Smaller Bulk Modulus Research Articles

Exploration of lead-free novel double perovskite halides Na2TlBiX6 (X = Cl, Br, I) for flexible memory devices: Using DFT approach

Flexible resistive switching random access memory (RRAM) devices have drawn significant attention due to their low power consumption, high speed and simple structure. Halide perovskites with excellent optoelectronic properties such as high charge carrier mobility and tunable bandgap emerge as the best material for flexible RRAM devices. In this study, for the first time, the structural, mechanical, optoelectronic and thermoelectric properties of Na2TlBiX6 (X = Cl, Br, I) were explored using the Wien2k package for flexible RRAM devices. Elastic parameters show that studied perovskites display mechanical stability, ductile nature and anisotropic behaviour with lower values of the bulk modulus. In comparison, Na2TlBiI6 has a smaller bulk modulus which makes it a suitable material for flexible RRAM devices. Furthermore, the large machinability index of Na2TlBiI6 among the studied perovskites highlights its feasibility for industrial applications. The semiconductor nature of Na2TlBiX6 (X = Cl, Br, I) is confirmed from band structure, total density of states (TDOS) and partial density of states (PDOS) calculation. The optical study shows that Na2TlBiI6 can absorb a wide range of ultraviolet and visible electromagnetic incident radiations making it an excellent material for flexible RRAM devices. Lastly, in terms of transport properties at room temperature, Na2TlBiI6 reveals a larger figure of merit (ZT) than other studied perovskites making it a suitable candidate for thermoelectric applications. The outcome of this study suggests that Na2TlBiI6 is the perfect candidate for flexible RRAM devices at room temperature.

Pressure-dependent electronic, optical, and mechanical properties of antiperovskite X3NP (X = Ca, Mg): A first-principles study

Hydrostatic pressure provides an efficient way to tune and optimize the properties of solid materials without changing their composition. In this work, we investigate the electronic, optical, and mechanical properties of antiperovskite X3NP (X2+ = Ca, Mg) upon compression by first-principles calculations. Our results reveal that the system is anisotropic, and the lattice constant a of X3NP exhibits the fastest rate of decrease upon compression among the three directions, which is different from the typical Pnma phase of halide and chalcogenide perovskites. Meanwhile, Ca3NP has higher compressibility than Mg3NP due to its small bulk modulus. The electronic and optical properties of Mg3NP show small fluctuations upon compression, but those of Ca3NP are more sensitive to pressure due to its higher compressibility and lower unoccupied 3d orbital energy. For example, the band gap, lattice dielectric constant, and exciton binding energy of Ca3NP decrease rapidly as the pressure increases. In addition, the increase in pressure significantly improves the optical absorption and theoretical conversion efficiency of Ca3NP. Finally, the mechanical properties of X3NP are also increased upon compression due to the reduction in bond length, while inducing a brittle-to-ductile transition. Our research provides theoretical guidance and insights for future experimental tuning of the physical properties of antiperovskite semiconductors by pressure.

Elastic, anisotropic, lattice dynamics and electronic properties of XNiM and XNi2M (X = Ti, Zr, Hf; M = Sn, Ge, Si): DFT comparison study

The ab-initio calculations were performed using density functional theory (DFT) as implemented in the Quantum ESPRESSO (QE) code. Lattice parameter mismatch was minimal (2.5 % - 3 %) in TiNiSn/TiNi2Sn, ZrNiSn/ZrNi2Sn, and HfNiSn/HfNi2Sn compounds. Half-Heusler compounds had shorter bond lengths and smaller bulk moduli than their full-Heusler counterparts. Directional dependencies of the Young’s and shear moduli were investigated using elastic constants. TiNiGe, HfNiSn, and ZrNiSn had a greater universal anisotropy index among the half-Heusler compounds. Half-Heusler, TiNi2Sn, TiNi2Si, ZrNi2Sn, and HfNi2Sn compounds are reported to be dynamically stable. The electronic structure computations show that TiNiSn, ZrNiSn, and HfNiSn compounds have the least indirect energy gaps of 0.4508 eV, 0.5019 eV, and 0.3847 eV respectively. TiNiSn, ZrNiSn, and HfNiSn compounds are good candidates for minimizing the thermal conductivity of thermoelectric devices. TiNi2Sn, ZrNi2Sn, and HfNi2Sn are good candidates as possible contact electrodes for TiNiSn, ZrNiSn, and HfNiSn.

