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1.
Choe W Pecharsky VK Pecharsky AO Gschneidner KA Young VG Miller GJ 《Physical review letters》2000,84(20):4617-4620
A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si2Ge2), across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si,Ge) dimers that are richer in Ge increase their distances by 0.859(3) A and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the Gd5(SixGe1-x)(4). 相似文献
2.
G. D. Saraiva W. Paraguassu M. Maczka P. T. C. Freire F. F. de Sousa J. Mendes Filho 《Journal of Raman spectroscopy : JRS》2011,42(5):1114-1119
Temperature–dependent Raman studies of disodium dimolybdate (Na2Mo2O7) crystal are reported. Lattice dynamical calculation was used to predict both wavenumbers and atomic displacements (eigenvectors) for the vibrational modes. These calculations were based on the classical rigid‐ion model. The high‐temperature Raman scattering study of the crystal showed that it remains in the orthorhombic structure in the 8–848 K range and undergoes a structural phase transition between 848 and 854 K. This phase transition is most likely connected with weak tiltings and/or rotations of both MoO4 (tetrahedra) and MoO6 (octahedra) units, which lead to a disorder in the oxygen sublattice. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
3.
G. D. Saraiva W. Paraguassu M. Maczka P. T. C. Freire J. A. Lima C. W. A. Paschoal J. Mendes Filho A. G. Souza Filho 《Journal of Raman spectroscopy : JRS》2008,39(7):937-941
The mode assignment of the cubic phase of anhydrous Na2MoO4 was carried out on the basis of lattice dynamic calculation using the classical rigid‐ion model. Temperature‐dependent studies indicate that this crystal remains in the cubic structure in the 15–773 K range and undergoes a phase transition at around 783 K. The behavior of the Raman modes indicates that this transition is strongly first‐order in nature and the phase above 773 K may have an orthorhombic symmetry. This transition is connected with tilting and/or rotations of the MoO4 tetrahedra, which lead to a disorder at the MoO4 sites. Our results give also evidence that the Mo O bond lengths decrease in the high‐temperature phase. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
4.
《中国科学:物理学 力学 天文学(英文版)》2020,(7)
Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition(MST) from antiferromagnetic(AFM) orthorhombic phase to ferromagnetic(FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect(MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn_(0.95)Co_(0.75)Cu_(0.25)Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic(PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators. 相似文献
5.
《中国科学:物理学 力学 天文学(英文版)》2020,(10)
Topological crystalline insulator(TCI) is one of the symmetry-protected topological states. Any TCI can be deformed into a simple product state of several decoupled two-dimensional(2 D) topologically nontrivial layers in its lattice respecting its crystalline symmetries called the layer construction(LC) limit. In this work, based on first-principles calculations we have revealed that both tetragonal LaSbTe(t-LaSbTe) and orthorhombic LaSbTe(o-LaSbTe) can be interpreted as stacking of 2 D topological insulators in each lattice space. The structural phase transition from t-LaSbTe to o-LaSbTe due to soft phonon modes demonstrates how the real space change can lead to the modification of topological states. Their symmetry-based indicators and topological invariants have been analyzed based on LC. We propose that LaSbTe is an ideal example demonstrating the LC paradigm, which bridges the crystal structures in real space to the band topology in momentum space. 相似文献
6.
We have investigated, by X-ray diffraction, a series of single crystals of Bi-based oxides with the nominal composition Bi2Sr2(Cu1-zFez)O6+δ(0≤z≤0.55). In this system we observed two structural phase transitions with the increase of the doping content. The first transition, from an incommensurate monoclinic phase to an incommensurate orthorhombic phase, occurs at a doping content of iron zFe=0.027. The second one corresponds to a phase transition from an incommensurate orthorhombic phase to a commensurate orthorhombic phase at zFe=0.34. The comparison of these results with those for more limited substitutions of Zn and Ni indicates the significant role of the insertion of the extra oxygen in the (Bi-O)∞ double layers. 相似文献
7.
