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1.
The optical absorption spectra from bismuth ferrite (BiFeO3) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ~1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe3+ ions near the transition. 相似文献
2.
The transition of Fe3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mössbauer spectroscopy) and FeK β high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover. 相似文献
3.
The magnetic behavior of a Bi57FeO3 powdered sample was studied at high pressures by the method of nuclear forward scattering (NFS) of synchrotron radiation. The NFS spectra from 57Fe nuclei were recorded at room temperature under high pressures up to 61.4 GPa, which were created in a diamond anvil cell. In the pressure interval 0 < P < 47 GPa, the magnetic hyperfine field HFe at the 57Fe nuclei increased reaching a value of ~52.5 T at 30 GPa, and then it slightly decreased to ~49.6 T at P = 47 GPa. As the pressure was increased further, the field HFe abruptly dropped to zero testifying a transition from the antiferromagnetic to a nonmagnetic state (magnetic collapse). In the pressure interval 47 < P < 61.4 GPa, the value of HFe remained zero. The field HFe recovered to the low-pressure values during decompression. 相似文献
4.
Optical absorption spectra of single crystals of the ferromagnetic semiconductor VBO3 are studied at high pressures up to 70 GPa achieved in a diamond-anvil cell. An electronic transition accompanied by sharp changes in the optical parameters and a decrease in the optical gap from E 0 = 3.02 eV to 2.25 eV is found at the pressure P C ~ 30 GPa. The gap does not disappear in the high-pressure phase and its value becomes typical of semiconductors. This is indicative of a semiconductor-semiconductor transition. The transition to the metallic state may occur at the critical pressure P met ≈ 290 GPa. 相似文献
5.
The vibrational frequencies of the BiFeO3 crystal lattice in the cubic phase (Pm3m) and the rhombohedral paraelectric phase (R3c) are calculated in terms of the ab initio model of an ionic crystal with the inclusion of the dipole and quadrupole polarizabilities. In the ferroelectric phase with the symmetry R3c, the calculated spontaneous polarization of 136 μC cm?2 agrees well with the experimental data. The dependences of the unit cell volume, the elastic modulus, and the vibrational frequencies on the pressure are calculated. It is found that the frequency of an unstable ferroelectric mode in both the cubic (Pm3m) and rhombohedral (R3c) phases are almost independent of the applied pressure, in contrast to classical ferroelectrics with a perovskite structure, where the ferroelectric instability is very sensitive to a variation in the pressure. 相似文献
6.
A. V. Nazarenko A. G. Razumnaya M. F. Kupriyanov Yu. V. Kabirov A. G. Rudskaya P. Yu. Teslenko N. B. Kofanova 《Physics of the Solid State》2011,53(8):1599-1602
The (1 − x)BiFeO3−x
YMnO3 solid solutions have been found to undergo the following sequence of phase transformations with increasing x: R3c → Pbnm → C2 → Pnma → P63
cm. It has been established that the Pbnm and Pnma phases have different orientations of atomic displacements and can exhibit antiferroelectric properties. 相似文献
7.
A. G. Gavriliuk I. A. Troyan R. Boehler M. I. Eremets I. S. Lyubutin N. R. Serebryanaya 《JETP Letters》2003,77(11):619-624
The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO3 orthoferrite single crystals is studied in diamond anvil cells. At P~37.5 GPa, an electronic transition at which the optical absorption edge jumps from ~2.2 to ~0.75 eV is observed. The equation of state V(P) is studied on the basis of the X-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at P~37 GPa with a jump of ~4% in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30–40 GPa is a transition of the insulator-to-semiconductor type. 相似文献
8.
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
9.
The polycrystalline samples of (Bi1?x K x ) (Fe1?x Nb x ) O3 (BKFN) for x = 0.0, 0.1, 0.2 and 0.3 were synthesized by a solid-state reaction method. The X-ray diffraction patterns of BKFN exhibit that the addition of KNbO3 in BiFeO3 gradually changes its structure from rhombohedral to pseudocubic. The analysis of scanning electron micrograph clearly showed that the sintered samples have well-defined and uniformly distributed grains. Addition of KNbO3 to BiFeO3 enhances the dielectric, ferroelectric and ferromagnetic properties of BiFeO3. Detailed studies of impedance and related parameters of BKFN using the complex impedance spectroscopic technique exhibit the significant contributions of grain and grain boundaries in the resistive and transport properties of the materials. Some oxygen vacancies created in the ceramic samples during high-temperature processing play an important role in the conduction mechanism. The leakage current or tangent loss of BiFeO3 is greatly reduced on addition of KNbO3 to the parent compound BiFeO3. Preliminary studies of ferroelectric and magnetic characteristics of the samples reveal the existence of ferroelectric, and weak ferromagnetic ordered ceramics. 相似文献
10.
