B-site Disorder Research Articles

Pushing magnetic spirals beyond room temperature by reducing the uniaxial pyramidal elongation in layered Cu/Fe perovskites

The impact of a 3d divalent substitution for Cu using M2+ ions not presenting elongated MO5 pyramids has been explored in YBaCuFeO5 as a new strategy to upgrade the magnetic properties of this high-temperature spiral magnet and potential multiferroic. In this work we investigate the incidence of the Cu2+ with Co2+ substitution in YBa(Cu1−xCox)FeO5 compounds in which B-site disorder is kept invariant (disorder nd≈36%). The study validates the reduction of the uniaxial elongation distortion at Cu sites as an efficient alternative way to disorder that enables one to enhance the thermal stability of the spiral order. In addition, the rotation plane of the helix is reoriented by the Cu/Co substitution and nearby the triple point (when TS is maximal) the spiral order adopts an almost complete cycloidal magnetic configuration. These results lay the foundation of a new mechanism to tune the stability of the magnetic spirals in layered perovskites based on reducing the uniaxial elongation associated to the (4+1)-coordination of Cu2+ ions in square CuO5 pyramids. Published by the American Physical Society 2024

The electronic structure and magnetic properties of stable Sr2BB′O5 (B = Fe, Co, Ni and B′ = Co, Ni, Mn) with the brownmillerite structure

First-principles calculations have been performed to investigate the structural, electronic, and magnetic properties of Sr2BB′O5 (B = Fe, Co, Ni; B′ = Co, Ni, Mn) brownmillerite materials. By considering the atomic arrangements and the tetrahedral rotated modes, including typical magnetic configurations, the ground states are found by comparing their total energy. The thermodynamic and dynamic stability studies revealed that two compounds, Sr2FeCoO5 and Sr2NiMnO5, could be stable in the brownmillerite structure. Further electronic structure calculations show that the ground state of Sr2FeCoO5 is a G-AFM insulator, while the ground state of Sr2NiMnO5 is an A-AFM half-metal with a half-metallic bandgap of about 1 eV, exhibiting excellent half-metallic properties. In addition, the magnetic interactions are analyzed, which suggest that Sr2FeCoO5 could suffer from the influence of the B-site atomic disorder to maintain the G-AFM state, while Sr2NiMnO5 is more sensitive to atomic disorder, so its A-AFM state would be easily destroyed by the B-site disorder. These results would be helpful for further research.

B-site disorder driven Griffiths-like phase and electrochemical behavior in Y2NiCrO6 double perovskite

In this investigation, nanoparticles of B-site disordered Y2NiCrO6 (YNCO) double perovskite were synthesized by the facile sol–gel method to evaluate their magnetic and electrochemical properties. Their crystallographic structure is monoclinic and the average size of the particles is 79±16 nm. XPS analysis indicated a mixed oxidation states of B-site transition metals Ni2+/Ni3+ and Cr2+/Cr3+. The mixed valence states of Ni and Cr, along with the mixed magnetic phases of YNCO, constitute a signature of the B-site disorder. This antisite disorder contributed to the observation of a Griffiths-like phase arising from ferromagnetic short-range interactions above the magnetic transition up to the Griffiths temperature, T G = 137 K. The synthesized YNCO double perovskite demonstrated a promising behavior as an electrode material for electrochemical supercapacitors. In a three-electrode system, it displayed a specific capacitance of 270 F g−1 at a current density of 0.5 A g−1. In a symmetric two-electrode system, YNCO exhibited a specific capacitance of 180 F g−1 at 0.5 A g−1, alongside an energy density of 6.25 Wh kg−1 at 250 W kg−1 power density. In both cases, we employed a mild 0.5 M neutral aqueous Na2SO4 solution as the electrolyte, in contrast to the typically employed corrosive and concentrated alkaline aqueous solution. The fascinating magnetic and charge storage properties of the B-site disordered YNCO double perovskite indicate its potential for use in spintronic devices and as efficient electrodes in symmetric hybrid supercapacitors.