High-pressure behavior of tetragonal barium carbodiimide, BaNCN

The high-pressure behavior of tetragonal barium carbodiimide (BaNCN) was investigated by in-situ high-pressure X-ray diffraction measurements and first-principles theoretical calculations. No phase transition or decomposition of tetragonal BaNCN was observed during compression to 6.0 GPa and subsequent decompression to ambient pressure. The bulk modulus estimated from the unit cell volume with respect to the applied pressure was 69(2) GPa, which is comparable to that estimated by first-principles calculations. The theoretical calculations indicate that contraction of the Ba-N bond length has a crucial effect on the volume change in response to pressure, whereas the C-N bonds in the NCN 2- anions remain robust against the change in pressure. • High-pressure behavior of tetragonal BaNCN was examined by synchrotron XRD and DFT calculations. • Tetragonal BaNCN exhibits a small bulk modulus of 69(2) GPa. • The large compressibility of the BaNCN originates from the contraction of Ba-N bond. • The C-N bond in NCN 2- anion is robust with respect to pressure.

Gold Nanoparticle Thin Film-Based Strain Sensors for Monitoring Human Pulse

A concise and economical way to build nanoparticle thin-film strain sensors of high gauge factors on a flexible polydimethylsiloxane substrate is developed. The gauge factors for resistance change and capacitance change are much larger than those of many other competing sensors. The small bulk modulus of the substrate provides the sensor a large responsivity for pressure with a frequency bandwidth of kHz. It can be used for tactile sensing that is capable of monitoring human pulse for pre-clinical diagnosis, by resolving minor structures such as peaks of dicrotic pulse and before dicrotic pulse in the waveforms.

Mathematical analysis of the acoustic imaging modality using bubbles as contrast agents at nearly resonating frequencies

<p style='text-indent:20px;'>We analyze mathematically the acoustic imaging modality using bubbles as contrast agents. These bubbles are modeled by mass densities and bulk moduli enjoying contrasting scales. These contrasting scales allow them to resonate at certain incident frequencies. We consider two types of such contrasts. In the first one, the bubbles are light with small bulk modulus, as compared to the ones of the background, so that they generate the Minnaert resonance (corresponding to a local surface wave). In the second one, the bubbles have moderate mass density but still with small bulk modulus so that they generate a sequence of resonances (corresponding to local body waves). <p style='text-indent:20px;'>We propose to use as measurements the far-fields collected before and after injecting a bubble, set at a given location point in the target domain, generated at a band of incident frequencies and at a fixed <i>single backscattering direction</i>. Then, we scan the target domain with such bubbles and collect the corresponding far-fields. The goal is to reconstruct both the, variable, mass density and bulk modulus of the background in the target region. <p style='text-indent:20px;'>1.We show that, for each fixed used bubble, the contrasted far-fields reach their maximum value at, incident, frequencies close to the Minnaert resonance (or the body-wave resonances depending on the types of bubbles we use). Hence, we can reconstruct this resonance from our data. The explicit dependence of these resonances in terms of the background mass density of the background allows us to recover it, i.e. the mass density, in a straightforward way. <p style='text-indent:20px;'>2.In addition, this measured contrasted far-fields allow us to recover the total field at the location points of the bubbles (i.e. the total field in the absence of the bubbles). A numerical differentiation argument, for instance, allows us to recover the bulk modulus of the targeted region as well.