S. V. Trukhanov A. V. Trukhanov A. N. Vasiliev H. Szymczak 《Journal of Experimental and Theoretical Physics》2010,111(2):209-214
The magnetic and thermal properties of the anion-deficient La0.70Sr0.30MnO2.85 manganite are investigated in wide temperature (4–350 K) range, including under hydrostatic pressure (0–1.1 GPa). Throughout
the pressure range investigated, the sample is spin glass with diffused phase transition into paramagnetic state. It is established,
that spin glass state is a consequence of exchange interaction frustration of the ferromagnetic clusters embeded into antiferromagnetic
clusters. The magnetic moment freezing temperature T
f
of ferromagnetic clusters increases under pressure, freezing temperature dependence on pressure is characterized by derivative
value ∼4.5 K/GPa, while the magnetic ordering T
MO temperature dependence is characterized by derivative value ∼13 K/GPa. The volume fraction of sample having ferromagnetic
state is V
fer ∼ 13% and it increases under a pressure of 1.1 GPa by ΔV
fer ≈ 6%. Intensification of ferromagnetic properties of the anion-deficient La0.70Sr0.30MnO2.85 manganite under hydrostatic pressure is a consequence of oxygen vacancies redistribution and unit cell parameters decrease.
The most likely mechanism of frustrated exchange interactions formation is discussed. 相似文献
11.
The density of liquid and undercooled BiFeO3 and high-temperature solid, liquid, and undercooled BaTiO3 was measured with an electrostatic levitation furnace. The density was obtained with an ultraviolet-based imaging technique that allowed excellent sample contrast throughout all phases of processing, including at elevated temperatures. Over the 1250- to 1490-K temperature range, the density of liquid BiFeO3 can be expressed as L(T)=6.70×103–1.31(T-Tm)(kgm-3) (±2 per cent) with Tm=1423 K, yielding a volume coefficient of thermal expansion L(T)=1.9×10-4 K-1. For BaTiO3, the density of the solid can be expressed as S(T)=5.04×103–0.21(T-Tm) (Tm=1893 K) over the 1220- to 1893-K range, yielding a volume coefficient of thermal expansion S(T)=4.2×10-5 K-1, whereas that of the liquid can be expressed as L(T)=4.04×103-0.34(T-Tm) over the 1300- to 2025-K range with L(T)=8.4×10-5 K-1. PACS 77.84.-s; 81.05.Je; 81.20.n 相似文献
12.
The crystal structure and magnetic properties of Bi1 − x
A
x
FeO3 − x/2 (A = Ca, Sr, Pb, Ba), Bi1 − x
A
x
(Fe1 − x
Ti
x
)O3, and Bi1 − x
A
x
(Fe1 − x/2Nb
x/2)O3 solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity
of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar
phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in
the polar and nonpolar phases differ only slightly. 相似文献
13.
I. O. Troyanchuk M. V. Bushinsky A. N. Chobot O. S. Mantytskaya N. V. Tereshko 《JETP Letters》2009,89(4):180-184
The magnetic properties of the Bi1 ? x Ln x FeO3 (Ln is a rare-earth ion), Bi1 ? x A x FeO3 ? x/2 (A is an alkali earth ion), and BiFe1 ? x Ti x O3 + δ solid solutions in magnetic fields up to 14 T have been studied. The concentration ranges of the existence of the ferroelectric phase described by the space group R3c have been determined. It is shown that the substitution of the rare-earth ions for the Bi3+ ions leads to a sharp decrease in the critical fields inducing the metamagnetic transition from a modulated antiferromagnetic state to a weakly ferromagnetic one; however, the modulated structure in the concentration range of the R3c phase is mainly retained. The substitution of the alkali earth ions (x ~ 0.1) for the bismuth ions leads to the total destruction of the modulated structure and to the implementation of the weakly ferromagnetic state within the R3c phase. A homogeneous weakly ferromagnetic state has been revealed when the Ti4+ ions (x = 0.1) are substituted for the Fe3+ ions in the ferroelectric R3c phase. 相似文献
14.