T. Thomsen H. G. Zachmann H. R. Kricheldorf 《Journal of Macromolecular Science: Physics》2013,52(1-2):87-99
The change of molecular mobility during the phase transition of PHB/3-C1-PHB copolymer (5:1) from the orthorhombic modification II into an orthorhombic modification which is close to hexagonal chain packing was investigated by means of 2H-NMR. It is shown that a flipping of the benzene rings takes place at temperatures far below the transition temperature. The phase transition starts after more than 60% of the benzene rings are flipping. Even in the newly formed phase close to the hexagonal chain packing, at temperatures up to 300°C, free rotation of the benzene rings can be excluded. 相似文献
8.
不同浓度的Cu元素掺杂会极大地影响TiNi二元合金的物理性质和相变行为.为了解释其中的物理机制,本文通过第一性原理计算,对TiNi和Ti_(50)Ni_(25)Cu_(25)的相变机制和相稳定性进行了计算和讨论.通过计算Cu掺杂前后立方相到正交相、再到单斜相过程中的相变路径和相变势垒,解释了Cu掺杂对二元合金TiNi相变过程的影响.计算结果表明:TiNi合金的正交相和单斜相之间存在一个大小为1.6meV的相变势垒;而对于Ti_(50)Ni_(25)Cu_(25),这两个相之间的相变势垒大小至少为10.3meV,如此大的一个相变势垒意味着Ti_(50)Ni_(25)Cu_(25)合金的正交相很难跨过势垒相变到单斜相. 相似文献
9.
We have investigated the influence of a phase transition in polycrystalline layers of lead tetroxide Pb3O4 on their diffuse reflection spectra in the wavelength range of 380–700 nm. We have determined differences in the character of the spectral dependences of the reflection coefficient and surface morphology that are related to the formation of a new high-temperature phase. We have determined values of the optical band gap from analysis of the diffuse reflection spectra of this metal oxide semiconductor compound under normal conditions and after its transition from the tetragonal to an orthorhombic modification. 相似文献
10.
Paul I 《Physical review letters》2011,107(4):047004
We examine the relevance of magnetoelastic coupling to describe the complex magnetic and structural behavior of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond the mean field, we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention: first, why iron telluride orders magnetically at a non-nesting wave vector (π/2,π/2) and not at the nesting wave vector (π,0) as in the iron arsenides, even though the nominal band structures of both these systems are similar, and second, why the (π,0) magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magnetoelastic interaction. 相似文献
11.
Structural stability of the perovskite-type GdMnO(3) has been investigated by the synchrotron angle-dispersive x-ray diffraction technique up to 63 GPa in a diamond anvil cell. GdMnO(3) stays in an orthorhombic structure but undergoes an isostructural phase transition with ~5% volume reduction at 50 GPa. In the parent orthorhombic phase, the compressions along a, b and c axes exhibit a large anisotropic behavior. With increasing pressure, our results show that the distortion and tilts of the MnO(6) octahedra are reduced continuously and the orthorhombic structure evolves towards higher symmetry. By fitting the observed pressure-volume data using the third-order Birch-Murnaghan equation of state, we obtain the bulk modulus B(0) = 156(3) GPa with B(0)' = 6.5(3) for the starting orthorhombic phase. Upon decompression, the starting orthorhombic phase is recovered. 相似文献
12.
E. M. Roginskii A. S. Krylov Yu. F. Markov M. B. Smirnov 《Bulletin of the Russian Academy of Sciences: Physics》2016,80(9):1033-1037
A theoretical model based on long-range dispersion corrections of the charge density functional is proposed for model Hg2Cl2 calomel crystals, typical representatives of molecular inorganic compounds where the intermolecular interaction is found to play an important role. This model successfully describes the electronic state and the phonon spectrum of the above crystal, predicts the earlier unstudied phase transition at high hydrostatic pressure. Study of the baric behavior of the phonon spectrum with Raman spectroscopy observes the soft mode in the low-symmetry orthorhombic phase with the frequency softening as the pressure rises. Pressures above 9 GPa considerably transform the Raman spectra, indicating a structural phase transition. 相似文献
13.
14.
Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations 下载免费PDF全文
The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained. 相似文献
15.
