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1.
Nonequilibrium phase transformations in D2O ices, including the solid-state amorphization of ice 1h (1h-hda) and the heating-induced transition cascade hda-lda-1c-1h from high-density amorphous (hda) ice to low-density amorphous (lda) ice followed by crystallization in cubic ice 1c and phase transition to ordinary hexagonal ice 1h, were studied using an ultrasonic technique. It has been shown that, as in H2O ice, the softening of a crystal lattice or an amorphous network precedes nonequilibrium transformations. However, noticeable isotopic differences in the behavior of the elastic properties of H2O and D2O, in particular, their 1h and hda modifications, call for a more detailed study of the structural features of these H2O and D2O phases.  相似文献   

2.
The transition of the hexagonal ice phase Ih to the clathrate phase sII has been found in the H2O-H2 system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the IhsII and sII → Ih transitions have been determined over a temperature range from ?36 to ?18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the Ih and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → Ih transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.  相似文献   

3.
The parameters of the long-wavelength exciton band for Rb2CdI4 films are investigated in the temperature range 90–410 K. It is found that the Rb2CdI4 films undergo a sequence of phase transitions at temperatures Tc1=380 K (paraphase → incommensurate phase), Tc2=290 K (incommensurate phase → ferroelastic phase I), and Tc3 = 210 K (ferroelastic phase I → ferroelastic phase II). The parameters of the exciton band (such as the spectral position and the half-width) measured during heating and cooling of the Rb2CdI4 film differ significantly. This is especially true for the incommensurate phase. Upon heating of the incommensurate phase, the domain boundaries become frozen, whereas the cooling of this phase is accompanied by the generation of solitons and their pinning, which, in turn, results in a first-order phase transition at the temperature Tc2. It is revealed that the oscillator strength of the exciton band anomalously increases in the range of existence of commensurate phase I (Tc3<-T<-Tc2) due to ordering of the Rb2CdI4 crystal lattice.  相似文献   

4.
In the absorption spectra of the hexagonal single-crystal manganite HoMnO3 in the paramagnetic ferroelectric state, lines near 1.1 and 2.0 μm were observed associated with the transitions 5 I 85 I 6 and 5 I 85 I 7, respectively, within the 4f 10 configuration of the Ho3+ ion. At T = 80 K, to the 5 I 85 I 7 transition corresponds one band at 1.9 μm for both polarizations Ec and Ec. As the temperature increases from 80 to 293 K, a low-energy band with a peak at 2.04 μm for Ec and a peak at 2.07 μm for Ec arises associated with transitions from an excited Stark level of the ground 5 I 8 multiplet to the Stark levels of the 5 I 7 multiplet and with an increase in the population of the initial Stark level, the energy of which is ~100 K.  相似文献   

5.
The results of x-ray structural studies of the [N(C2H5)4]2CdBr4 crystal at low temperatures are presented. The unit cell parameters and the thermal expansion coefficients along the main crystallographic directions are measured at temperatures in the range from 90 to 320 K. The integrated intensities of the diffraction reflections are investigated as a function of the temperature. It is shown that the curves a = f(T), c = f(T), I 500 = f(T), and I 006 = f(T) at temperatures T 1 ≈ 174 K and T 2 ≈ 226 K exhibit anomalies in the form of abrupt changes in the lattice parameters and the diffraction reflection intensities. This indicates that the [N(C2H5)4]2CdBr4 crystal undergo phase transitions at these temperatures. Moreover, there is an anomaly in the form of a small maximum at the temperature T 3 = 293 K.  相似文献   

6.
The temperature behavior of I-U curves and the field and temperature dependences of the electrical resistivity and dielectric permittivity of crystals of the LiCu2O2 phase have been studied. It was established that the crystals belong to p-type semiconductors and that their static resistivity in the range 80–260 K follows the Mott law ρ=Aexp(T0/T)1/4 describing variable-range hopping over localized states. At comparatively low electric fields, the crystals exhibit threshold switching and characteristic S-shaped I-U curves containing a region of negative differential resistivity. In the critical voltage region, jumps in the conductivity and dielectric permittivity are observed. Possible mechanisms of the disorder and electrical instability in these crystals are discussed.  相似文献   

7.
(NH4)3ZrF7 single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T 1↑ = 280 K, T 2↑ = 279.6 K, T 3↑ = 260–265 K, and T 4↑ = 238 K on heating and at T 1↓ = 280 K, T 2↓ = 269–270 K, T 3↓ = 246 K, and T 4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O h 5 (Z = 4) ? D 2h 25 (Z = 2) ? C 2h 3 (Z = 2) ? C i 1 (Z = 108) ? monoclinic2(Z = 216).  相似文献   

