Phase Transitions In Na0 Research Articles

Crystallographic direction dependence of direct current field induced strain and phase transitions in Na0.5Bi0.5TiO3-x%BaTiO3 single crystals near the morphotropic phase boundary

The crystallographic dependence of the DC electric (E) field induced strain and phase transition in Na0.5Bi0.5TiO3-5.6%BaTiO3 (NBT-5.6%BT) single crystals has been investigated. An induced transition between pseudocubic and tetragonal structures was observed under E = 10 kV/cm for fields applied along the 〈001〉 direction, whereas an induced transition between pseudocubic and rhombohedral structures was observed when the E-field was applied along 〈111〉. Our results show near the morphotropic phase boundary that the phase stability of NBT-x%BT is dependent on the crystallographic direction along which E was applied.

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

Thermal non-Gaussianity in Near-Milne universe

The thermal non-Gaussianity in Near-Milne universe is investigated in this Letter. Through classifying thermal fluctuations into two types, one characterized by a phase transition and the other without phase transition, we show that for fluctuations undergoing a phase transition, the non-Gaussianity depends linearly on k, but in non-phase transition case it is proportional to k−3γ, γ>0. Moreover, if the phase transition scenario is similar to that in holographic cosmology, we find that the non-Gaussianity fNLequil can reach O(1) or larger than O(1) by fine tuning of the equation of state ωC. Especially at the limit of ωC→−5/3, the non-Gaussianity can be very large. On the other hand, for non-phase transition case, the non-Gaussianity estimator fNLequil is approximately fNLequil≃5/108 when the energy is sub-extensive, especially when γ→0+. However, when γ→4, large non-Gaussianity can be obtained.

Doping-Induced Structural Phase Transition in Na1.6-xCo[Fe(CN)6]0.902.9H2O

Lattice structure of hole-doped Na 1.6- x Co[Fe(CN) 6 ] 0.90 2.9H 2 O was systematically investigated against doping level ( x ) and temperature ( T ) by means of synchrotron-radiation powder diffraction. We found that the hole-doping procedure on the Co site induces a rhombohedral–cubic structural phase transition. The rhombohedral phase (cubic phase) is observed in x ≤0.02 and 100 ≤ T ≤300 K ( x ≥0.14 and 100 ≤ T ≤300 K), while two lattice structures coexist in the intermediate- x region (0.02 ≤ x ≤0.14). We interpreted the two-phase region of the x – T phase diagram in terms of intrinsic phase separation into large- x (Na + -poor) and small- x (Na + -rich) regions, because the Na + ions can migrate within the crystal to minimize the total free energy.

NMR Studies of Successive Phase Transitions in Na0.5CoO2and K0.5CoO2

59 Co- and 23 Na-NMR measurements have been carried out on polycrystalline and c -axis aligned samples of Na 0.5 CoO 2 , which exhibits successive transitions at temperatures T = 87 K (= T c1 ) and T = 53 K (= T c2 ). 59 Co-NMR has also been carried out on c -axis aligned crystallites of K 0.5 CoO 2 with similar successive transitions at T c1 ∼60 K and T c2 ∼20 K. For Na 0.5 CoO 2 , two sets of three NMR lines of 23 Na nuclei, explained by considering the quadrupolar frequencies ν Q ∼1.32 and 1.40 MHz, have been observed above T c1 , as is expected from the crystalline structure. A rather complicated but characteristic variation of the 23 Na-NMR spectra has been observed with varying T through the transition temperatures, and the internal fields at two crystallographically distinct Na sites are discussed on the basis of the magnetic structures reported previously. The internal fields at two distinct Co sites observed below T c1 and the 59 1/ T 1 – T curves of Na 0.5 CoO 2 and K 0.5 CoO 2 are also discusse...

Reentrant superconductivity on a triangular lattice: application to cobalt oxide superconductor

It is possible that a triplet pairing is responsible for superconductivity in the recently discovered wet cobalt-based compounds. In such a case, the Zeeman coupling does not put limits on the upper critical field ( H c 2 ) and its value is determined solely by the diamagnetic effects. In the absence of the Pauli pair-breaking mechanism reentrant superconducting phase may set on. It was shown by Rasolt and Tešanović that the Landau level quantization in isotropic superconductors could enhance superconductivity in a very strong magnetic field. In the present paper, we investigate superconducting phase transition in Na x CoO 2 · y H 2 O in the presence of a strong magnetic field. Special attention is put on a possible occurrence of the reentrant phase.

