin, orbit, lattice, and other related degrees of freedom. Here, we report our findings on the pressure-induced coevolution of superconductivity and Hall coefficient in KCa2Fe4As4F2, an iron-based superconductor possessing a hybrid crystal structure combining KFe2As2 and CaFeAsF. Our investigation, involving high-pressure resistance, Hall effect and x-ray diffraction (XRD) measurements, allows us to observe the connection of the superconductivity and Hall coefficient with the anisotropic lattice shrinkage. We find that its ambient-pressure tetragonal (T) phase presents a collapse starting at around 18 GPa, where the sign of the Hall coefficient (R H) changes from positive to negative. Upon further compression, both superconducting transition temperature (T c) and R H exhibit a monotonous decrease. At around 41 GPa, the superconductivity is completely suppressed (T c = 0), where the parameter a begins to decline again and the Hall coefficient remains nearly unchanged. Our experiment results clearly demonstrate that the pressure-induced anisotropic lattice collapse plays a crucial role in tuning the interplay among multiple degrees of freedom in the superconducting system and, correspondingly, the stability of the superconductivity.

Pressure-induced core defects and photoluminescent quenching in carbon quantum dots

Carbon quantum dots (CDs) with favorable luminescent features for biphotonic applications have attracted much interest in modulating their photoluminescence (PL) efficiency. A surface state with various defects is believed to play a key role in the emissive intensity. Here, pressure-induced quenching of PL is observed in red emissive CDs (R-CDs) and is ascribed to defects in carbon cores upon compression. In the power-law fitting to the excitation power-dependent PL of R-CDs at high pressure, the coefficient k parameter related to the emissive mechanism decreases from 1 under ambient pressure to much less than 1 under the application of pressure, suggesting a transition from single exciton recombination to defect-related emission. With the k parameter decreasing to 0.69 at 1.6 GPa, the pressure-induced defects reduce the PL intensity by approximately one order of magnitude. Furthermore, the attenuation and broadening of the G band characterizing the sp2 hybrid structure of carbon cores in the Raman spectra for R-CDs at high pressure support that the pressure-induced lattice relaxation impairs the crystalline symmetry of the carbon core and results in the dramatic quenching of PL. Our results highlight the importance of the well-crystallized carbon core in designing CDs with high quantum yields.

Thermal expansion and compressibility of alamosite (PbSiO3) determined by in-situ synchrotron X-ray diffraction

This manuscript presents an investigation of alamosite, PbSiO3, obtained by crystallization of lead metasilicate glass, PbO·SiO2. Rietveld refinement of synchrotron X-ray diffraction (SXRD) measurements as a function of temperature and pressure confirmed the stable crystalline structure of PbSiO3, with a monoclinic crystal system in the P12/n1 (13) space group. The samples were heated to 973 K at ambient pressure and compressed to 6 GPa at room-temperature. Furthermore, the in situ temperature-induced lattice expansion and pressure-induced lattice compression are reported, providing the thermal expansion coefficients, compressibility and bulk modulus, important parameters determining physical properties of glass-ceramic materials.

Atomic-scale mapping of pressure-induced deformations and phase defects in the charge density wave order parameter

Understanding the intricate interplay between multiple electronic phases in quantum materials such as charge density wave (CDW), superconducting, and metallic phases is a challenging issue. Systematic introduction of pressure is one approach that has been used to probe this interplay. However, the influence of pressure together with the intricate interaction between electronic and lattice degrees of freedom can trigger complex structural evolution and distribution of various electronic phases at the atomic scale, the characterization of which demands high spatial resolution. We investigate the atomic-scale response of the charge density waves and the underlying atomic lattice in $1T\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ after exposure to hydrostatic pressure. High-resolution transmission electron microscopy images show that the CDW order parameter reacts with an elasticlike strain response to pressure-induced stacking faults and dislocations in the lattice. This is characterized by a proliferation of phase defects including CDW dislocations, discommensurations, and domain walls. Our results evidence the importance of pressure-induced lattice deformations and defects in modulating, stabilizing, or destroying electronic phases at the atomic scale.

Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression.

Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2–3 times faster at 0.3 GPa than at ambient pressure. Our electron–phonon coupling calculations reveal ∼2-fold stronger electron–phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high charge-carrier densities.

High-Pressure Band-Gap Engineering and Metallization in the Perovskite Derivative Cs3 Sb2 I9.

Among photovoltaic materials, the antimony-based, perovskite-like structure Cs3 Sb2 I9 stands out owing to its low toxicity and air stability. Here, changes in the optoelectronic properties and crystal structure of the lead-free perovskite derivative Cs3 Sb2 I9 are reported, caused by pressure-induced lattice compression. At 20.0 GPa, Cs3 Sb2 I9 with a wide band gap (2.34 eV) successfully broke through the Shockley-Queisser limit (1.34 eV), accompanied by clear piezochromism from orange-yellow to opaque black. Additionally, Cs3 Sb2 I9 experienced completely reversible amorphization at 20.0 GPa. These optical changes could be attributed to atomic-orbital overlap enhancement caused by contraction of the Sb-I bond length and diminution of the Sb-I bond angle. In addition, Cs3 Sb2 I9 underwent a transition from semiconductor to conductor upon compression and obtained metallic properties at 44.3 GPa, indicating new electronic properties. The obtained results may further broaden the research prospects of halide perovskite materials in the field of photovoltaics.

