Transitions In Na Research Articles

Phase Transitions in Na(Nb0.9Me0.1)O3–δ (Me = Bi, Fe) Solid Solutions

Phase Transitions in Na(Nb0.9Me0.1)O3–δ (Me = Bi, Fe) Solid Solutions

HCN and HCO+ in Planetary Nebulae: The Next Level

Abstract Observations of HCN and HCO+ have been carried out toward 13 planetary nebulae (PNe) using the facilities of the Arizona Radio Observatory (ARO). These nebulae represent a wide range of morphologies and ages (∼2000–28,000 yr). For both molecules, the J = 1 → 0 transitions at 88–89 GHz and the J = 3 → 2 lines at 265–267 GHz were measured, together with CO lines (J = 1 → 0, 2 → 1, and 3 → 2, depending on the source), using the ARO 12 m and Submillimeter Telescopes. HCN and HCO+ were detected with at least one transition in 10 nebulae: He 2-459, Hu 1-1, K3-52, K3-65, M1-8, M1-40, M1-59, M2-53, M4-17, and NGC 6445. HCO+ was additionally identified via two transitions in Na 2. Some observed line profiles were complex, with multiple velocity components tracing varied outflows. From radiative transfer modeling, column densities were established for HCN and HCO+: N tot(HCN) = 0.005–1.1 × 1014 and N tot(HCO+) = 0.008–9.5 × 1013 cm−2. Gas densities of n(H2) ∼ 105–107 cm−3 were also determined for all PNe. Fractional abundances with respect to H2, calculated using CO as a proxy, are f(HCN) ∼ 0.2–1.5 × 10−7 and f(HCO+) ∼ 0.3–5.1 × 10−8. The abundances of HCN and HCO+ did not significantly vary with nebular age to 28,000 yr. Combined with previous observations, at least 30 PNe contain HCN and/or HCO+, indicating that polyatomic molecules are common constituents of these objects. The data strongly support a scenario where dense ejecta from PNe seed the interstellar medium with molecular material.

Significance of uncertain phasing between the onsets of stadial–interstadial transitions in different Greenland ice core proxies

Abstract. Different paleoclimate proxy records evidence repeated abrupt climate transitions during previous glacial intervals. These transitions are thought to comprise abrupt warming and increase in local precipitation over Greenland, sudden reorganization of the Northern Hemisphere atmospheric circulation, and retreat of sea ice in the North Atlantic. The physical mechanism underlying these so-called Dansgaard–Oeschger (DO) events remains debated. A recent analysis of Greenland ice core proxy records found that transitions in Na+ concentrations and δ18O values are delayed by about 1 decade with respect to corresponding transitions in Ca2+ concentrations and in the annual layer thickness during DO events. These delays are interpreted as a temporal lag of sea-ice retreat and Greenland warming with respect to a synoptic- and hemispheric-scale atmospheric reorganization at the onset of DO events and may thereby help constrain possible triggering mechanisms for the DO events. However, the explanatory power of these results is limited by the uncertainty of the transition onset detection in noisy proxy records. Here, we extend previous work by testing the significance of the reported lags with respect to the null hypothesis that the proposed transition order is in fact not systematically favored. If the detection uncertainties are averaged out, the temporal delays in the δ18O and Na+ transitions with respect to their counterparts in Ca2+ and the annual layer thickness are indeed pairwise statistically significant. In contrast, under rigorous propagation of uncertainty, three statistical tests cannot provide evidence against the null hypothesis. We thus confirm the previously reported tendency of delayed transitions in the δ18O and Na+ concentration records. Yet, given the uncertainties in the determination of the transition onsets, it cannot be decided whether these tendencies are truly the imprint of a prescribed transition order or whether they are due to chance. The analyzed set of DO transitions can therefore not serve as evidence for systematic lead–lag relationships between the transitions in the different proxies, which in turn limits the power of the observed tendencies to constrain possible physical causes of the DO events.

Dynamics of phase transitions in Na2TiO3 and its possible utilization as a CO2 sorbent: a critical analysis

Thermally-driven structural changes of Na2TiO3 and their effect on its carbonation behaviour.

The translational landscape of ground state pluripotency

Translational control plays a central role in regulation of gene expression and can lead to significant divergence between mRNA- and protein-abundance. Here, we used genome-wide approaches combined with time-course analysis to measure the mRNA-abundance, mRNA-translation rate and protein expression during the transition of naïve-to-primed mouse embryonic stem cells (ESCs). We find that the ground state ESCs cultured with GSK3-, MEK-inhibitors and LIF (2iL) display higher ribosome density on a selective set of mRNAs. This set of mRNAs undergo strong translational buffering to maintain stable protein expression levels in 2iL-ESCs. Importantly, we show that the global alteration of cellular proteome during the transition of naïve-to-primed pluripotency is largely accompanied by transcriptional rewiring. Thus, we provide a comprehensive and detailed overview of the global changes in gene expression in different states of ESCs and dissect the relative contributions of mRNA-transcription, translation and regulation of protein stability in controlling protein abundance.

