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1.
Uniaxial strain induced ferroelectric phase transitions in rutile TiO 2 are investigated by first-principles calculations. The calculated results show that the in-plane tensile strain induces rutile TiO 2, paraelectric phase with P4-2/mnm ( D4h) space group, to a ferroelectric phase with Pm ( Cs) space group,driven by the softening behaviour of the E _u1 mode. In addition, the out-of-plane tensile strain, vertical to the ab plane, leads to a ferroelectric phase with P42nm ( C4v) space group, driven by the softening behaviour of the A 2u mode. The critical tensile strains are 3.7% in-plane and 4.0% out-of-plane, respectively. In addition, the in-plane compression strain, which has the same structure variation as out-of-plane tensile strain due to Poisson effect, leads the paraelectric rutile TiO 2 to a paraelectric phase with Pnnm ( D2h) space group driven by the softening behaviour of the B 1g mode. These results indicate that the sequence ferroelectric (or paraelectric) phase depends on the strain applied. The origin of ferroelectric stabilization in rutile TiO 2 is also discussed briefly in terms of strain induced Born effective charge transfer. 相似文献
2.
In a study of the elastic behaviour of the InPb alloys, the elastic stiffness tensor components of crystals of each of the three phases (fct, ; fct, ; fcc) have been obtained as a function of temperature from pulse superposition measurements of ultrasonic wave velocities. Comparison of the elastic stiffness constants obtained for a fct ( ) 5 atm.% Pb alloy with those of In itself and those of InTl and InCd alloys, establishes for this phase that alloying with Pb, as with TI and Cd, enhances the softening of the acoustic [110] phonon mode, polarization [1 0] near the Brillouin zone centre. The elastic properties of a 17 atm.% Pb crystal, which is in the fct ( ) phase, are quite different from those shown by In alloys in the fct ( phase; in particular the response to a shear stress is remarkably isotropie: there is no phonon mode softening in this alloy. Neither is there softening of this mode (which corresponds at the zone centre to the shear stiffness (C 11;C 12)) in crystals of the fee phase — in complete contrast to the dominating influence of the softening of (C 11;C 12) in the InTl and InCd fee alloys. In fact for a fcc In-75 atm.% Pb alloy the anistropy ratio for shear is close to unity. The transitions between the three phases of the InPb alloys are markedly first order and acoustic mode softening has a much smaller influence on the elastic behaviour of the fct ( ) and fcc InPb alloys than it has on the fct ( ) InPb, InCd and InTl alloys. 相似文献
3.
Phase diagram and thermodynamic parameters of the random field Ising model (RFIM) on spherical lattice are studied by using mean field theory. This lattice is placed in an external magnetic field (B). The random field (hi) is assumed to be Gaussian distributed with zero mean and a variance 相似文献
4.
The velocities and the attenuation of ultrasonic waves have been investigated as a function of temperature (255K T350K) for the cubic phase of K 2SnCl 6 which undergoes a structural transition at T
c1263K. An anomalous decrease of the shear stiffness constant c=1/2( c
11
–c
12) of about 30% between room temperature and T
c1 is found in this cubic high temperature phase when T
c1 is approached from above. Whereas the softening of c extends over a substantial temperature range (about 50 K), the other shear constant c
44 shows only a weak decrease, which is an order of magnitude smaller and restricted to T–T
c110 K. Near T
c1, a strong increase occurs in the attenuation of the shear acoustic wave propagated along the [111] direction, while such an anomalous attenuation is not observed for the transverse acoustic wave propagated along [100]. Furthermore, at T
c1 a hysteresis is detected for the longitudinal sound wave velocity v
L[100], which is restricted to the temperature region 262K T263.5 K. In addition, the room-temperature elastic constants of (NH 4) 2SiF 6 and the hydrostatic pressure derivatives of the elastic constants of K 2SnCl 6 and (NH 4) 2SiF 6 at room-temperature are presented and discussed in terms of mode softening behaviour. 相似文献
5.