Pressure-Engineered Structural and Optical Properties of Two-Dimensional (C4H9NH3)2PbI4 Perovskite Exfoliated nm-Thin Flakes.

Resolving the structure-property relationships of two-dimensional (2D) organic-inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0-10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42- layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb-I-Pb⟩ bond angle and increase in Pb-I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/a phase above 1.4 GPa increases the ⟨Pb-I-Pb⟩ bond angle and decreases the Pb-I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P21/a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C4H9NH3)2PbI4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Pressure-Induced Destabilization and Anomalous Lattice Distortion in TcO2.

Tc-based oxides are of interest because of their complex crystalline structures. In this work, the phonon dispersions, lattice distortions, and elastic constants of TcO2 at external pressures up to 120 GPa were comprehensively studied using first-principles calculations. It is found that the lattice dynamic stability of TcO2 can be assessed by fitting the Γ-Z acoustical phonon branch. The applied external pressure can be divided into three ranges: the low-pressure stable range, the middle-pressure buckling range, and the high-pressure unstable range. Interestingly, the variation tendency of the low-pressure stable range is very close to that of the high-pressure unstable range. On the other hand, the TcO2 lattice responds intensely to external pressure in the middle-pressure buckling range, which can be sustained under about 71 GPa pressure. More importantly, we have unraveled the pressure-induced lattice distortion in TcO2, which leads to anomalous behaviors for the lattice constants, Tc-O bond lengths, and elastic constants at 10 and 20 GPa external pressures.

Pressure-Temperature Phase Diagram Reveals Spin-Lattice Interactions in Co[N(CN)2]2.

Diamond anvil cell techniques, synchrotron-based infrared and Raman spectroscopies, and lattice dynamics calculations are combined with prior magnetic property work to reveal the pressure-temperature phase diagram of Co[N(CN)2]2. The second-order structural boundaries converge on key areas of activity involving the spin state exposing how the pressure-induced local lattice distortions trigger the ferromagnetic → antiferromagnetic transition in this quantum material.

Theoretical study of B3-to-B1 phase transition in ZnS

The pressure-induced phase transformation from B3 to B1 structures in ZnS using first-principle projector-augmented wave method is studied. To understand the nature and driving force behind the transition, the interesting properties in both phases, including enthalpy, phonon dispersion curves and elastic constants, are systematically investigated. The results show that the calculated transition pressure is within the range of 16.33 GPa to 19.04 GPa, which is in good agreement with the available experimental and theoretical data. The transition process can be viewed as the appearance and disappearance of very slight lattice distortion accompanied by the movement of Zn and S atoms along the [111] crystallographic axis. The physical driving force of the B3–B1 phase transition is confirmed to be a coupling effect between the mechanical instability of B3 phase under pressure and the softening acoustic phonon mode resulting from the pressure-induced lattice deformation. For B1 phase, it is further predicted that a new phase transition takes place at about 59.9 GPa.

Local Lattice Distortions in Mn[N(CN)2]2 under Pressure.

We combined synchrotron-based infrared spectroscopy, Raman scattering, and diamond anvil cell techniques with complementary lattice dynamics calculations to reveal local lattice distortions in Mn[N(CN)2]2 under compression. Strikingly, we found a series of transitions involving octahedral counter-rotations, changes in the local Mn environment, and deformations of the superexchange pathway. In addition to reinforcing magnetic property trends, these pressure-induced local lattice distortions may provide an avenue for the development of new functionalities.

Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles.

NaYF4:Yb(3+),Er(3+) 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.

Pressure induced lattice anomalies in pnictides

Hydrostatic pressure Raman and synchrotron XRD measurements at room temperature have been carried out on a series of NdFeAsO1−xFy (Nd1111) oxypnictides in order to investigate pressure-induced lattice modifications. The synchrotron XRD data indicate that there is an increased deviation of the lattice constants from smooth pressure dependence in the superconducting compound, in close agreement with the results from SmFeAsO1−xFx (Sm1111), although the effect is less pronounced in Nd1111. As in Sm1111 the hydrostatic pressure Raman measurements show that the A1g mode of the rare earth atom deviates from the linear pressure dependence. Anomalous pressure dependence and a hysteresis is observed in the phonon width of the phonon modes. The calculated Grüneisen parameter for the Nd phonon is very similar to the corresponding value for SmFeAsO1−xFx compound and it does not vary with substitution. For the As mode it has a lower value indicating a stiffer phonon with the increased ion size. In connection with the XRD measurements the Raman data show a sudden increase of the pressure-induced lattice anomalies close to doping where the compounds become superconducting.