Pressure-Induced Structural Transitions in Na–Li Silicate Glasses under Compression

The atomic structures of alkali silicate glasses with more than two types of nonframework cations (e.g., Li and Na) at high pressure provide insights into the effects of pressure on the transport p...

A comparative analysis of surface hopping acceptance and decoherence algorithms within the neglect of back-reaction approximation.

We have implemented a Python-based software package within the Libra software for performing nonadiabatic molecular dynamics (NA-MD) within the neglect of back reaction approximation (NBRA). Available in the software are a wide variety of proposed hop acceptance (PHA) and decoherence methodologies. Using Libra, a comparative analysis of PHA schemes and decoherence methods is performed to examine thermal equilibrium in NA-MD simulations within the NBRA. The analysis is performed using 3 model systems, each of which highlights the effects of the different decoherence methods and PHA schemes on NA transitions. We find that the interplay between decoherence and PHA schemes is important for achieving detailed balance in the NBRA and discuss the conditions by which the detailed balance is achieved for each model. We discuss the qualitative features of NA dynamics computed using various combinations of decoherence and PHA schemes for a wide range of model and condition parameters such as temperature, energy gap magnitude, and dephasing times. Furthermore, we extend the analysis to include the Boltzmann corrected Ehrenfest methodology of Bastida and co-workers and compare the dynamics produced with it with that obtained using the surface hopping-based approach.

Pressure-induced phase transitions in Na2B12H12, structural investigation on a candidate for solid-state electrolyte

closo-Borates, such as Na2B12H12, are an emerging class of ionic conductors that show promising chemical, electrochemical and mechanical properties as electrolytes in all-solid-state batteries. Motivated by theoretical predictions, high-pressure in situ powder X-ray diffraction on Na2B12H12 was performed and two high-pressure phases are discovered. The first phase transition occurs at 0.5 GPa and it is persistent to ambient pressure, whereas the second transition takes place between 5.7 and 8.1 GPa and it is fully reversible. The mechanisms of the transitions by means of group theoretical analysis are unveiled. The primary-order parameters are identified and the stability at ambient pressure of the first polymorph is explained by density functional theory calculations. Finally, the parameters relevant to engineer and build an all-solid-state battery, namely, the bulk modulus and the coefficient of the thermal expansion are reported. The relatively low value of the bulk modulus for the first polymorph (14 GPa) indicates a soft material which allows accommodation of the volume change of the cathode during cycling.

Quantification of charge carriers participating antiferromagnetic to weak ferromagnetic phase transition in Na doped LaFeO3 nano multiferroics

Multiferroic La1-xNaxFeO3 (x = 0.0, 0.05, 0.10, and 0.15) nanoparticles have been synthesized by the citrate sol-gel auto-combustion method. It is uniquely known for the homogeneous distribution of particle size due to the low temperature synthesis condition. The Rietveld refinement of the X-ray diffraction pattern has exhibited the single phase orthorhombic perovskite structure. It is confirmed from micro-Raman analysis that hole doping (Na) produces a structural crossover from the Pnma to Pbnm space group. The red shift observed in UV-visible spectroscopy indicates the shrinkage of bandgap. Photoluminescence analysis supports independently the bandgap shrinkage by the existence of violet-blue emission due to charge disproportion by oxygen related defects. A changeover from semiconductor to metallic nature accompanied by the hopping of charge carriers from Fe3+ to Fe2+/Fe4+ exchange interaction has revealed a considerable increase in conductivity and dielectric constant. Finally, the doping helps to induce transition from an antiferromagnetic to a weak ferromagnetic state due to free carrier concentration. It is found that the hole driven Fe3+ to Fe2+/Fe4+ exchange interactions with change in the Bohr magneton values from 5.9 μB to 4.9/4 μB are responsible for variation in the magnetization values.