Research on optimized piezoelectric properties of PNN-PMN-PT ceramics and corresponding underlying factors based on component regulation

To meet the increasing demands for developing miniaturized high-precision modern piezoelectric devices, researchers are focusing their attention on PbTiO3 (PT) relaxor ferroelectrics having excellent comprehensive performance. Although the partial substitution of Pb(Mg1/3Nb2/3)O3 (PMN)-PT with Pb(Ni1/3Nb2/3)O3 (PNN)-PT has been reported to improve dielectric and piezoelectric properties, the factors underlying the performance improvement of the PNN-PMN-PT ternary system remain unclear. Therefore, based on PT content regulation, the 0.128PNN-(0.872-x)PMN-xPT (x = 0.316, 0.326, 0.336, 0.346, and 0.356) ternary ceramics were fabricated, and the crystal structure and electrical properties were systematically investigated during the rhombohedral-tetragonal phase transition. The 0.128PNN-0.536PMN-0.336PT composition exhibited optimum performance with the piezoelectric charge coefficient, effective piezoelectric strain coefficient, dielectric constant were 691 pC/N, 1060 pm/V and 4545 respectively at a maximum permittivity temperature of 136 ℃, signifying its potential for use in transducers and actuators. The outstanding properties of this system were mainly derived from the lattice distortion and reversible domain wall motion. The results indicated that the multiphase coexistence and enhanced B-site disorder in the perovskite structure contribute to the improvement in the piezoelectric properties, which would facilitate the design and development of novel piezoelectric materials.

Magnetization sign reversal, spin-glass-like state and Griffiths-like phase driven by B-site disorder in double perovskite Gd2CrMnO6

The crystal structure and magnetic order of the double perovskite Gd2CrMnO6 have been investigated systematically by employing magnetization and ESR measurement for the first time. Structure analysis revealed that the Gd2CrMnO6 crystallizes in the adopted B-site disorder orthorhombic structure with space group of Pbnm. Magnetization measurements indicated that the system possesses a long-range antiferromagnetic ordering at TN = 40 K. In Gd2CrMnO6, the magnetization sign reversal was observed below Tcomp = ∼ 27 K due to the Dzyaloshinskii-Moriya antisymmetric interaction of Cr3+-O2--Mn3+ and the different sublattice of Gd3+ and Cr3+/Mn3+. The competing magnetic interaction is greatly enhanced by FM 3d-4f interaction and should be responsible for the Griffiths-like phase (TG ∼ 260 K) and spin-glass-like state.

Understanding the structural and optical characteristics in Sr2xLa2-2xLi2xTe2-2xO6 perovskites

Perovskites of the type Sr2xLa2-2xLi2xTe2-2xO6; x = 0.25, 0.4, 0.5, 0.6, 0.75 were analyzed for the first time for their structural and optical characteristics. All the compounds were found to be monoclinic with the P21/n structure thereby indicating a double perovskite structure formation. The x = 0.5 composition with Li+ and Te6+ was found to represent an ordered double perovskite having the least B-site disorder while x = 0.4 and x = 0.6 were found to have the highest disorder based on the refined occupancies. The tolerance factor calculations indicated that the symmetry is lowered on account of the octahedral tilting and Li-O-Te bending in making room for any divergence in symmetry that may arise due to a change to monoclinic from cubic structure. A greater tilting of the octahedral were observed in all compositions except x = 0.5 in terms of the tilting angle resulting in a drop in the bond angle of Li-O-Te. FTIR measurements indicate the existence of symmetric and antisymmetric stretching vibrations of the Li(Te)O6 octahedra. Energy band gap measurements indicated a depletion in the conduction band width and an increase in the energy gap. The ability for conducting is found to be the best in x = 0.25 and x = 0.6 as these compositions have the least value of energy band gap. This behavior can be accounted for by the possible presence of polarons in these compositions. Also, an increase in the energy band gap is observed when going from x = 0.25 to x = 0.5 due to an increase in cell volume for these compositions. SEM and EDX measurements carried out for x = 0.25, 0.5 and 0.75 indicate rough and agglomerated particles with a near uniform distribution in terms of their shape and the presence of elements as per the stoichiometric formula.