Minor Squirt in Unconsolidated Sands versus Strong Squirt in Compressed Glass Beads

Squirt driven by local pressure imbalance between contact of grains (or throat) and the main pore space is a mechanism of P-wave attenuation in consolidated rocks. In this paper, we investigate squirt in unconsolidated and consolidated porous media (represented by Toyoura sands and compressed glass beads, respectively). The former sample has very small bulk modulus and shear modulus, manifested by relatively free/mobile grains. As such, solid stress on the surface tends to be uniform and squirt should be minor. Biot’s theory improved with dynamic permeability successfully predicts the ultrasonic velocity and quality factor of P wave measured in the unconsolidated sands, confirming the aforementioned judgment. Dynamic permeability inverted at a high frequency is far lower than Darcy permeability. However, the improved model remains to be incapable of predicting velocity and attenuation measured in the latter sample; the reason is that the compressed beads allows a high pressure at compliant throat which drives strong squirt between throat and the main pore space.

Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination.

Low-cost solution-based synthesis of metal halide perovskites (MHPs) invariably introduces defects in the system, which could form Shockley-Read-Hall (SRH) electron-hole recombination centers detrimental to solar conversion efficiency. Here, we investigate the nonradiative recombination processes due to native point defects in methylammonium lead halide (MAPbI3) perovskites using ab initio nonadiabatic molecular dynamics within surface-hopping framework. Regardless of whether the defects introduce a shallow or deep band state, we find that charge recombination in MAPbI3 is not enhanced, contrary to predictions from SRH theory. We demonstrate that this strong tolerance against defects, and hence the breakdown of SRH, arises because the photogenerated carriers are only coupled with low-frequency phonons and electron and hole states overlap weakly. Both factors appreciably decrease the nonadiabatic coupling. We argue that the soft nature of the inorganic lattice with small bulk modulus is key for defect tolerance, and hence, the findings are general to other MHPs.

Size and morphology effects on the high pressure behaviors of Mn3O4 nanorods.

The high-pressure behaviors of Mn3O4 nanorods were studied by high pressure powder synchrotron X-ray diffraction and Raman spectroscopy. We found that the initial hausmannite phase transforms into the orthorhombic CaTi2O4-type structure, and then to the marokite-like phase upon compression. Upon decompression, the marokite-like phase is retained at the ambient pressure. Compared with Mn3O4 bulk and nanoparticles, Mn3O4 nanorods show obviously different phase transition behaviors. Upon compression, the phase transition sequence of Mn3O4 nanorods is similar with the nanoparticles, while the decompression behavior is consistent with the bulk counterparts. The hausmannite phase shows higher stability and smaller bulk modulus in Mn3O4 nanorods than those of the corresponding bulk and nanoparticles. We proposed that the higher phase stability and compressibility of the nanorods are concerned with their nanosize effects and the rod morphology. Both the growth orientation and the suppressed Jahn–Teller distortion of the Mn3O4 nanorods are crucial factors for their high pressure behaviors.

Element-selective elastic properties of Fe65Ni35 Invar alloy and Fe72Pt28 alloy studied by extended X-ray absorption fine structure

ABSTRACTWe present the element-selective local structure analysis of ferromagnetic FeNi Invar alloy and FePt alloy under high pressures as a recent application of nano-polycrystalline diamond (NPD) to extended X-ray absorption fine structure (EXAFS). The Invar effect is a phenomenon where thermal expansion becomes nearly zero in a wide range of temperatures. FePt is an alloy exhibiting negative thermal expansion. Our EXAFS analysis successfully revealed significant differences in element-selective elastic properties between local structures around Fe and Ni/Pt. The local structure around Fe has a smaller bulk modulus than that of Ni. Hence, the more compressive behavior around Fe is responsible for the anomalously soft elastic property that is characteristic of Invar alloy. In the case of FePt, Fe has a smaller bulk modulus than Pt; however, the difference in bulk modulus in the ferromagnetic phase is small compared with that of Invar alloy. We also report different trends of pressure-induced decrease in Debye-Waller factors toward paramagnetic phases.