Exploration of Iron-Based Mixed Polyanion Cathode Materials for Thin Film Sodium-Ion Batteries

Li-ion batteries are promising power sources for portable electronics and electric vehicles due to their high energy density. However, its application in grid storage is limited due to high cost and low abundance of Li resources. Next to Li-ion, Na-ion batteries (NIBs) are getting prominent interest due to its low cost, intercalation properties similar to Li and electrochemical potential of Na/Na+ (-2.71 V vs SHE) [1]. Recent studies based on layered NaMnO2 showed high specific capacity of 140 mA h g-1 by six biphasic transitions. However, the layered materials deliver poor cycling performance due to multiple phase transitions in Na insertion/extraction reactions [2]. On the other hand, Polyanion-type compounds are the most promising electrode materials for NIBs due to their stability, safety, and suitable operating voltages. Recently, Na4M3(PO4)2P2O7 (M=Co, Mn, Ni and Fe) are evaluated as electrode materials for energy storage applications due to its Na-ion intercalating properties, low cost and environmental friendliness [3,4]. In this work, the bulk powder of carbon coated Na4Fe3(PO4)2P2O7 material was prepared by solution combustion synthesis technique using ascorbic acid. The crystal structure was identified to be orthorhombic with Pna21 symmetry from Rietveld refinement. It has a 3D intercalating structure for Na-ion with a theoretical capacity of 129 mAh g-1. The Na-ion cell with the carbon coated nano-Na4Fe3(PO4)2P2O7 cathode delivered a discharge capacity of ~110 mAh g-1 at a 0.1C rate (Fig. 1a). The Na4Fe3(PO4)2P2O7 showed excellent rate capability with good cycling stability 100 cycles. Overall the voltage profile shows two plateau regions with the average voltage of ~3.1 V (Fig. 1b). The electrochemical results motivated us to synthesize the Na4Fe3(PO4)2P2O7 thin films for all-solid-state Na-ion batteries, which can be useful to design thin film micro-batteries. Thin films of Na4Fe3(PO4)2P2O7 electrode was deposited on stainless steel substrates using pulsed laser deposition (PLD) using KrF Laser source. Various deposition parameters were optimized to grow uniform thin films on substrates. From X-ray diffraction it was observed that films were crystallized into orthorhombic structure with Pna21 symmetry. Nano size and thickness of Na4Fe3(PO4)2P2O7 thin film grains were identified by field emission electron microscopy (FE-SEM) and atomic force microscopy (AFM) (Fig. 1d). Here, Na-ion intercalating properties of iron-based mixed-polyanion Na4Fe3(PO4)2P2O7 thin films was investigated. The electrochemical properties of both bulk and thin films of Na4Fe3(PO4)2P2O7 were studied. The thin films with thickness of ~200 nm delivered a maximum capacity and similar voltage profiles to the bulk material (Fig. 1d). The work demonstrates the capability of the mixed polyanionic material as high performance thin film Na-ion battery cathode. The electrochemical performances of the bulk and thin films of Na4Fe3(PO4)2P2O7 will be presented. Finally, an all-solid-state thin film micro-battery will be demonstrated using the full cell configuration having Na4Fe3(PO4)2P2O7 as cathode, NASICON Na3Zr2Si2O12 as solid electrolyte and Na3Ti2(PO4)3 as anode [5]. Figure 1. Electrochemical performances of Na4Fe3(PO4)2P2O7 in sodium half-cell architecture. (a) Galvanostatic charge/discharge curves of Na4Fe3(PO4)2P2O7 at a C/10 current rate. (b) Differential capacity vs. voltage (dQ/dV) plots, (c) Rate capability at different C rates and (d) Galvanostatic charge/discharge curves of Na4Fe3(PO4)2P2O7 thin film (inset) AFM and SEM images of the thin films grown by PLD technique [5]. Acknowledgement Author B.S. gratefully acknowledges the DST (SERB), New Delhi, India (PDF/2015/00217) for providing Fellowship. Figure 1

Metal Transition in Sodium–Ammonia Nanodroplets

AbstractThe famous nonmetal‐to‐metal transition in Na–ammonia solutions is investigated in nanoscale solution droplets by photoelectron spectroscopy. In agreement with the bulk solutions, a strong indication for a transition to the metallic state is found at an average metal concentration of 8.8±2.2 mole%. The smallest entity for the phase transition to be observed consists of approximately 100–200 solvent molecules. The quantification of this critical entity size is a stepping stone toward a deeper understanding of these quantum–classical solutions through direct modeling at the molecular level.

Reflections on the long nineteenth century and Indian Removal

Those of us who study American Indian history have long worked with a different set of chronological parameters and periodicities than conventional historians of American history. Transitions in Na...

Thermal conductivity across the metal-insulator transition in the single-crystalline hyperkagome antiferromagnetNa3+xIr3O8

The hyperkagome antiferromagnet Na$_{4}$Ir$_3$O$_8$ represents the first genuine candidate for the realisation of a three-dimensional quantum spin-liquid. It can also be doped towards a metallic state, thus offering a rare opportunity to explore the nature of the metal-insulator transition in correlated, frustrated magnets. Here we report thermodynamic and transport measurements in both metallic and weakly insulating single crystals down to 150 mK. While in the metallic sample the phonon thermal conductivity ($\kappa^{ph}$) is almost in the boundary scattering regime, in the insulating sample we find a large reduction $\kappa^{ph}$ over a very wide temperature range. This result can be ascribed to the scattering of phonons off nanoscale disorder or off the gapless magnetic excitations that are seen in the low-temperature specific heat. This works highlights the peculiarity of the metal-insulator transition in Na$_{3+x}$Ir$_3$O$_8$ and demonstrates the importance of the coupling between lattice and spin degrees of freedom in the presence of strong spin-orbit coupling.