Phase transition and thermodynamic properties of ThO2:Quasi-harmonic approximation calculations and anharmonic effects 下载免费PDF全文
The thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are investigated using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon calculations. The anharmonic contribution to quasi-harmonic free energy is accounted for by using an effective method(2010 Phys. Rev. B 81 172301). The results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.45 GPa, which is consistent with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly. By comparing the experimental results with the calculation results, it is shown that the thermodynamic properties of ThO2 at high temperature improve substantially after including the anharmonic correction to quasi-harmonic free energy. 相似文献
16.
V. Srivastava M. Rajagopalan S. P. Sanyal 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):131-139
A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method
within the local-density approximation is used to calculate the total
energy, lattice parameter, bulk modulus, magnetic moment, density of states
and energy band structures of half-metallic CrO2 at ambient as well as
at high pressure. The magnetic and structural stabilities are determined
from the total energy calculations. From the present study we predict a
magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM)
state at 65 GPa, which is of second order in nature. We also observe from
our calculations that CrO2 is more stable in tetragonal phase
(rutile-type) at ambient conditions and undergoes a transition to an
orthorhombic structure (CaCl2-type) at 9.6 GPa, which is in good
agreement with the experimental results. We predict a second structural
phase transition from CaCl2- to fluorite-type structure at 89.6 GPa
with a volume collapse of 7.3%, which is yet to be confirmed
experimentally. Interestingly, CrO2 shows half metallicity under
ambient conditions. After the first structural phase transition from
tetragonal to orthorhombic, half metallicity has been retained in CrO2 and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a
pressure of 65 GPa. 相似文献
17.
Yoshikazu Mizuguchi Kentaro Hamada Kazuki Goto Hiroshi Takatsu Hiroaki Kadowaki Osuke Miura 《Solid State Communications》2012,152(12):1047-1051
The low-temperature crystal structure of Fe1.13Te, which exhibits an anomalous two-step magnetic transition, was clarified by the systematic x-ray diffraction measurements. It was found that two-step structural phase transition, tetragonal–orthorhombic–monoclinic, occurred correspondingly to the two-step magnetic transition. The detailed analysis of the profile at 5 K indicated the coexistence of the minor orthorhombic area inside the major, monoclinic lattice, which could explain the lower-shift (suppression) of the antiferromagnetic transition temperature in Fe1.13Te and suggest a possibility of superconductivity at the domain boundary. 相似文献
18.
Y. K. Sharma L. Iannarella F. E. Wagner C. A. Taft N. C. Furtado T. P. Arsenio 《Hyperfine Interactions》1988,41(1):517-520
Mössbauer experiments performed on CsFeS2 at temperatures between 4.2 K and 300 K show that the orthorhombic high temperature phase undergoes a second order magnetic phase transition near 69 K, when the previously reported first order magnetic and structural transition to a triclinic modification near 75 K is suppressed by lattice defects or internal stresses. The saturation values of the hyperfine fields are 19.1 T for the triclinic and 15.5 and 14.1 T for the orthorhombic phase. 相似文献
19.
Hg2Cl2 undergoes a second order phase transition around 185 K. All the elastic constants have been measured above (tetragonal phase) and below (orthorhombic phase) the transition point. The results support the hypothesis that the transition is induced by the condensation of a transversal acoustical mode at an X point of the first Brillouin zone in the tetragonal phase. The coefficients of the coupling terms in the free energy are evaluated and the relative values of the spontaneous deformations are evaluated. 相似文献
20.
Etgens VH de Camargo PC Eddrief M Mattana R George JM Garreau Y 《Physical review letters》2004,92(16):167205
Magnetic and structural properties of CrAs epilayers grown on GaAs(001) by molecular beam epitaxy have been studied. CrAs epilayers are orthorhombic for all thicknesses investigated but show a structural transition from a metastable phase for very thin films, to the usual bulk MnP-type orthorhombic phase at higher thicknesses. At intermediate thicknesses, there is a predominance of the new phase, although a contribution from the usual CrAs bulk phase remains clearly present. These results strongly suggest that the ferromagnetic signal measured at room temperature comes from the new metastable orthorhombic structure with an expanded b-axis induced by the substrate strain. 相似文献