Structural, electronic, and magnetic properties of CrMnX (X = Ge, Se, Si, and Sn) compounds

We have studied the structural, electronic, and magnetic properties of CrMnX (X = Ge, Se, Si, and Sn) compounds. The first principles band structure calculation within the framework of density functional theory was used to explore these properties. The full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the Wien2k software package has been used. We investigated the effect of compositional variation on lattice constants, bulk modulus, electronic, and magnetic properties. CrMnSi has the largest while CrMnSe has the smallest bulk modulus among the studied compounds. Our calculated electronic and magnetic properties for CrMnX (X = Ge, Se, Si, and Sn) compounds show that CrMnGe, CrMnSe, and CrMnSi are half-metallic materials with integer magnetic moments while CrMnSn has metallic behavior. These compounds are fascinating for spintronic devices due to their half-metallic properties.

Silver Oxalate: Mechanical Properties and Extreme Negative Mechanical Phenomena

AbstractThe crystal structure and mechanical properties of silver oxalate, Ag2C2O4, are determined using first‐principles solid‐state methods. The set of calculated mechanical properties include the bulk modulus and its pressure derivatives, the Young and shear moduli, the Poisson's Ratio and the ductility, hardness, and anisotropy indices. Silver oxalate is a highly anisotropic brittle material possessing a small bulk modulus of ≈9.6 GPa. It displays the negative Poisson's ratio (NPR) phenomenon, the value of the lowest NPR having a very large magnitude, −1.27. Besides, it exhibits the most extreme form of the negative linear compressibility (NLC) phenomenon found to date. Silver oxalate displays anisotropic NLC for external pressures in the range −0.1 to −2.4 GPa directed along the [010] crystallographic direction and isotropic NLC for isotropic pressures in the range −0.51 to 13.4 GPa. The lowest value of the negative compressibility, −831.9 ± 10 TPa−1, is found for an isotropic pressure of −0.16 GPa. The absolute value of the computed lowest NLC is extremely large, about three times larger than the absolute value of the lowest NLC found so far. The NLC pressure range is also very wide, its width being more than two times the largest range found to date.

Three-dimensional honeycomb carbon: Junction line distortion and novel emergent fermions

Carbon enjoys a vast number of allotropic forms, each possessing unique properties determined by the lattice structures and bonding characters. Here, based on first-principles calculations, we propose a new three-dimensional carbon allotrope—hC28. We show that hC28 possesses excellent energetic, dynamical, thermal, and mechanical stability. It is energetically more stable than most other synthesized or proposed carbon allotropes. The material has a relatively small bulk modulus, but is thermally stable at temperatures as high as 2000 K. The structural, mechanical, X-ray diffraction, and electronic properties are systematically investigated. Particularly, we show that its low-energy band structure hosts multiple unconventional emergent fermions, including the quadratic-contact-point fermions, the birefringent Dirac fermions, and the triple-point fermions. We construct effective models to characterize each kind of fermions. Our work not only discovers a new carbon allotropic form, it also reveals remarkable mechanical and electronic properties for this new material, which may pave the way towards both fundamental studies as well as practical applications.

Insights into the Structure of Liquid Water from Nuclear Quantum Effects on the Density and Compressibility of Ice Polymorphs.

Nuclear quantum effects lead to an anomalous shift of the volume of hexagonal ice; heavy ice has a larger volume than light ice. Furthermore, this anomaly in ice increases with temperature and persists in liquid water up to the boiling point. To gain more insight, we study nuclear quantum effects on the density and compressibility of several ice-like structures and crystalline ice phases. By calculating the anisotropic contributions to the stain tensor, we analyze how the compressibility changes along different directions in hexagonal ice, and find that hexagonal ice is softer along the x- y plane than the z-direction. Furthermore, by performing ab initio density functional theory calculations with a van der Waals functional and with the quasiharmonic approximation, we find an anomalous isotope effect in the bulk modulus of hexagonal ice: heavy ice has a smaller bulk modulus than light ice. In agreement with the experiments, we also obtain an anomalous isotope effect for clathrate hydrate structure I. For the rest of the ice polymorphs, the isotope effect is (i) anomalous for ice IX, Ih, Ic, clathrate, and low density liquid-like (LDL-like) amorphous ice; (ii) normal at T = 0 K and becomes anomalous with increasing temperature for ice IX, II, high density liquid-like (HDL-like) amorphous ices, and ice XV; and (iii) normal for ice VIII up to the melting point. There is a transition from an anomalous isotope effect to a normal isotope effect for both the volume and bulk modulus, as the density (compressibility) of the structures increases (decreases). This result can explain the anomalous isotope effect in liquid water: as the compressibility decreases from the melting point to the compressibility minimum temperature, the difference between the volumes of the heavy and light water rapidly decreases, but the effect stays anomalous up to the boiling temperature as the hydrogen bond network is never completely broken by fully filling all of the interstitial sites.