8.
Temperature dependences of specific heat Cp(T) and coefficient of thermal expansion ;(T) for Na0.95Li0.05NbO3 sodium-lithium niobate ceramic samples are investigated in the temperature range of 100–800 K. The Cp(T) and α(T) anomalies at T3 = 310 ± 3 K, T2 = 630 ± 8 K, and T1 = 710 ± 10 K are observed, which correspond to the sequence of phase transitions N ? Q ? S(R) ? T2(S). The effect of heat treatment of the samples on the sequence of structural distortions was established. It is demonstrated that annealing of the samples at 603 K leads to splitting of the anomaly corresponding to the phase transition QR/S in two anomalies. After sample heating to 800 K, the only anomaly is observed in both the Cp(T) and ;(T) dependence. Possible mechanisms of the observed phenomena are discussed.  相似文献   

9.
Polarization-optical study of twinning and measurements of the Raman spectra and birefringence in oxyfluoride (NH4)3Ti(O2)F5 were carried out over the temperature range 90–350 K. Phase transitions were detected at temperatures T 01 = 266 K (second-order transition) and T 02 = 225 K (first order). It is assumed that the crystal symmetry is changed as follows: Fm3m ? I4/mmm ? I4/m. Anomalies of the spectral parameters are established in the frequency range of internal vibrations of ammonium ions and Ti(O2)F5 complexes. An analysis of the results shows that the transition at T 01 is likely due to small shifts of the tetrahedral groups from their position on the triad axis and that the transition at T 02 is due to fluorine-oxygen ordering of Ti(O2)F5 complexes.  相似文献   

10.
The sequence of the ground states in SrTiO3 films subjected to epitaxial strain and fixed mechanical stress in the [001] and [110] directions is calculated from first principles within the density functional theory. Under the fixed-strain conditions, an increase in the substrate lattice parameter results in the following sequence of the ground states: I4cmI4/mcmIma2 → CmFmm2 → Ima2(II). When moving to the fixed-stress conditions, the phase sequence changes significantly and depends on how the stress is applied. It is revealed that the simultaneous presence of two types of the lattice instability (the ferroelectric and structural ones) in strontium titanate leads to the formation of a whole system of metastable phases whose number increases abruptly under the fixed-stress conditions. The stability of these phases changes with pressure and phase transitions occur between them. The appearance of broad bistability regions in certain parts of the phase diagram enables the use of this phenomenon for developing nonvolatile phase-change memory.  相似文献   

11.
Electrical transport and structural characterizations of isoelectronically substituted Ba(Fe0.9Ru0.1)2As2 have been performed as a function of pressure up to ~ 30 GPa and temperature down to ~ 10 K using designer diamond anvil cell. Similar to undoped members of the AFe2As2 (A = Ca, Sr, Ba) family, Ba(Fe0.9Ru0.1)2As2 shows anomalous a-lattice parameter expansion with increasing pressure and a concurrent ThCr2Si2 type isostructural (I4/mmm) phase transition from tetragonal (T) phase to a collapsed tetragonal (cT) phase occurring between 12 and 17 GPa where the a is maximum. Above 17 GPa, the material remains in the cT phase up to 30 GPa at 200 K. The resistance measurements show evidence of pressure-induced zero resistance that may be indicative of high-temperature superconductivity for pressures above 3.9 GPa. The onset of the resistive transition temperature decreases gradually with increasing pressure before completely disappearing for pressures above ~ 10.6 GPa near the T-cT transition. We have determined the crystal structure of the high-T c phase of Ru-doped BaFe2As2 to remain as tetragonal (I4/mmm) by analyzing the X-ray diffraction pattern obtained at 10 K and 9.7 ± 0.7 GPa, as opposed to inferring the structural transition from electrical resistance measurement, as in a previous report [S.K. Kim, M.S. Torikachvili, E. Colombier, A. Thaler, S.L. Bud’ko, P.C. Canfield, Phys. Rev. B 84, 134525 (2011)].  相似文献   

12.
The spins of Ru5+ ions in Sr2YRuO6 form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/JI/J~10?3 values correspond to the experimental T N and sublattice magnetic moment values.  相似文献   

13.
The spinel structure of lithium titanate Li4Ti5O12 is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li4Ti5O12 compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li4Ti5O12 compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)8a[Li0.33Ti1.67]16dO4 → [Li□]16c[Li1.33Ti1.67]16dO4 → [Li1.330.67]16c[Ti1.670.33]16dO4. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....  相似文献   

