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1.
A. I. Lebedev 《Physics of the Solid State》2011,53(12):2463-2467
First-principles density functional theory is used to calculate the phonon spectrum in the paraelectric phase, the ground-state
structure and polarization distribution in the polar phase, and energies of ferro- and antiferroelectrically ordered phases
of free-standing (KNbO3)1(KTaO3)
n
ferroelectric superlattices with n = 1–7. It is established that quasi-two-dimensional ferroelectricity with polarization oriented in the layer plane, which
weakly interacts with polarization in neighboring layers, appears in potassium niobate layers with a thickness of one unit
cell in the superlattices. The possibility of using of such ferroelectric superlattices as a medium for three-dimensional
information recording is shown. 相似文献
2.
Y. Yan J. Y. Zhang T. Cui Y. Li Y. M. Ma J. Gong Z. G. Zong G. T. Zou 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):397-403
The lattice dynamics of lithium nitride (Li3N) under high pressure are extensively investigated to probe its phase transformations by using the pseudopotential plane-wave
method within the density functional theory. A new second order α↦α′-Li3N phase transition is identified for the first time. The newly proposed α′-phase possesses a hexagonal symmetry with four ions in the unit cell having a space group of P-3m1. Further enthalpy and
phonon calculations support the existence of this phase, which stabilizes in a narrow pressure range of 2.8 – 3.6 GPa at zero
temperature. Upon further compression, transitions to denser packed phases of β-and γ-Li3N are typical first order. The analysis of the electronic densities of states suggests that all the high pressure modifications
of Li3N are insulators and, interestingly, the typical behavior of compression is to broaden the band gap. 相似文献
3.
The phase transition during chemical lithium insertion into α-MoO3 was investigated by chemical analysis, X-ray diffraction (XRD) and electrochemical characterisation. The samples have been
prepared by reaction of various amounts of water-free lithium iodide with fine-particulate orthorhombic molybdenum trioxide
in n-hexane (non-aqueous media), which yielded materials with different Li/Mo ratio. XRD investigations of these materials proved
that the crystal structure of the layered α-MoO3 has been changed after the chemical lithiation. The phase transition ranged from 0.25 < x < 0.5 in Li
x
MoO3 upon chemical lithium insertion into α-MoO3. The XRD lines of lithium inserted phase Li
x
MoO3 grew at the expense of the XRD lines of the pristine α-MoO3 as lithium ions were chemically inserted until the disappearance of lines related to α-MoO3. The electrochemical performance of the lithiated samples is improved in comparison with the starting material (non-lithiated
α-MoO3). 相似文献
4.
S. G. Jabarov D. P. Kozlenko S. E. Kichanov A. V. Belushkin B. N. Savenko R. Z. Mextieva C. Lathe 《Physics of the Solid State》2011,53(11):2300-2304
The crystal structure of lead titanate PbTiO3 was investigated by energy dispersive X-ray diffraction at high pressures up to 4 GPa in a temperature range of 300–950 K.
At the ambient conditions, the PbTiO3 structure is tetragonal with the space group P4mm (ferroelectric phase). A structural phase transition into the cubic phase with a space group Pm[`3]mPm\bar 3m is observed at T = 747 K. It was found that the phase transition temperature decreases upon applying the high pressure with the coefficient
dT
C
/dP = -65 K/GPa. Dependences of parameters and volume of the unit cell on the pressure and temperature was found, and the bulk
modulus and thermal expansion coefficients for the tetragonal and cubic phases of lead titanate have been calculated. 相似文献
5.
S. A. Gridnev A. A. Kamynin 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(10):1371-1374
The dispersion of complex permittivity in ferroelectric PbFe1/2Nb1/2O3 with diffused phase transition at frequencies of 20 Hz-10 kHz was studied. Dielectric measurements revealed that polarization
1/f noise occurs in the region of coexistence of polar and nonpolar phases. It was found that the temperature dependence of parameter
α exhibits a singularity when the temperature of the local order parameter rises. Our analysis of the experimental data enables
us to say that the source of the noise is the fluctuations of spontaneous polarization. 相似文献
6.