本文利用BCS波函数及投影波函数计算了 84Po的偶-偶同位素及 85At的奇-偶同位素的α约化宽度γ α2的绝对值。文中仔细研究γ α2的绝对值及相对趋势和参数b,R 0,G的关系;b为谐振子势阱参数,R 0为道半径,G为对关联相互作用常数。计算结果的分析表明:(1)对关联有利于α的形成,它可以使α约化宽度的绝对值增加一个数量级,且可以使相对趋势与实验符合得更好。(2)BCS波函数的粒子数不守恒的缺点,对于满壳外粒子数不多的核的α衰变来说相当严重。用了粒子数守恒的投影波函数,绝对值可以增加1—2倍,且可以使相对趋势变好。(3)考虑了阻塞效应后,宽度约减小10%,使奇A核的阻碍因子的理论值和实验值一致。(4)绝对值对参数的选择很敏感,而相对趋势对参数则不太敏感,要使理论的绝对值接近实验值,必须取R 0在8f以内。 相似文献
6.
The combination of micro‐Raman spectroscopy and an advanced universal fibre tester (UFT) made it possible to probe at the nanoscale (through monitoring the modification of chemical bonds) the change in conformation (α‐helix, β‐sheet, etc.), macromolecular fibroin chain orientation and coupling during the application of stress, quantitatively. Different single fibres of silkworms ( Bombyx mori, Gonometa rufobrunea, Gonometa postica) and a spider ( Nephila madagascariensis) were tested in a dry environment and compared with the behaviour of keratin fibre. As observed previously for single keratin fibres, a direct relationship is observed between nano‐ and micro‐mechanical tensile behaviour. The phase transition plateau, well defined for some pristine B. mori fibres, disappears in degummed fibres, which indicates a structural modification and increasing disorder with chemical treatments. Stress‐controlled micro‐Raman analysis shows that a few modes involving CH 2 and/or amide groups of β‐conformation chains undergo a wavenumber softening during the elastic behaviour (∼0–3%), although most of the modes are not affected. A different behaviour is observed for modes associated with ‘ordered’ and ‘disordered’ β‐sheets and helical chains. Larger softening is observed for lattice modes with increasing stress/strain, as expected. Structural changes and relationships with mechanical behaviour are discussed. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
7.
The elastic transition in s-triazine (C 3N 3H 3) from a trigonal ( ) high temperature (low pressure) structure to a monoclinic ( C2/ c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition. 相似文献
8.
The synthesis of a specific isotopomer, C 6D 4H(ortho)-H(ortho)D 4C 6 of biphenyl is reported. The intramolecular dipolar coupling of the protons leads to a well-resolved single-crystal proton
nuclear magnetic resonance (NMR) spectrum and allows one to study the dynamics of the phenyl rings in a unique way. At room
temperature and above, the most conspicuous dynamical mode consists of 180° ring flips. The present data together with previous
measurements of the total flip rate allow us to conclude that the rings flip almost exclusively independently of each other.
Between the incommensurate (IC) phase transition of biphenyl at 38 K and T=250 K, the prominent namical mode consists of oscillatory twists ϕ( t) of the two rings. The data allow us to infer the mean square, (φ 2), of these twists. (φ 2) is found to grow linearly with T for 50< T<200 K. From the slope of (φ 2) vs. T the frequency (the wave number[( v)\tilde]\tilde v) is derived. The result is[( v)\tilde] = 20\tilde v = 20 cm −1. For T<38 K, the spectra give direct evidence of the IC phase transition and its nature (stripelike rather than quiltlike). The
temperature dependence of the magnitude of the order parameter of the IC phase is obtained. 相似文献
9.
The paper reports the first measurements of the single crystal elastic constants of the heavy rare earth metal thulium as a function of temperature and magnetic field. The constants were obtained from ultrasonic velocity measurements over a temperature range of 4.2–296 K and in applied magnetic fields of up to 5 T. The elastic constants; C 11, C 33, C 44 and C 66=(C 11–C 12)/2 were determined from the ultrasonic velocities. Anomalies in the elastic constants were observed at 58 K from the c-axis propagated shear wave measurements and at 55 K from the c-axis propagated longitudinal wave measurements. Significant softening of the elastic constants C 33 and C 44 was observed close to TN. Application of a magnetic field (>2 T) along the c-axis direction induced further softening of the material. Electromagnetic acoustic transducers (EMATs) were also employed in addition to conventional piezoelectric quartz transducers. A marked increase in the EMATs acoustic coupling efficiency (generation and detection efficiency) occurred close to TN. 相似文献
10.