Mode-crystallography analysis of the crystal structures and the low- and high-temperature phase transitions in Na0.5K0.5NbO3

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

Topological tuning in three-dimensional dirac semimetals.

We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na_{3}Bi_{1-x}Sb_{x} and Cd_{3}[As_{1-x}P_{x}]_{2} alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands.

Detuning-induced stimulated Raman adiabatic passage in atoms with hyperfine structure

We theoretically study the generation of coherence in generalized two-level atoms with hyperfine structure by utilizing the detuning-induced stimulated Raman adiabatic passage (D-STIRAP). As expected, the degree of attainable coherence between the ground and excited states cannot be as large as that for the ideal two-level atoms without hyperfine structure. However, we find that the substantial degree of coherence can still be produced with small modulations, and the modulation period is essentially determined by the hyperfine splittings in the ground and excited states. The D-STIRAP scheme in generalized two-level atoms is sufficiently robust against the various parameters such as intensities of lasers, initial detuning, time delay between the laser pulses, and Doppler broadening. As a specific example, we show realistic results for the ${D}_{1}$ transition of Na.

Effect of chloride substitution on the order–disorder transition in NaBH4 and Na11BD4

Phase transition associated with anion disordering over two orientations in Na11BD4 (NaBH4) and its solid solutions with NaCl, Na(11BD4)1−xClx, is investigated with powder diffraction (neutron and synchrotron radiation), differential scanning calorimetry and Raman spectroscopy. Upon heating, the transition temperature extrapolated to zero rate of heating is 192.2 K for Na11BD4, ΔS = 4.41 J/mol K, hysteresis 1.7 K and the volume increase 0.43%. Thermal parameters of the transition in Na(11BD4)1−xClx follow a colligative-property model of an ideal solution, with x = 0.158(1) as the critical concentration at which the ordering interactions and the transition itself are eliminated. On approaching this limit, the tetragonal distortion of the ordered structure decreases somewhat towards the cubic average, and this is associated with a partial disorder of the tetrahedral anions seen by diffraction methods. In fact, a 3% disorder is already present in the pure solvent of the solid solution (Na11BD4) at 8 K.

NMR study of thermally activated paramagnetism in metallic low-silica X zeolite filled with sodium atoms

We report a ${}^{23}$Na and ${}^{27}$Al nuclear magnetic resonance (NMR) investigation of low-silica X (LSX) zeolite with chemical formula Na${}_{12}$Al${}_{12}$Si${}_{12}$O${}_{48}$ (Na${}_{12}$-LSX) loaded with $n$ additional guest sodium atoms. Na${}_{n}$/Na${}_{12}$-LSX exhibits an insulator-to-metal transition around $n=11.6$, which is accompanied by a significant enhancement of bulk magnetic susceptibility. Paramagnetic moments are in the metallic Na${}_{12}$/Na${}_{12}$-LSX thermally activated with an activation energy of around 0.1 eV. Simultaneously a new shifted component appears in the ${}^{23}$Na NMR, whose large and positive NMR shift scales with bulk magnetic susceptibility. Its spin-lattice relaxation rate $1/{T}_{1}$ is governed by the local-field fluctuations characterized by the same activation energy as obtained from the bulk magnetic susceptibility data. The time scale of these fluctuations is typical for atomic motions, which suggest strong electron-phonon coupling, a hallmark of polaron states. The insulator-to-metal transition in Na${}_{n}$/Na${}_{12}$-LSX is thus discussed within a polaron model.

Similar zone-center gaps in the low-energy spin-wave spectra of Na1−δFeAs and BaFe2As2

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na${}_{1\ensuremath{-}\ensuremath{\delta}}$FeAs and BaFe${}_{2}$As${}_{2}$. Despite their very different values of the ordered magnetic moment and N\'eel temperatures, ${T}_{\mathrm{N}}$, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below ${T}_{\mathrm{N}}$, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na${}_{1\ensuremath{-}\ensuremath{\delta}}$FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe${}_{2}$As${}_{2}$ along the out-of-plane momentum component, ${q}_{z}$. At the magnetic zone boundary (${q}_{z}=0$), spin excitations in the ordered state persist down to $\ensuremath{\sim}$20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, ${J}_{\mathrm{c}}$, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.

Pressure-induced phase transitions in Na–Au intermetallic compounds

Pressure-induced phase transitions in Na–Au intermetallic compounds