R. Mittal S. L. Chaplot A. I. Kolesnikov C. -K. Loong O. D. Jayakumar S. K. Kulshreshtha 《Pramana》2004,63(2):405-408
We report here measurements of phonon spectrum and lattice dynamical calculations for GaPO4. The measurements in low-cristobalite phase of GaPO4 are carried out using high-resolution medium-energy chopper spectrometer at ANL, USA in the energy transfer range 0–160 meV.
Semiempirical interatomic potential in GaPO4, previously determined using ab-initio calculations have been widely used in studying the phase transitions among various
polymorphs. The calculated phonon spectrum using the available potential show fair agreement with the experimental data. However,
the agreement between the two is improved by including the polarisability of the oxygen atoms in the framework of the shell
model. The lattice dynamical models are also exploited for calculations of various thermodynamic properties of GaPO4. 相似文献
7.
A. G. Gamzatov A. M. Aliev K. Sh. Khizriev I. K. Kamilov A. S. Mankevich I. E. Korsakov 《Physics of the Solid State》2011,53(11):2271-2274
The heat capacity of the manganite La0.87K0.13MnO3 has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties
of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters
near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity
with allowance for corrections on the scaling (α = −0.13 and A
+/A
− = 1.178) correspond to the critical behavior of the 3D Heizenberg model. 相似文献
8.
Dunmin Lin K.W. Kwok H.L.W. Chan 《Applied Physics A: Materials Science & Processing》2008,91(1):167-171
(K0.5Na0.5)(Nb1-xTax)O3 lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique. The results of X-ray diffraction
reveal that Ta5+ diffuses into the K0.5Na0.5NbO3 lattices to form a solid solution with an orthorhombic perovskite structure. Because of the high melting temperature of KTaO3, the (K0.5Na0.5)(Nb1-xTax)O3 ceramics can be sintered at higher temperatures. The partial substitution of Ta5+ for the B-site ion Nb5+ decreases both paraelectric/cubic–ferroelectric/tetragonal and ferroelectric/tetragonal–ferroelectric/orthorhombic phase
transition temperatures, TC and TO-T. It also induces a relaxor phase transition and weakens the ferroelectricity of the ceramics. The ceramics become ‘softened’,
leading to improvements in d33, kp, kt and εr and a decease in Ec, Qm and Np. The ceramics with x=0.075–0.15 become optimum, having d33=127–151 pC/N, kp=0.43–0.44, kt=0.43–0.44, εr=541–712, tanδ=1.75–2.48% and TC=378–329 °C.
PACS 77.65.-j; 77.84.Dy; 77.84.-s 相似文献
9.
V. V. Gorbatenko V. I. Kudryash B. N. Prasolov S. A. Gorbatenko 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(10):1313-1316
The contributions to the amplitudes of the harmonic components of the output signal of the Soyer-Tower circuit from charging
the reference capacitor with capacitive current and the conduction current of an investigated specimen are separated via the
harmonic analysis of repolarization in a Rb2ZnCl4 crystal in the ferroelectric phase near the phase transition temperature T
c
. Harmonic analysis of the dielectric hysteresis loops obtained when the specimen was affected by an electric field whose
intensity E
in changed according to harmonic law with amplitude E
m
showed that at E
m
> E
coer (where E
coer is the intensity of the coercive field), the process of repolarization starts and stops when E
in reaches values of E
m
sin(φ0 − α0) and E
m
sin(φ0 + α0), respectively (where φ0 is the shift of phases between the voltage of the harmonic signal source and the voltage of the first harmonic in the reference
capacitor in the Soyer-Tower circuit, and α0 is the phase corresponding to the normalized duration of repolarization). When the current value of intensity E
in coincides with E
c
= E
m
sinφ0, the direction of polarization reverses. The increase in E
c
when T
c
is approached is shown to be the reason for the anomalous temperature-independent behavior of E
coer. 相似文献
10.
A. Audzijonis 《Phase Transitions》2013,86(5):389-395
Using density functional theory methods, the phonon density of states, Helmholtz free energy, internal energy, and entropy of ferroelectric and paraelectric phases are investigated. The temperature dependence of the free energy indicates that vibrational entropy contributes to the destabilization of the ferroelectric phase. The vibrational entropy of Sb, S, and Br atoms is attributed to the stabilization of ferroelectric SbSBr at the temperature T c. Calculations indicated that SbSBr in ferroelectric phase become more stable than in paraelectric phase at temperatures lower than 22.8 K. The calculated temperature of ferroelectric phase transition is in good agreement with the experimental data. 相似文献
11.