Summary Dielectric measurements on Na 2B 4O 7(99.5%)−V 2O 5(0.5%) glass system, in the frequency range 10 −3 to 10 4 Hz and temperature range 300 to 500 K, have been carried out. The normalized plots of complex capacitance have shown a single
mechanism responsible for conduction for both volume and surface measurements with their close values of activation energies
(0.67±0.03) eV and (0.64±0.03) eV, respectively. The low-frequency dispersion (LFD) behaviour has been observed to be perturbed
by the presence of more than one competing process. The impedance plots have shown a parallel combination of a capacitor ( C) and a resistor ( R), with some contribution of a dispersive element due to charge accumulation in the vicinity of the electrodes. The values
of R and C were found to be of the same order of magnitude, for both surface and volume measurements. The observed R has shown a decrease with an increase in temperature due to an increase in mobility of Na + ions, whereas C remains practicaly constant. The complex capacitance surface behaviour is dominated by volume, due to hygroscopy of this
glass system. 相似文献
11.
The recently obtained analytical result [1] for renormalization of the jump of the heat capacity ( C
S– C
N)/C N by anisotropy of the order parameter is applied to the layered superconductors. The graph of ( C
S– C
N)/ C
N vs. the anisotropy of the order parameter allows a direct determination of the gap anisotropy in MgB 2 using available experimental data 相似文献
12.
A one-dimensional diagonal tight binding electronic system with dichotomic correlated disorder in the presence of external
d.c field is investigated. It is found numerically that the conductance distribution obeys fairly well to log-normal distribution
in weak disorder strength in localized regime, which indicates validity of single parameter scaling theory in this limit.
Contrary to the universal cumulant relation C
1 = 2 C
2 in the absence of d.c. field, we demonstrated numerically that C
1 ≫ 2 C
2 in the presence of the field in localized regime. We interpret this result as suppression of the fluctuation effects by the
external field. In addition, it is obtained that the quantity NF
c
, here N is the system size and F
c
is the crossover field, decreases as the as the system energy E increases. Moreover, we find numerically a simple linear relation between the average logarithm of the conductance 〈ln( g)〉 and the field strength as 〈ln( g)〉 = C( N, λ) F, here C( N, λ) is a constant for particular values of N and λ, which is the Poisson parameter of the dichotomic process. 相似文献
13.
A group-theoretical analysis is performed for the complete condensation of order parameters at structural phase transitions (SPT's) in the high-temperature superconductors belonging to the D 17 4h -14/mmm space group in the high-symmetry phase. As a rule, such transformations are due to a successive softening of phonons with wave vectors k 1 = 1/2 b 3 and k 2 = 1/2 ( b 1 - b 3) belonging to the K13(X) star of the Brillouin zone of a tetragonal body-centered Bravais cell. SPT's in system La 2-x Ba x CuO 4 are considered in detail. 相似文献
14.
We measured Raman spectra in crystals of molecular donor–acceptor fullerene complexes {Me(nPr2dtc)2} · (C60)2 (Me = Ni, Cu, Pt). In the spectra of the {Pt(nPr2dtc)2} · (C60)2 complex under prolonged irradiation with a laser with λ = 532 nm, characteristic changes in the photopolymerization of fullerene are observed, associated with the splitting of degenerate phonon Hg modes and softening of Ag modes of the C60 molecule. The kinetics of photopolymerization under conditions of weak irradiation at room temperature is studied. It was found that thermal destruction of the photopolymer with increasing temperature leads to a decrease in its concentration in the final photopolymerization product. The kinetics of thermal destruction is described by the Arrhenius equation, with the activation energy EA of (0.68 ± 0.03) eV; the dimers are destructed to a concentration of 1% within 15 min at ~114°C. 相似文献
15.
The interaction of an amino-functionalized ionic liquid, 1-(2-aminoethyl)-3-butylimidazolium bromide ([NH 2C 2C 4im]Br), with cytochrome c (cyt c) at pH 7.4 was investigated using fluorescence and UV-Vis absorption spectroscopic techniques. From the experimental results, it is found that cyt c has a strong ability to quench the intrinsic fluorescence of [NH 2C 2C 4im]Br and the quenching mechanism is considered as a static quenching process. The binding constants and the number of binding sites ( n) were calculated at different temperatures. The thermodynamic parameters such as free energy change (Δ G), enthalpy change (Δ H), and entropy change (Δ S) were calculated by thermodynamic equations. According to the results, the values of Δ G, Δ H, and Δ S are all negative, suggesting that interaction between [NH 2C 2C 4im]Br and cyt c is spontaneous and mainly driven by hydrogen bonding and van der Waals forces. 相似文献
16.
