Pbam Phase Research Articles

Electric relaxation and Mn3+/Mn4+ charge transfer in Fe-doped Bi12MnO20\u2013BiMn2O5 structural self-composite

Fe-doped Bi12MnO20–BiMn2O5 ceramics was sintered at 1130 K for 6 h in ambient air. Two centro-symmetric phases formed thermodynamically stable self-composite material that was deduced from X-ray pattern analysis. The lattice parameters were a = 10.147(8) Å—for the cubic I23 Bi12MnO20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å—for the orthorhombic Pbam BiMn2O5 phase. The 57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe3+ ions distributed in two sites, i.e., octahedral Fe4+O6 and square pyramidal Fe3+O5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe3+ in tetrahedral Fe3+O4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity ρDC(T) dependence on temperature has shown thermally activated features, and the value of Ea,DC varied in the range of 0.22–0.37 eV. The electric impedance was measured in the f = 20 Hz–1 MHz and 100–690 K range. Two electrical relaxations were determined using the electric modulus formalism M″(T). Low-temperature relaxation has shown the temperature-dependent activation energy EA,1 = 0.14–0.20 eV and characteristic time values of τ01 = 10−10–10−12 s in 100–200 K range. It was attributed to the charge transfer between Mn4+/Mn3+ sites. The other relaxation occurred in the 170–220 K range, and it exhibited the following values: τ02 = 10−11 s, and EA,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for ρDC(T) and for electric relaxation processes.

Quantitative phase separation in multiferroic Bi0.88Sm0.12FeO3 ceramics via piezoresponse force microscopy

BiFeO3 (BFO) is a classical multiferroic material with both ferroelectric and magnetic ordering at room temperature. Doping of this material with rare-earth oxides was found to be an efficient way to enhance the otherwise low piezoelectric response of unmodified BFO ceramics. In this work, we studied two types of bulk Sm-modified BFO ceramics with compositions close to the morphotropic phase boundary (MPB) prepared by different solid-state processing methods. In both samples, coexistence of polar R3c and antipolar Pbam phases was detected by conventional X-ray diffraction (XRD); the non-polar Pnma or Pbnm phase also has potential to be present due to the compositional proximity to the polar-to-non-polar phase boundary. Two approaches to separate the phases based on the piezoresponse force microscopy measurements have been proposed. The obtained fractions of the polar and non-polar/anti-polar phases were close to those determined by quantitative XRD analysis. The results thus reveal a useful method for quantitative determination of the phase composition in multi-phase ceramic systems, including the technologically most important MPB systems.

Structural evolution and enhanced magnetization of Bi1−xPrxFeO3

The structural, ferroelectric, vibrational, and magnetic properties of polycrystalline Bi1−xPrxFeO3 (0≤x≤0.50) powders are investigated by the measurements of X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering spectroscopy, and magnetization at room temperature. Our results reveal that the rare-earth ion Pr substitution at Bi site causes the structural transformations from rhombohedral R3c phase to orthorhombic Pbam phase at x≈0.15, and then to orthorhombic Pnma phase at higher Pr concentration x≈0.25, accompanying the ferroelectric–antiferroelectric–paraelectric phase transition. Measurements of magnetic properties confirm that Pr substitution can improve the magnetization of BiFeO3 before approaching the first phase transition. We find that an inverse behavior occurs after passing a maximum across the antiferroelectric–paraelectric phase boundary. We also obtain the unique switching behavior in low magnetic field of Bi0.5Pr0.5FeO3 from the field dependence on magnetization, indicating the existence of the antiferromagnetic ordering.

Synthesis-phase–composition relationship and high electric-field-induced electromechanical behavior of samarium-modified BiFeO3 ceramics

Solid-state (non-activated) and mechanochemical activation (activated) synthesis methods were used to produce Sm-modified BiFeO3 ceramics of composition Bi0.88Sm0.12FeO3. The first part shows that the formation of Bi0.88Sm0.12FeO3 using the two synthesis methods followed a different reaction pathway on annealing the powders. The non-activated ceramics reacted by forming two intermediate phases, isostructural to BiFeO3 and SmFeO3, and then inter-diffusing, forming the final Bi0.88Sm0.12FeO3 solid solution. Unlike the non-activated samples, the activated ceramic powders formed Bi0.88Sm0.12FeO3 phase on annealing the powders, without apparent intermediate phases. As revealed by transmission electron microscopy, the non-activated reaction pathway caused the Pbam phase to form as chemical inhomogeneous (Sm-rich) isolated nano-sized grain inclusions in the final ceramics. Conversely, the activated reaction pathway caused the Pbam phase to form chemically homogeneous nano-regions within the R3c phase grains. The results demonstrate the important role of processing in the appearance of the frequently discussed anti-polar Pbam phase in this system. In the second part, the high electric-field-induced polarization and strain behaviors of these ceramics were studied by means of polarization–electric (P–E) and strain–electric field (S–E) hysteresis loops, and the S–E loops were compared with those of unmodified BiFeO3. Bipolar S–E loops of Bi0.88Sm0.12FeO3 had a distinctive butterfly shape with less frequency dependence relative to BiFeO3 at driving-field frequencies of 0.1–100Hz. BiFeO3 ceramics exhibite strong driving electric-field-frequency-dependent domain switching, the origins of which were previously attributed to a domain-wall pinning mechanism and “hardening” behavior. This study shows that Sm-modification induces a “hardening–softening” transition in BiFeO3 ceramics.