Sayan Bhattacharyya Sajith Kurian S. M. Shivaprasad N. S. Gajbhiye 《Journal of nanoparticle research》2010,12(4):1107-1116
A new ternary nitride, CoMoN2, was prepared in the nanosize regime of 9.0 ± 2.0 nm, by nitridation of the precursor intermetallic nitride Co3Mo3N. XRD–Rietveld analysis revealed the presence of 0.60 (±0.02) mass % of Co impurity phase. The calculated space groups of
CoMoN2 and Co are P6
3
/mmc and Fm-3m, respectively. The N atoms lie at the interstitial sites and the 12 calculated nitrogen sites indicate the presence of a
layered structure. The XPS studies indicated the presence of the nitride and surface oxynitride/oxide phases. CoMoN2 is an interstitial nitride with Co and Mo in the zero oxidation state. The room temperature susceptibility is estimated after
subtracting the ferromagnetic contribution from cobalt and found to be 2.7 × 10−4 emu g−1 Oe−1, indicating the Pauli-paramagnetic nature. The ferromagnetic exchange interactions between the Co atoms in CoMoN2 are reduced due to the presence of Mo and N in the crystal lattice. The hysteresis loop shift 19 Oe is attributed to the
demagnetizing dipolar fields created in the soft CoMoN2 phase by the hard Co phase. 相似文献
12.
B. P. Gorshunov E. S. Zhukova E. G. Maksimov A. S. Prokhorov V. I. Torgashev T. Zhang D. Wu M. Dressel 《JETP Letters》2010,91(7):336-340
The nature of the low-energy excitations of polycrystalline and nanostructured La0.25Ca0.75MnO3 samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that
the electrodynamic response spectra of La0.25Ca0.75MnO3 in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed
to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies
of 20–40 and 80–100 cm–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance
of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression
indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem. 相似文献
13.
A. G. Gamzatov A. M. Aliev Sh. B. Abdulvagidov A. B. Batdalov O. Yu. Gorbenko 《Russian Physics Journal》2007,50(4):383-386
Temperature dependences (77–300 K) of the thermal capacity, diffusion, and conductivity are investigated for the Nd
0.5
Sr
0.5
MnO
3 and Nd
0.55
Sr
0.45
MnO
3 polycrystalline samples. The examined characteristics show anomalous behavior in the magnetic phase transition and transition
to the charge-ordered state. It is demonstrated that the main reason for a sharp decrease in the thermal conductivity during
Nd
0.5
Sr
0.5
MnO
3 transition into the antiferromagnetic charge-ordered state is a change in the phonon spectrum caused by the lattice compression.
A temperature dependence of the free phonon path is calculated for the examined temperature interval based on the thermal
diffusion obtained and the literature data on the sound propagation velocity.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 72–75, April, 2007. 相似文献
14.
A. V. Malakhovskii S. L. Gnatchenko I. S. Kachur V. G. Piryatinskaya A. L. Sukhachev V. L. Temerov 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,80(1):1-10
Optical absorption spectra of trigonal crystal TbFe3(BO3)4 have been studied in the region of 7F6 → 5D4 transition in Tb3+ ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb3+ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice,
have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the
absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be
due to the abrupt change of equilibrium geometry of the local Tb3+ ion environment only in the excited state of the Tb3+ ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb3+ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has
been shown that the ground state (in D
3 symmetry approximation) consists of two close singlet states of A
1 and A
2 type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of
the excited electronic states relative to that of the ground state have been
experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines
has been observed in the vicinity of the TbFe3(BO3)4 structural phase transition. The temperature interval of coexistence of the phases is about 3 K. 相似文献
15.
G. A. Komandin A. S. Prokhorov V. I. Torgashev E. S. Zhukova B. P. Gorshunov A. A. Bush 《Physics of the Solid State》2011,53(4):736-744
The spectra of complex permittivity of a Ba2Mg2Fe12O22 single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters
of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm−1). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the
experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E
u
symmetry, which are allowed by the symmetry of the Ba2Mg2Fe12O22 crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response
exhibits new absorption lines due to magnetic excitations. 相似文献
16.
