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1.
Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs2CdI4 and Cs2ZnI4. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P21/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs2ZnI4 crystal in a broad pre-transition region (T - T i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).  相似文献   

2.
The magnetic layer structure of TlCo2Se2−xSx has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo2Se2 form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.  相似文献   

3.
We have investigated the structural, mechanical and lattice dynamical properties of ZrW2 and HfW2 compounds in cubic C15 (space group Fd-3m), hexagonal C14 (space group P63/mmc) and C36 (space group P63/mmc) phases using generalized gradient approximation within the plane-wave pseudo-potential density functional theory. We have found that ZrW2 and HfW2 in cubic C15 phase are the most stable among the considered phases. From calculated elastic constants, it is shown that all phases are mechanically stable according to the elastic stability criteria. The related mechanical properties, such as bulk, shear and Young moduli, Poisson’s ratio, Debye temperature and hardness have been also calculated. The results show that ZrW2 and HfW2 compounds are ductile in nature with respect to the B/G and Cauchy pressure analysis. The phonon dispersion curves, phonon density of states and some thermodynamic properties are computed and discussed exhaustively for considered phases.  相似文献   

4.
We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi2B2C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T N, but no changes are seen at T F.  相似文献   

5.
A rich sequence of structural modulations in Cs2HgCl4 as a function of temperature was studied by means of X-ray diffraction. Accurate satellite-position measurements on the cooling and heating paths of the crystal revealed abnormal thermal hystereses for incommensurate phases and coexistences of neighboring commensurate phases. A well-defined X-ray picture of the a-axis modulated phases in the range of 221–184 K were observed on the heating path, while the c-axis modulated phases existing below 184 K were definitely detected on the cooling path. The proper conditions for a precise phase diagram of Cs2HgCl4 can be correlated with relatively defect-free transformations of a-axis modulations at heating and of c-axis modulations at cooling. The peculiarity of Cs2HgCl4 to switch modulation direction among the a- and c-axes at 184 K allows us deliberately accumulate and thus control a majority of mobile defects on the mutually perpendicular (100) or (001) planes by possessing crystal within temperature domain of a- or c-axes modulations, respectively.  相似文献   

6.
The structural phase transitions of the layer compound BaMnF4 were studied by high-resolution X-ray diffraction using synchrotron radiation. The intensities and profiles of two kinds of superlattice reflections having incommensurate reduced wave vectors q 1= (~ ± 1/5,0,0) p and q 2 =(~ ± 2/5,1/2,1/2)p, respectively, were measured as a function of temperature from 25 K to 280 K. These temperature dependencies show that incommensurate structural phase transitions of second order occur at 234 K and 244 K. These structural phase transitions are interpreted as successive condensations of a folding-screen-like incommensurate plane-distortion mode and a commensurate anti-ferro-distortive tilting mode of the MnF6 octahedra around the primitive a0 - and b0 -axes, when cooled down. It is also found that there is another structural phase transition at about 45 K related to a precursor structural distortion for the antiferromagnetic transition occurring at about 26 K.  相似文献   

7.
Several new phases containing platinum in Y-Ba-Cu-O system are reported in this paper. When Ba-Y-Cu-O system containing platinum is sintered at 950°C, two hexagonal phases YBa2(Pt, Cu)3O9-δ and Ba4Pt1+x Cu2-x Cu2-x O9-δ (x = 0.1 – 0.9) are found, respectively. The super-K2NiF4 structure Ba x Y2-x (Cu, Pt)O4 is obtained above 1000°C in the Y-Ba-Cu-O system containing platinum. Electrical properties of these phases are also reported.  相似文献   

8.
Various solid solutions TlCo2−xMexSe2 (Me=Fe, Ni and Cu) have been investigated by neutron powder diffraction, supplemented by magnetometry. The incommensurate spin-helix running along the c-axis in tetragonal TlCo2Se2 prevails for low concentrations of copper and iron but changes pitch. In the copper case, only cobalt carries a magnetic moment. On nickel substitution, however, collinear antiferromagnetic coupling between the ferromagnetic layers occurs. The magnetic moment distribution between the two transition metals in the solid solution TlCo2−xNixSe2 was tentatively probed with first principle calculations on fictive ordered TlCoNiSe2, modelled by two types of superstructures. Also the ternary mother compounds, Pauli paramagnetic TlNi2Se2 and antiferromagnetic TlCo2Se2, were investigated with the same LMTO method.  相似文献   