Structure and growth of single crystal SiP2 using flux method

A new orthorhombic phase of silicon diphosphide has been grown in Sn flux by using Gd as a mineralizer. It is a needle-like crystal and its structure has been determined through single crystal X-ray diffraction and elemental analysis. It crystallizes in the orthorhombic space group Pnma (No. 62, Z = 8) with cell parameters: a = 10.0908(19) Å, b = 3.4388(6) Å and c = 13.998(3) Å and the final R value is 0.0294. It has a layered structure that is closely related to the Pbam phase of SiP2. Its optical band gap is 1.45 eV and it decomposes at 1002 K.

BaC: a thermodynamically stable layered superconductor.

To predict all stable compounds in the Ba-C system, we perform a comprehensive study using first-principles variable-composition evolutionary algorithm USPEX. We find that at 0 K the well-known compound BaC2 is metastable in the whole pressure range 0-40 GPa, while intercalated graphite phase BaC6 is stable at 0-19 GPa. A hitherto unknown layered orthorhombic Pbam phase of BaC has structure consisting of alternating layers of Ba atoms and layers of stoichiometry Ba2C3 containing linear C3 groups and is predicted to be stable in the pressure range 3-32 GPa. From our electron-phonon coupling calculations, the newly found BaC compound is a phonon-mediated superconductor and has a critical superconductivity temperature Tc of 4.32 K at 5 GPa. This compound is dynamically stable at 0 GPa and therefore may be quenchable under normal conditions.

Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral (R3c) phase of BiFeO3 first transforms to monoclinic (C2/m) phase at 7 GPa, then to orthorhombic (Pnma) phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5–7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic (Pbam) phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.

Особенности структурных превращений в сегнетоэлектрической керамике PbZr1-xTixO3 (ЦТС) при х≤0,08

Dielectric and pyroelectric properties of ceramics PbZr1-xTixO3 (PZT) in the concentration range 0.01<x<0.08 were investigate. As a matter of record it was found that in the PZT there is a region wherein appear three inhevent for the entire family of perovskite instabilities of original cubic lattice. It is shown that the antiferroelectric phase in PZT exists up to x=0.07. Studies of a composition with x = 0.08 in the temperature range -200ºC<T<300ºC had not revealed existence of antiferroelectric phase that makes evident the existence of vertical boundary between the Pbam and R3c phases.

Antipolar phase in multiferroic BiFeO3at high pressure

The crystal and magnetic structures of BiFeO${}_{3}$ have been studied at high pressures up to 8.6 GPa by means of neutron powder diffraction at ambient temperature. Upon compression, a reduction of the spontaneous ferroelectric polarization is evidenced from the analysis of atomic displacements in the rhombohedral phase. A structural phase transition from the polar rhombohedral $R$3$c$ phase to the antipolar orthorhombic Pbam phase with antiferroelectic character of atomic displacements was revealed at $P$ = 3 GPa. The lattice parameters of the Pbam phase at $P$ = 4.1 GPa are $a$ = 5.5177(4), $b$ = 11.159(1) and $c$ = 7.7550(6) \AA{}. The variation of the structural parameters of both phases as a function of pressure was determined. The initially incommensurate G-type antiferromagnetic structure of BiFeO${}_{3}$ becomes commensurate due to the structural phase transition.

Phase Transitions in PbZr 1−x Sn x O3 Single Crystals

The phase diagram of PbZr 1−xSnxO3 (0 ≤ x ≤ 0.4) has been investigated for single crystals by dielectric and optic measurements. The stability of the antiferroelectric Pbam phase was observed for 0 ≤ x ≤ 0.05. Au Sn4+ content above x > 0.05 induces another antiferroelectric P2221 phase. At concentrations higher than x = 0.3 a phase of ferroelectric character (R3m) becomes stable. Above x = 0.15 the crossover from sharp to diffuse behaviour of the phase transition takes place. The changes in the phase transitions character and the mechanism leading to the appearance of new anti- and ferroelectric phases are discussed.

S-type domain boundaries in orthorhombic phases of PbZr1-xSnxO3crystais

Abstract The paper concentrates on the optical and crystallographic studies of the S-type domain boundaries occured in the Pbam (or Pba2), P2221, and Cm2m phases of the plate-like PbZr1-xSnxO3 crystals (x=0.37, 0.48, and 0.56) with the complicated domain structure.

Optical and crystallographic investigations of rhombohedral-orthorhombic phase transition in PbZrO3crystals with complicated domain structure

Abstract The results of investigations of a domain struuture rearrangement during the first-order R3m↔Pbam phase transition in the PbZrO3 single crystals have been discussed. Crystallographic interpretation of some versions for this rearrangement in the crystals with the complicated domain structure has been proposed.

The S-type domain and twin boundaries in plate-like PbZrO3 crystals having complicated twinned structures

Four kinds of S-type domain and twin boundaries in the ferroelectric R3m and antiferroelectric Pbam phases of plate-like PbZrO3 crystals with complicated twinned structures have been described. Conditions for thermal stability of S-boundary orientations in perovskites have been formulated.

A Phenomenological Theory of the Low Temperature Transition in K2Ba(NO2)4

The low-temperature transition in K 2 Ba(NO 2 ) 4 is analyzed using a Landau-type thermodynamic potential function, and it is found that the low-temperature phase must be monoclinic if the transition is induced by the instability of a mode at k 0 =(1/2, 1/2, 1/2) of the Pbam phase.