14.
The thermoelectric properties of n-Bi2 ? x Sb x Te3 ? y ? z Se y S z solid solutions are studied in the temperature range 300–550 K. It is shown that an increase in the parameter β determining the figure-of-merit Z of the material is observed in compositions with the optimally related effective mass of the density of states m/m 0, the carrier mobility μ0, and the lattice thermal conductivity κ L . Within the temperature range 300–350 K, the parameter β and the figure-of-merit Z are found to increase in solid solutions with substitutions in both bismuth telluride sublattices Bi → Sb and Te → Se, S (x = 0.16, y = z = 0.12) for optimum electron concentrations. An increase in the electron concentration and substitutions of atoms only in the tellurium sublattice bring about an increase in the β parameter and the value of Z at higher temperatures. Within the range 350–450 K, the parameters β and Z are observed to increase in a solid solution with a low content of substituted atoms in the tellurium sublattice Te → Se, S for y = z = 0.09 and, at higher temperatures up to 550 K, in compositions with tellurium substituted by selenium only, with increasing content of substituted atoms.  相似文献   

15.
The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La0.82Ca0.18MnO3 single crystal with the Curie temperature T C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.  相似文献   

16.
We investigate the Josephson effect in SFXSF junctions, where SF is a superconducting material with a ferromagnetic exchange field, and X is a weak link. The critical current Ic increases with the (antiparallel) exchange fields, if the distribution of transmission eigenvalues of the X layer has its maximum weight at small values. This exchange-field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an SFISF junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference in the exchange fields h1-h2 in an SFISF junction corresponds to the potential difference V1-V2 in an SIS junction; i.e., the singularity in Ic [in an SFISF junction] at |h1-h2|=Δ12 is the analogue of the Riedel peak. We also discuss the AC Josephson effect in SFISF junctions.  相似文献   

17.
The thermopower, S, magnetothermopower, ΔS/S, resistivity, ρ, and magnetoresistivity, Δρ/ρ, depending on the temperature T and magnetic field H, have been studied in an Nd0.5Sr0.5MnO3 single crystal consisting of three types of clusters: an antiferromagnetic CE-type with charge-orbital ordering (below the Neel temperature TNCE ~ 145 K) and an A-type with TNA ~ 220 K; a ferromagnetic at 234 ≤ T ≤ 252 K, and a ferromagnetic metal phase below the Curie temperature TC = 248 K. The thermopower was found to be negative, indicating the dominance of the electronic type of conductivity. In the S(T) curves, a sharp minimum is observed in the temperature range of 100 K ≤ T ≤ 133 K, close to TNCE, where the absolute S value attains 53 μV/K. With a further increase in temperature, the absolute S value decreases rapidly; at 200 K it is equal to 7 μV/K. It then slightly increases, reaching its maximum value of 15 μV/K at a temperature of 254 K, which is close to TC. The absolute thermopower decreased under the influence of the magnetic field; i.e., a negative magnetothermopower occurs. In {ΔS/S}(T) curves, a sharp minimum is observed at T = 130 K close to TNCE, where the magnetothermopower reaches a huge value of ~45% at H = 13.23 kOe. A broad minimum in the {ΔS/S}(T) curves is observed near the Curie temperature and its value is also high, viz., ~15% in the maximum measuring magnetic field of 13.23 kOe. The extremely high magnetothermopower values mean that the charge-orbital ordered nanoclusters or ferron type make the main contribution to the thermopower of the entire sample. The behavior of the ρ(T) and {Δρ/ρ}(T) curves is similar to that of the S(T) and {ΔS/S}(T) dependencies, which is in agreement with this conclusion.  相似文献   

18.
The spectra of magnetic circular dichroism in the range of the 7 F 65 D 4 absorption band and the spectra of magnetic circular polarization of luminescence in the range of the 5 D 47 F 5 band in the terbium-gallium garnet Tb3Ga5O12 are studied at a temperature of 80 K. The optical transitions between the Stark sublevels of the 7 F 6, 7 F 5, and 5 D 4 multiplets are identified based on the analysis of the magneto-optical and optical spectra. It is shown that the experimentally determined symmetry and energy of the Stark sublevels of these multiplets confirm the results of numerical calculations of the energy spectrum of the Tb3+ rare-earth ion in terbium-gallium garnet.  相似文献   

19.
The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co2CrAl Heusler alloy (HA) films has been investigated. We show that Co2CrAl films with L21 order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T 3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L21 → B2 → A2 → amorphous state in Co2CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ?(T = 293 K) ~ 200 μΩ cm in comparison to ?(T = 293 K) ~ 230 μΩ cm typical for the Co2CrAl films with L21 order. The magnetic-field dependence of MR of the Co2CrAl films with L21 order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co2CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.  相似文献   

20.
The unit cell parameters a, b, and c of [N(CH3)4]2ZnCl4 have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients αa, αb, and αc along the principal crystallographic axes and of the unit cell thermal expansion coefficient αV were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T1=161 K and T2=181 K and that the phase transition occurring at T3=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T1=161 K and T2=181 K in [N(CH3)4]2ZnCl4 were established to be first-order.  相似文献   

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