Comparative study on structural, elastic, dynamical, and thermodynamic properties of Weyl semimetals MX (M = Ta or Nb; X = As or P)

We present a comparative investigation on structural, elastic, dynamical and thermodynamic properties of Weyl semimetals MX (M = Ta or Nb; X = As or P) using density functional theory (DFT) within the generalized gradient approximation. The elastic properties of NbAs, TaP and NbP are obtained for the first time, then we compared them with each other and with some well-studied materials. Among four Weyl semimetals, TaP and NbAs possess the largest and smallest bulk modulus B, shear modulus G, and Young's modulus E, respectively, while NbP and TaAs own the maximum and minimum elastic Debye temperature. Through the analysis of three dimensional (3D) representations and two dimensional (2D) projections of Young's modulus, MX series exhibit distinct elastic anisotropy, especially for TaAs and NbAs. The calculated phonon dispersions of four Weyl semimetals show no imaginary frequency throughout the Brillouin zone, indicating they are dynamically stable. In addition, compared with other theoretical results, our calculated Brillouin-zone-center frequencies of MX series are more in line with experimental data. Furthermore, Phonon velocities are obtained using phonon spectra, and anisotropic phonon group velocities are responsible for their anisotropic lattice thermal conductivity. Additionally, thermodynamic properties are also predicted using the calculated phonon density of states. The results are in good agreement with available experimental values. We expect our work can provide more information for further experimental studies.

Negative thermal expansion in isostructural cubic ReO3 and ScF3: A comparative study

Negative thermal expansion (NTE) in isostructural cubic ReO3 and ScF3 were studied using first-principles calculations within the quasiharmonic approximation. The reproduced coefficients of NTE (CNTE) is in reasonable agreement with experiment and the NTE behavior was attributed to the vibrational modes which have the minimum negative values of Grüneisen parameters at M(0.5, 0.5, 0) and R(0.5, 0.5, 0.5) in Brillouin zone. The calculated CNTE of ReO3 is an order of magnitude smaller than that of ScF3 due to the stiffer covalent bonds in ReO3 as well as the vibrational modes which will generate a sharp energy-increasing antibonding state. Consequently, the NTE was suppressed in ReO3. Meanwhile, larger lattice constant and smaller bulk modulus than those of ReO3 also imply that ScF3 is expected to have larger coefficient of negative thermal expansion.

Structural Phase Transition and Photoluminescence Properties of YF3:Eu3+ Nanocrystals under High Pressure

High-pressure behaviors of YF3:Eu3+ nanocrystals with an average grain size of 40 nm were investigated by in situ high-pressure synchrotron radiation X-ray diffraction measurements up to 31.1 GPa at ambient temperature. The pressure-induced structural phase transition starts at 11.8 GPa and completes at 23.3 GPa. YF3:Eu3+ nanocrystals with a starting phase of orthorhombic structure transform into a high-pressure phase, which is inferred to be hexagonal structure. The high-pressure structure returned to the orthorhombic phase after release of pressure. The transition pressure is enhanced in nanosized YF3:Eu3+ as compared to submicrometer size samples, which is due to the surface energy differences between submicrometer size and nanosized materials. The nanosized samples of high-pressure phase were easier to compress with smaller bulk modulus than the submicrometer size samples. The photoluminescence properties of YF3:Eu3+ have also been studied in the pressure range from ambient pressure to 25.0 GPa at room temperature. Accompanied by the structure transformation, the Eu3+ ion luminescence from 5D0 → 7F1,2,3,4 transition in YF3:Eu3+ nanocrystals emerges obvious changes, which indicate the variation of the local symmetry of Eu3+ ions.