D. P. Kozlenko T. A. Chan S. E. Kichanov Z. Jirák L. S. Dubrovinsky B. N. Savenko 《JETP Letters》2011,92(9):590-594
The crystal structure and Raman spectra of Pr0.7Ca0.3MnO3 manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural
phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have
been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state
(T
C = 120 K) to the A-type antiferromagnetic state (T
N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering
have been discussed. 相似文献
15.
Kh. G. Bogdanova A. R. Bulatov V. A. Golenishchev-Kutuzov R. F. Mamin A. A. Potapov 《Physics of the Solid State》2007,49(3):519-522
The specific features of a phase transition from a disordered orbital state to an ordered orbital state in an La0.875Sr0.125MnO3 single crystal are investigated using acoustic methods at a frequency f = 500 MHz. The phase transition is accompanied by a distortion of MnO6 octahedra due to the cooperative Jahn-Teller effect and is a first-order phase transition, as judged from the sharp change observed in the damping of acoustic pulses, the acoustic wave velocity, and the temperature hysteresis. It is revealed that the parameters of the acoustic waves change significantly throughout the temperature range of existence of the cooperatively distorted structure. In an external magnetic field, the structural phase transition is shifted toward lower temperatures. 相似文献
16.
Within a nonempirical model of an ionic crystal with the inclusion of the dipole and quadrupole ion polarizations, the lattice vibrational frequencies, high-frequency dielectric constant, Born dynamic charges, and the elasticity moduli of the BiFeO3 crystal have been calculated and their dependencies on the hydrostatic pressure in the cubic, rhombic, and rhombohedral phases have been determined. The results indicate the presence of the ferroelectric instability, which depends weakly on the pressure in all of the phases investigated. The dependence of the crystal lattice dynamics on the applied pressure for the cubic phases of BiAlO3, BaTiO3, and PbTiO3 has been calculated for comparison. 相似文献
17.
K.?S.?Aleksandrov V.?N.?Voronov A.?N.?Vtyurin A.?S.?Krylov M.?S.?Molokeev M.?S.?Pavlovski? S.?V.?Gorya?nov A.?Yu.?Likhacheva A.?I.?Ancharov 《Physics of the Solid State》2009,51(4):810-816
Pressure-induced phase transitions in the ScF3 crystal were studied using synchrotron radiation diffraction, polarization microscopy, and Raman spectroscopy. The phase existing in the range 0.6–3.0 GPa is optically anisotropic; its structure is described by space group R 3 c (Z = 2), and the transition is due to rotation of ScF6 octahedra around a threefold axis. The pressure dependence of the structural parameters and angle of rotation are determined. The number of Raman spectral lines corresponds to that expected for this structure; above the phase transition point, a recovery of soft modes takes place. At a pressure of 3.0 GPa, a transition occurs to a new phase, which remains metastable as the pressure decreases. The results are interpreted using an ab initio method based on the Gordon-Kim approach. 相似文献
18.
V. A. Ryzhov P. L. Molkanov A. V. Lazuta V. V. Runov V. P. Khavronin I. O. Troyanchuk 《Bulletin of the Russian Academy of Sciences: Physics》2010,74(10):1477-1480
We present the results from studying the magnetic properties (linear and nonlinear susceptibilities and the depolarization
of polarized neutrons) of Nd1 − x
Ba
x
MnO3 manganite, x = 0.3, with Curie temperature T
C ≈ 140 K and dielectric-to-metal transition temperature T
DM ≈ 129 K. Its critical behavior corresponds to that of an isotropic 3-D ferromagnet at temperatures above T*≈ 144 K, but a strong nonlinear response in weak magnetic fields and depolarization are observed at temperatures below T*. It is shown that this nontraditional behavior is related to the generation of ferromagnetic clusters in the paramagnetic
matrix that form a conducting percolative network at temperatures near T
DM. 相似文献
19.
The isothermal magnetization of the Bi1 – xHo x FeO3 (x = 0?0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at PC ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO3 (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: PC decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi1 – xHo x FeO3 with x = 0.2, the transition pressure PC = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition. 相似文献
20.
The crystal structure and magnetic properties of the Bi1 ? x Ca x Fe1 ? x/2Nb x/2O3 system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe0.5Nb0.5O3, the Fe3+ and Nb5+ ions are partially ordered and the unit cell is monoclinic (space group P21/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed. 相似文献