ABSTRACTIn this study, ultrasonic measurements were performed on a single crystal of cubic PrNi 2Cd 20, down to a temperature of 0.02?K, to investigate the crystalline electric field ground state and search for possible phase transitions at low temperatures. The elastic constant ( C11? C12)/2, which is related to the Γ 3-symmetry quadrupolar response, exhibits the Curie-type softening at temperatures below ~30?K, which indicates that the present system has a Γ 3 non-Kramers doublet ground state. A leveling-off of the elastic response appears below ~0.1?K toward the lowest temperatures, which implies the presence of level splitting owing to a long-range order in a finite-volume fraction associated with Γ 3-symmetry multipoles. A magnetic field–temperature phase diagram of the present compound is constructed up to 28?T for H || [110]. A clear acoustic de Haas–van Alphen signal and a possible magnetic-field-induced phase transition at H ~26?T are also detected by high-magnetic-field measurements. 相似文献
17.
This work was devoted to X-ray diffraction study and investigations of temperature changes of the optical absorption edge of (NH 2(C 2H 5) 2) 2CoCl 4 crystals in the region of possible phase transitions. The X-ray powder diffraction data revealed the monoclinic phase at room temperature – space group P2/n. The cobalt atom was found to be square-plane coordinated by four chlorine atoms resulting [CoCl 4] 2– anion, which is surrounded by two DEA + cations. It was shown that the low-energy tail of the absorption edge in these materials possesses an exponential shape. In the temperature range above 255?K it follows the empirical Urbach’s rule. The obtained experimental data confirmed the existence of the ferroelastic phase in (NH 2(C 2H 5) 2) 2CoCl 4 in the temperature range between 255 and 326?K. The anomalous behaviour of the investigated parameters observed at the temperatures below 255?K would be related to earlier unknown phase transitions. 相似文献
18.
Crystalline [CuNd 2(C 4O 4) 4(H 2O) 16]·2H 2O constructed of complexes of trivalent neodymium and divalent copper, has been synthesized and studied by EPR. The square
anion groups (C 4O 4) enter as bridge ligands, forming chains of neodymium ions interconnected by (C 4O 4)Cu(C 4O 4) fragments. It is found that the relaxation rate of the neodymium subsystem at room temperature significantly exceeds the
exchange interaction rate between copper and neodymium ions. Under these conditions the magnetic properties of the crystal
are determined by two magnetically nonequivalent chains of copper ions, which do not interact. The intrachain exchange interaction
via hydrogen bonds is estimated to be ∼0.1 cm −1. As one proceeds from the high-temperature (250< T<300 K) to the low-temperature region ( T<40 K), a substantial change in the nature of the interaction is revealed. An unusual magnetic structure given in a crystal
is observed at low temperatures, which is determined by the presence of two magnetically nonequivalent “ribbons,” formed by
the interacting copper and neodymium ions: chains of copper ions are framed on two sides by chains of neodymium ions. The
magnitude of the parameter of the exchange interaction between the copper and neodymium ions is estimated as J
Cu-Nd⩾0.2 cm −1. An exchange interaction between magnetically nonequivalent neodymium ions is not revealed in the EPR spectra.
Fiz. Tverd. Tela (St. Petersburg) 39, 2057–2061 (November 1997) 相似文献
19.
The elastic constants of the molecular crystal adamantane (C 10H 16) have been measured at 10 MHz as a function of temperature in the cubic plastic phase, with special attention given to the region just above the order-disorder transition into the tetragonal rigid phase. Both the velocity and attenuation of the C′ shear wave exhibit incipient critical behavior near this transition. The C′ stiffness also shows a pretransitional softening when this transition is approached by increasing the pressure along several isotherms. 相似文献
20.
Computation of non-linearity parameter ( B/A), molecular radius ( r
m) and intermolecular free length ( L
f) for H 2O, C 6H 6, C 6H 12, CH 3OH, C 2H 5OH and their deuterium-substituted compounds have been carried out at four different temperatures, viz., 293.15, 303.15, 313.15
and 323.15 K. The aim of the investigation is an attempt to study the isotopic effects on the non-linearity parameter and
the physicochemical properties of the liquids, which in turn has been used to study their effect on the intermolecular interactions
produced thereof. 相似文献
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