Structural phase transitions in Na, Mg and Al crystals: dominant role of the valence in local pseudopotential theory

Within a perturbative treatment of realistic local electron–ion pseudopotentials for the simple metals, we study structural phase transitions under pressure in Na and other monovalent metals, Mg and other divalent metals, and Al and other trivalent metals. For the “good local pseudopotential” metals Na, Mg, and Al, our results are in reasonable agreement with experiment. The sequence of predicted transitions between crystal structures, and the volume compression ratios V/V0 at which these transitions are predicted, are determined largely by the valence z. The valence z also determines the dependence of the total energy upon the tetragonal c/a ratio along the Bain path from bcc to fcc. This path shows the divalent metals unstable in both fcc and bcc structures, and the trivalent metals unstable in bcc.

Crystal lattice and phase transitions in Na4TiP2O9 (NTP) and Na4.5FeP2O8(O,F) (NFP) superionic conductors as a function of high pressures and temperatures.

The lattice dynamics of Na(4)TiP(2)O(9) (tetrasodium titanium diphosphorus nonaoxide, NTP) and Na(4.5)FeP(2)O(8)(O,F) (nonasodium diiron tetraphosphorus difluoride octadecaoxide, NFP) crystals, which are superionic conductors with Na(+)-ion conductivity, were studied under high pressures. Lattice constants as a function of hydrostatic pressure were measured on a four-circle diffractometer using a high-pressure cell with diamond anvils. At 1.78 +/- 0.15 GPa NTP undergoes a reversible phase transition from the modulated monoclinic (pseudo-orthorhombic) modification which is stable under atmospheric conditions. A similar phase transition in NTP is observed at 523 K. For NFP, it may be assumed that at least three phase transitions occur when the pressure increases from atmospheric to 12 GPa, at 1.39 +/- 0.08, 4.52 +/- 0.32, and 6.02 +/- 0.02 GPa, as concluded from the change in the unit-cell parameters and in the color of the crystals: the color changes from ginger (dark orange) to pink at ~1.5-2.0 GPa pressure and to violet at ~6.0 GPa.

Fragmentation phase transition in atomic clusters IV

Within the micro-canonical ensemble it is well possible to identify phase-transitions in small systems. The consequences for the understanding of phase transitions in general are discussed by studying three realistic examples. We present micro-canonical calculations of the fragmentation phase transition in Na-, K-, and Fe- clusters of N = 200 to 3000 atoms at a constant pressure of 1 atm. The transition is clearly of first order with a backbending micro-canonical caloric curve T P (E, V (E,P)) = {∂S(E, V (E,P))/∂E P}−1. From the Maxwell construction of βP (E/N,P) = 1/T P one can simultaneously determine the transition temperature T tr, the specific latent heat q lat, and the specific entropy-loss Δs surf linked to the creation of intra-phase surfaces. T trΔs surf*N/(4πr ws 2 N eff 2/3 ) = γ gives the surface tension γ. Here 4πr ws 2 N eff 2/3 = ΣN i*4πr ws 2 m i 2/3 is the combined surface area of all fragments with a mass m i ≥ 2 and multiplicity N i. All these characteristic parameters are for ∼1000 atoms similar to their experimentally known bulk values. This finding shows clearly that within micro-canonical thermodynamics phase transitions can unambiguously be determined without invoking the thermodynamic limit. However, one has carefully to distinguish observables which are defined for each phase-space point, like the values of the conserved quantities, from thermodynamic quantities like temperature, pressure, chemical potential, and also the concept of pure phases, which refer to the volume of the energy shell of the N-body phase-space and thus do not refer to a single phase-space point. At the same time we present here the first successful microscopic calculation of the surface tension in liquid sodium, potassium, and iron at a constant pressure of 1 atm.

Peculiarities of phase transitions in Na0,5Bi0,5TiO3

The measurements of time dependencies of dielectrics and structural properties of Na0,5Bi0,5TiO3 have been performed. The results of this investigations confirm the prior data, that phase transition in Na0,5Bi0,5TiO3 from tetragonal to rhombohedral phase takes place in wide temperature range, in which both phases coexist. Moreover, this phase transition indicate the features of isothermal phase transformation.