Mechanics of nanowire buckling on elastomeric substrates with consideration of surface stress effects

In this paper, the surface stress effects are considered in the analysis of nanowire buckling on elastomeric substrates. Theoretical solutions of the buckling wavenumber, amplitude, and the critical strain for the out-of-surface buckling are derived firstly. After that, the influences of the surface properties, size, bulk modulus, and the shape of the cross section of the nanowire on the out-of-surface and in-surface buckling behavior are systematically discussed. Our study indicates that the buckling mode of the nanowire can be greatly influenced by the surface effects. For nanowires with small cross section sizes and low bulk modulus, the buckling mode may be varied by tuning the surface properties. Based on the energy criterion, we explicitly determine the range of the parameters where the two buckling modes take place.

Structural, phonon and thermodynamic properties of fcc-based metal nitrides from first-principles calculations

Structural, phonon, and thermodynamic properties of fcc-based metal nitrides MN (M=Ti,Al,Zr,Hf) have been investigated by first-principles calculations within the local density approximation (LDA) and the generalized gradient approximation (GGA). The supercell method for lattice dynamics is utilized to calculate the phonon density of states, and the thermodynamic properties (heat capacity, enthalpy, entropy, and Gibbs energy) of these nitrides at elevated temperatures are predicted via the quasiharmonic approximation. We find that the lattice parameters predicted by GGA agree well with experiments compared to those from LDA. The GGA predicts a larger bond length and a smaller bulk modulus than those from LDA. It is found that the vibrational contribution to Gibbs energy is larger for HfN in comparison with other metal nitrides due to its higher population of phonon density of state in the low frequency region. Calculated structural, phonon, and thermodynamic properties are in good agreement with the available experiments and CALPHAD-type evaluations.

A new approach to the equation of state of silicate melts: An application of the theory of hard sphere mixtures

A comparison of compressional properties of silicate solids, glasses, and liquids reveals the following fundamental differences: (1) Liquids have much smaller bulk moduli than solids and glasses and the bulk moduli of various silicate melts have a narrow range of values; (2) Liquids do not follow the Birch’s law of corresponding state as opposed to solids and glasses; (3) The Grüneisen parameter increases with increasing pressure for liquids but decreases for solids; (4) The radial distribution functions of liquids show that the interatomic distances in liquids do not change upon compression as much as solids do. The last observation indicates that the compression of silicate melts occurs mostly through the geometrical arrangement of various units whose sizes do not change much with compression, i.e., the entropic mechanism of compression plays a dominant role over the internal energy contribution. All of the other three observations listed above can be explained by this point of view. In order to account for the role of the entropic contribution, we propose a new equation of state for multi-component silicate melts based on the hard sphere mixture model of a liquid. We assign a hard sphere for each cation species that moves in the liquid freely except for the volume occupied by other spheres. The geometrical arrangement of these spheres gives the entropic contribution to compression, while the Columbic attraction between all ions provides the internal energy contribution to compression. We calibrate the equation of state using the experimental data on room-pressure density and room-pressure bulk modulus of liquids. The effective size of a hard sphere for each component in silicate melts is determined. The temperature and volume dependencies of sphere diameters are also included in the model in order to explain the experimental data especially the melt density data at high pressures. All compressional properties of a silicate melt can be calculated using the calibrated sphere diameters. This equation of state provides a unified explanation for most of compressional behaviors of silicate melts and the experimental observations cited above including the uniformly small bulk moduli of silicate melts as well as the pressure dependence of Grüneisen parameters. With additional data to better constrain the key parameters, this equation of state will serve as a first step toward the unified equation of state for silicate melts.