9.
《Molecular physics》2012,110(17):2003-2018
An algebraic model to describe inelastic collisions between two anharmonic diatomic molecules in the semiclassical approximation is presented. The interactions for the diatomic systems are modelled in terms of Morse potentials, while an exponential repulsive potential is taken for the interactions between the nearest atoms of the diatomic systems. This problem is treated in the interaction potential framework, where an approximation in terms of the generators of three SU(2) groups is proposed, two corresponding to the Morse oscillators and the other to the interaction. The transition probabilities are given in terms of a sum of the products of three Wigner's d(β) functions corresponding to the three SU(2) groups. As an example the systems N2?+?N2 and H2?+?H2 are described and compared with exact quantum mechanical calculations.  相似文献   

10.
The phase diagram of ferroelastic Pb3(PO4)2-Pb3(AsO4)2 has been reinvestigated and four new phase transitions have been found from birefringence measurements at low temperatures. The first-order phase transition between the phases b (C2/c) and c (P21/c) in Pb3(AsO4)2 becomes continuous with increasing P-content.

In Pb3(P0.2As0.8O4)2, a monoclinic intermediate phase bc was identified from the measurements of the spontaneous strain, the morphic birefringence and the orientation of the optical indicatrix.

The experimental results are interpreted using the Landau theory of biquadratically coupled order parameters. Phase b is related to an order parameter of L-point symmetry in the paraelastic phase a (3). The phase c is due to a critical F-point in 3. The intermediate phase bc is the consequence of the biquadratic coupling of order parameters with L-and F-point symmetry.

Model calculations for Pb3(P0.2As0.8O4)2 relate the free energies of the phases b, bc and c and the temperature dependence of the order parameters with the measured optical birefringence. They explain well the occurrence of an intermediate phase with monoclinic symmetry from the movement of the thermodynamic stability point in the vector space diagram of order parameters.  相似文献   

11.
It is shown that the first order quadrupole split NMR satellite transition frequencies of the87Rb nucleus can be detected in the paraelectric, incommensurate and ferroelectric phases of Rb2ZnCl4. From rotation patterns the electric field gradient tensor at the Rb sites is determined for the paraelectric phase. The data demonstrate a considerable influence of the structural changes in the incommensurate and ferroelectric phases on the observed NMR transition frequencies. For some crystal orientations the satellite transitions are followed through the incommensurate into the ferroelectric phase. Whereas in the former typical quasi continuous spectra are observed in the latter several sharp lines appear. The results are discussed in relation to the structural changes at the phase transitions.  相似文献   

12.
采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法, 通过自旋极化的广义梯度近似(GGA)电子结构计算对梯形化合物NaV2O4F进行了研究. 考虑了四种假想的自旋有序态,计算结果表明该化合物的磁基态具有二维反铁磁(AFM)结构, 即沿梯阶和梯腿方向都表现为AFM作用. 能带结构显示NaV2O4F为绝缘体材料, 带隙约为1.0eV. 方锥体中的晶体场劈裂使得VO4F方锥体中的 V4+ 关键词: 2O4F')" href="#">NaV2O4F 梯形化合物 第一性原理计算 电子结构  相似文献   

13.
Eneroth  E.  Bender Koch  C. 《Hyperfine Interactions》2004,158(1-4):423-427
The perturbed angular correlation method (PAC) was applied to investigate the lattice location of implanted 111In probe ions in Hf2Ni and Zr2Ni intermetallic compounds. It is concluded that the 111In/111Cd probe nuclei experiencing the highly asymmetric electric field gradient (EFG) occupy the unique hafnium or zirconium 8(h) sites in the investigated phases. Above room temperature, the EFGs decrease linearly with temperature. The results are compared with that of previous PAC measurements with 181Ta probes.  相似文献   