Pressure-induced structural phase transition in Na 0.5Bi 4.5Ti 4O 15 by Raman scattering

Raman spectra of Na 0.5Bi 4.5Ti 4O 15 have been investigated under hydrostatic pressure up to 10 GPa. Two pressure-induced phase transitions have been observed at about 1.94 GPa and 9.0 GPa, respectively.

NMR Studies on Orientational Ordering Phase Transition in Na2CsC60

Sodium-containing alkali metal fulleride, Na 2 CsC 60 , was studied by 13 C and 23 Na NMR below and above the face-centered cubic ( f c c ) to simple cubic ( s c ) phase transition temperature, T m . It is shown that the C 60 molecules rotate faster than the rotational correlation time of 1.6×10 -5 sec above ∼140 K and 2.4×10 -9 sec around 300 K. Using 23 Na NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e 2 q Q / h , was estimated to be 940±20 kHz and less than 0.1 kHz below and above T m , respectively. The detailed analysis suggests that Na atoms in s c phase have a covalent character with C 60 molecules. It is also suggested that Na 2 CsC 60 is the first metallic plastic crystal.

X-ray diffraction study of the phase transitions in Na0.5Bi0.5TiO3

X-ray investigations of Na05Bi0.5TiO3 crystals showed the coexistence of rhombohedral and tetragonal phases in the wide temperature range. The temperature dependence of phase concentrations can explain the anomaly of dielectric properties observed at 280°C and 320°C temperatures.

Phase transition of Na2SO4and structure refinements at several temperatures

Phase transition of Na₂SO₄and structure refinements at several temperatures

Diffuse phase transition in Na, Li and Bi modified PLZT ceramics

The polycrystalline samples of PLZT composition (Pb0.92La0.08) (Zr0.65Ti0.35)O3 have been synthesized by chemical methods. X-ray and SEM studies of the compounds suggest the formation of single phase material. Studies of the dielectric constant as a function of temperature (room temperature to 200°C) and frequency, suggest that the compound has relaxor behaviour and undergoes a diffuse type of phase transition.

On the universality of the nonphase transition singularity in hard-particle systems

Activity series for hard-particle lattice gases and hard particles in continuous space are examined with respect to the singularity on the negative activity axis. For approximately spherical particles it is found that the nature of the singularity depends only on the dimensionality of space.

Physical Properties Related to Phase Transitions in Na2CO3 Crystals

Several physical properties related to phase transitions in Na 2 CO 3 crystal were experimentally studied. Ferroelasticity was confirmed above 200°C. The temperature dependence of birefringence was measured. Pressure dependence of transition temperatures were studied above room temperature. In addition, specific heat and electric capacity in low temperature region were measured, but no distinct anomalies associated with the reported lock-in transition could be found.

Swelling and collapse of polymer networks

The swelling of a polymer network in a good solvent and of the collapse of this sample in a poor solvent is discussed theoretically for networks prepared in the presence of a large amount of diluent. The notation used is convenient for comparison with the theory of the swelling and collapse of a single macromolecule. The reference state for the networks under consideration must be chosen near the θ temperature for the corresponding linear macromolecule. For a free network and a network with fixed dimensions along one or two axes we obtain asymptotic results for the swelling of the network in a good solvent; these do not coincide with the classical Flory results. Results for the collapse of the network, which takes place upon cooling below the θ-point are also obtained. The collapse can occur either as a discrete first order phase transition (if the network chains are stiff or if it is stretched along one or two axes), or as a continuous non-phase transition (located in a narrow temperature range).

X-ray study of phase transitions in alkali tetracyanoquinodimethanides (TCNQ)

The phase transitions in Na-, K-, and Rb-tetracyanoquinodimethanide (TCNQ) single crystals have been investigated by the x-ray critical scattering technique. The intensities of superlattice reflections, which appeared below the phase-transition temperature ${T}_{c}$, decrease with increasing temperature toward ${T}_{c}$ and the diffuse scattering intensities observed above ${T}_{c}$ increase with decreasing temperature. The intensity distribution of x-ray diffuse scattering in the momentum space shows a very strong anisotropy belonging to the ${a}^{*}$ planes, which comes from the one-dimensional crystal instability. These results are discussed in terms of the one-dimensional electron-phonon interaction.