The Ac electrical conductivity and the dielectric relaxation properties of the [(C3H7)4N]2Cd2Cl6 polycrystalline sample have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies
and temperatures, 209 Hz–5 MHz and 361–418 K, respectively. The purpose is to make a difference between the electrical and
dielectric properties of the polycrystalline sample and single crystal. Besides, a detailed analysis of the impedance spectrum
suggests that the electrical properties of the material are strongly temperature-dependent. Plots of (Z" versus Z') are well
fitted to an equivalent circuit model consisting of a series combination of grains and grains boundary elements. Moreover,
the temperature dependence of the electrical conductivity in the different phases follows the Arrhenius law and the frequency
dependence of σ (ω) follows the Jonscher’s universal dynamic law. Furthermore, the modulus plots can be characterized by full
width at half height or in terms of a nonexperiential decay function φ(t) = exp(t/t)β. Finally, the imaginary part of the permittivity constant is analyzed with the Cole–Cole formalism. 相似文献
17.
Vladimir A. Kulbachinskii Boris M. Bulychev Vladimir G. Kytin Alexey V. Krechetov Valeriy P. Tarasov Elizaveta A. Konstantinova Yuriy A. Velikodnyi Yuriy B. Muravlev Andrey V. Zoteev 《Central European Journal of Physics》2010,8(1):101-112
Sodium fullerides Na
n
C60 (n = 2, 3) have been synthesized by a liquid phase reaction and investigated with X-ray diffraction (XRD), nuclear magnetic
resonance (NMR), electron paramagnetic resonance, and differential thermal analysis. XRD data indicate that the crystal structure
of Na2C60 at 300 K is face centered cubic (FCC). A phase transition from primitive cubic to FCC crystal structure has been observed
in this work in Na2C60 fulleride at 290 K. The transition is accompanied by the step-like change of paramagnetic susceptibility. The crystal structure
of Na3C60 is more complicated than, and different from, what has been reported in the literature. A nearly seven-fold increase of paramagnetic
susceptibility with increasing temperature has been observed in the Na3C60 fulleride at 240–260 K. In the same temperature range, a new line at about 255 ppm appears in the 23Na NMR spectrum, indicating a significant increase of electron density near the Na nucleus. The observed effect can be explained
by a metal-insulator transition caused by a structural transition. 相似文献
18.
K. P. Meletov V. A. Davydov J. Arvanitidis D. Christofilos K. S. Andrikopoulos G. A. Kourouklis 《Journal of Experimental and Theoretical Physics》2008,107(4):620-631
Stability of the linear orthorhombic polymer of C60 under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum
at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm2 of the 514.5 nm line of an Ar+ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show
similar irreversible changes even at the laser intensity as low as 470 W/cm2. The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that
the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates
a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from
those of the other known polymeric phases of C60. The Raman spectra of the phototransformed linear orthorhombic polymer of C60 were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near
4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable
disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase
in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C60, after pressure release and exposure to ambient conditions for 30 h.
The text was submitted by the authors in English. 相似文献
19.
A. M. Balagurov I. A. Bobrikov V. Yu. Pomjakushin E. V. Pomjakushina D. V. Sheptyakov I. O. Troyanchuk 《JETP Letters》2011,93(5):263-268
The magnetic and crystal structures of the Pr0.5Sr0.5CoO3 metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound
is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of
the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase
occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen
atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying
in the basal plane of the structure (μCo ≈ 1.7 μ
B
at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation
of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical
and magnetic characteristics of this compound observed earlier at temperatures about 120 K. 相似文献
20.
N. V. Shpilevaya M. F. Kupriyanov B. S. Kul’buzhev Yu. V. Kabirov 《Physics of the Solid State》2004,46(12):2263-2267
The phase transition from an orthorhombic phase (space group Pnma) to a rhombohedral phase (space group R3m) of the CdHfO3 hafnate is investigated using methods of structural analysis. It is shown that crystal lattices of both phases contain polar structural units (octahedra, cubooctahedra). On this basis, it is assumed that the orthorhombic and rhombohedral phases of the CdHfO3 compound are the antiferroelectric and ferroelectric phases, respectively. 相似文献