14.
A comprehensive investigation of the structural, elastic, and lattice dynamical properties for ZrMo2 and HfMo2 with C14, C15, C36, and CeCu2 phases are conducted using density functional total energy calculations. The results have showed that C15 phase for both materials is energetically more stable than C14, C36 and CeCu2 phases. We have also estimated the mechanical behaviours of these compounds, including mechanical stability, bulk modulus, Young's modulus, shear modulus, Poisson's ratio, ductility, and anisotropy. Additionally, the lattice dynamical properties are analyzed and discussed exhaustively for these phases. The calculated properties agree well with available experimental and theoretical data.  相似文献   

15.
The TiO2-Mn-TiO2 multilayers are successfully grown on glass and silicon substrates by alternately using radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and the magnetic behaviours of these films are characterised with x-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer, and superconducting quantum interference device (SQUID). It is shown that the multi-film consists of a mixture of anatase and rutile TiO2 with an embedded Mn nano-film. It is found that there are two turning points from ferromagnetic phase to antiferromagnetic phase. One is at 42 K attributed to interface coupling between ferromagnetic Mn3O4 and antiferromagnetic Mn2O3, and the other is at 97 K owing to the interface coupling between ferromagnetic Mn and antiferromagnetic MnO. The samples are shown to have ferromagnetic behaviours at room temperature from hysteresis in the M-H loops, and their ferromagnetism is found to vary with the thickness of Mn nano-film. Moreover, the Mn nano-film has a critical thickness of about 18.5 nm, which makes the coercivity of the multi-film reach a maximum of about 3.965×10 2 T.  相似文献   

16.
We report Cu-NQR results on Ge-doped heavy-fermion superconductor CeCu2(Si1–x Ge x )2 (0<x0.2) and undoped Ce0.99Cu2.02Si2. The main effect of the Ge doping is considered to be a negative pressure, since the strength of hybridization decreases by the Ge doping. With increasing x, the dynamical characteristics of the magnetic order at x=0 change to more static ones which suggests a localized regime above x0.25. From the derived Tx phase diagram, it is suggested that the magnetic and the superconducting phases are almost degenerate in undoped CeCu2Si2. An exotic interplay between the magnetism characterized by the slow fluctuations and the superconductivity is implied in the region of small x.  相似文献   

17.
Previous work on Ba2TiGe2O8 crystals has shown an unusual low-temperature (~ 223 K on cooling, ~ 273 K on heating) phase transition. Precession x-ray photographs on Ba2TiGe2O8 single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.  相似文献   

18.
On the basis of temperature dependences of the acoustic wave velocity and the acoustic attenuation in Sn2P2(Se0.28S0.72)6 crystals under normal conditions and hydrostatic pressures, we show that increasing pressure causes these crystals to move away from the tricritical point and enter into the region of first-order phase transitions (PTs) in (P,?T,?x)-space. Still higher hydrostatic pressures lead to an increase in the attenuation of the acoustic wave with the velocity v 22 and broadening of its anomaly in the PT region, thus hinting at splitting of the PT, with the appearance of an intermediate incommensurate phase approximately at P?=?3.3?kbar. We offer a (P,?T,?x)-phase diagram for the solid solutions Sn2P2(Se x S1? x )6, which includes two special lines of PTs, tricritical and triple ones, which can intersect at the Lifshitz point at negative hydrostatic pressures. The variations of the surface of acoustic wave velocities occurring with changing temperature are obtained for Sn2P2(Se0.28S0.72)6 crystals under the atmospheric pressure.  相似文献   

19.
K Shah  D K Aswal  Ajay Singh  L C Gupta  S K Gupta  V C Sahni 《Pramana》2002,58(5-6):1085-1088
The layered manganite Nd2−2x Sr1+2x Mn2O7, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd2−2x Sr1+2x Mn2O7 do not form the single-phase layered compound with A3B2O7 structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd1−z Sr z MnO3, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.  相似文献   

20.
High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al2O3:SiO2 ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature Tc > 3000°C.  相似文献   

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