Bandgap closure and reopening inCsAuI3at high pressure

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound ${\mathrm{CsAuI}}_{3}$, where Au adopts ${\mathrm{Au}}^{\mathrm{I}}$ and ${\mathrm{Au}}^{\mathrm{III}}$ valency. Raman spectroscopy shows softening with pressure of the vibration modes in the ${\mathrm{Au}}^{\mathrm{III}}$-${\mathrm{I}}_{4}$ square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds ${\mathrm{CsAuCl}}_{3}$ and ${\mathrm{CsAuBr}}_{3}$. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of ${\mathrm{CsAuI}}_{3}$, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.

Pressure-Induced Local Lattice Distortions in α-Co[N(CN)2]2

This work brings together diamond anvil cell techniques, vibrational spectroscopies, and complementary lattice dynamics calculations to investigate pressure-induced local lattice distortions in α-Co[N(CN)2]2. Analysis of mode behavior and displacement patterns reveals a series of pressure-driven transitions that modify the CoN6 counter-rotations, distort the octahedra, and flatten the C-N(ax)-C linkages. These local lattice distortions may be responsible for the low temperature magnetic crossover. We also discuss prospects for negative thermal expansion and show that there is not a straightforward low pressure pathway between the pink α and blue β ambient pressure phases of Co[N(CN)2]2.

Direct Observation of a Pressure-Induced Precursor Lattice in Silicon

Detailed knowledge of atomic-scale structural change is essential for understanding the process and mechanism of phase transitions in solids. We present the direct experimental evidence of a precursor lattice in silicon at high pressures. The precursor lattice may appear to coexist dynamically with the host lattice over a large pressure range through rapid lattice fluctuations. The first-principles calculations are used to elucidate a dynamic lattice-fluctuation mechanism that accounts for the experimental observations. This precursor lattice-fluctuation mechanism for the phase transition goes beyond previously considered reconstructive or displacive processes and provides a novel picture of the underlying dynamics.

Pressure-induced lattice instabilities and superconductivity inYBa2Cu4O8and optimally dopedYBa2Cu3O7−δ

Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate the pressure-induced lattice instabilities that are accompanied by T$_{\rm c}$ anomalies in YBa$_{\rm 2}$Cu$_{\rm 4}$O$_{\rm 8}$, in comparison with the optimally doped YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 7-\delta}$ and the non-superconducting PrBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm 6.92}$. In the first two superconducting systems there is a clear anomaly in the evolution of the lattice parameters and an increase of lattice disorder with pressure, that starts at $\approx3.7 GPa$ as well as irreversibility that induces a hysteresis. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. In complete agreement with the structural data, the micro-Raman data of the superconducting compounds show that the energy and width of the A$_{\rm g}$ phonons show anomalies at the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O$_{pl}$ bond length exhibits a strong contraction and correlate with the non-linear pressure dependence of T$_{\rm c}$. This is not the case for the non superconducting Pr sample, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.

Pressure-induced lattice collapse in the tetragonal phase of single-crystallineFe1.05Te

Pressure-induced lattice collapse was discovered in tetragonal (T) phase of single crystal ${\text{Fe}}_{1.05}\text{Te}$ at room temperature through x-ray and neutron-diffraction measurements. A remarkable compression along the $c$ axis $(\ensuremath{\sim}5%)$ was observed upon increasing pressure from the ambient condition to 4 GPa. Indexed results demonstrate that the crystallographic structure remains unchanged after the collapse, revealing that the collapse does not break symmetry of crystal structure. The Fe-spin state change was proposed to account for the lattice collapse. The equations of state for the T phase and pressure-induced collapsed T phase were determined from the diffraction measurements.

Pressure-induced melting of charge-order in the self-doped Mott insulatorYNiO3

The stability under pressure of charge order $({\mathrm{Ni}}^{3+\ensuremath{\delta}}/{\mathrm{Ni}}^{3\ensuremath{-}\ensuremath{\delta}})$ in ${\mathrm{YNiO}}_{3}$ has been investigated by infrared spectroscopy and x-ray synchrotron diffraction studies up to 23 GPa. We report the observation of a sudden electronic and structural transition in the pressure dependence of ${\mathrm{YNiO}}_{3}$ at $\ensuremath{\approx}14\mathrm{GPa}.$ The pressure-induced lattice evolution is consistent with a ${P2}_{1}/n$ to $\mathrm{Pbnm}$ structural transformation similar to the previously reported thermal melting of the charge-ordered state. Phonon screening indicating rapid electron hopping in the high-pressure phase has been observed from infrared measurements. However, although the conductivity increases due to partial charge delocalization, a metallic phase was not confirmed above the pressure-induced transition.