Effect of B-site disorder on the electrical properties of Barium Zirconium Titanate Ceramics composites

Effect of B-site disorder on the electrical properties of Barium Zirconium Titanate Ceramics composites

B-site disorder induced Griffiths phase evolution and high magnetocaloric effect in La0.7Sr0.3A0.05Mn0.95O3 (A= Si, Ti)

We report on evaluation of Griffiths phase and enhanced magnetic entropy change at low magnetic fields in B-site disordered perovskite La0.7Sr0.3A0.05Mn0.95O3 (A= Si, Ti) bulk samples. 5 mol% of Si and Ti doping at the Mn site of La0.7Sr0.3MnO3 leads to a decrease in Curie temperature (TC) towards room temperature, which is observed to be 365.2 K and 314.7 K respectively. Griffiths phase arises as a result of doping either Si or Ti in Mn site. Ti doped La0.7Sr0.3MnO3 exhibits higher entropy change (1.08 J/kg K) than Si (0.83 J/kg K) at room temperature. The increase in magnetic entropy change and high reduction in TC towards room temperature caused by Ti doping in the La0.7Sr0.3MnO3 implies that these materials might be exploited for magnetic cooling in low magnetic fields near room temperature. Critical exponent analysis indicates the possibility of 3D Heisenberg spin glass like system in both compounds.

The lattice distortion, mechanical and thermodynamic properties of A(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3 (A = Sr, Ba) high-entropy perovskite with B-site disorder: First principles prediction

High-entropy perovskite oxides are novel high-entropy ceramics developing recently. In this work, the local lattice distortion, mechanical and thermodynamic properties of perovskites A(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3 (A = Sr, Ba) are explored using the first principle investigation. The equilibrium lattice parameter and bulk moduli of high-entropy perovskites obtained by fitting approximately satisfy the rule of mixture of the components, while the entropy effect of component mixing is very small. The bond length distribution reveals the B-site disorder results in large local lattice distortion. The atomic displacement indicates larger average displacement of O atoms contributes mainly to the wider bond length distribution for most B-O bond. The distortion degree at B-site is naturally associated with A-site. The obtained elastic constants of high-entropy perovskites are also closed to the rule of mixing of the components. Compared with ternary SrTiO3 and BaTiO3 perovskites, B-site mixing for high-entropy perovskites enhances their toughness at expense of strength and stiffness, showing the elastic moduli can be modified and adjusted through multi-component design strategy. Relevant thermodynamic property reveals the B-site disorder is benefit to improve the resistance to softening and suppress volume expansion at high temperature. Electronic structures show the insulator–metal transition takes place, also uncover that the interaction of B-O bond is stronger.

Role of B-site disorder in the properties of lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 ceramic: A possible electrocaloric material with low leakage current

The electrocaloric (EC) effect in ferroelectric materials has been studied extensively to develop materials for solid-state cooling applications. The Na0.5Bi0.5TiO3-based ferroelectric material is selected for this particular application. The Ti4+ of the lead-free Na0.5Bi0.5TiO3 is completely replaced by (Mg1/3Nb2/3)4+ to enhance the dielectric breakdown strength and to observe the EC effect. The lead-free Na0.5Bi0.5(Mg1/3Nb2/3)O3 is synthesized via the columbite route with optimized processing conditions. The material shows a frequency-independent high ferroelectric–paraelectric transition temperature (460 °C) with a slim polarization vs electric field loop. The adiabatic temperature change (ΔT) of the compound is evaluated from the temperature-dependent polarization behavior, and calculated ΔT is ≈ 0.135 K at room temperature. The room-temperature EC strength and leakage current of the material are examined to check its suitability for solid-state refrigeration.

Ambient pressure synthesis and properties of LaCu3Fe2TiSbO12: New A-site ordered ferrimagnetic quadruple perovskite

A large number of quadruple perovskites are synthesized and reported under high pressure while ambient pressure synthesized quadruple perovskites are limited in number. Recently, ambient pressure synthesis of quadruple perovskites has gained much interest.1 The A-site ordered quadruple perovskite, LaCu3Fe2TiSbO12, reported for the first time, is synthesized by high-temperature solid-state reactions at ambient pressure. Rietveld structure refinement using P-XRD data reveal formation of LaCu3Fe2TiSbO12 in body centered cubic Im-3 space group with lattice parameter of 7.4465(7) Å. The compound shows complete 1:3 order of A-site La with A′-site Cu and B/B′-site disorder of Fe with Ti and Sb instead of both A- and B-site order in CaCu3Fe2Sb2O12 (Larregola et al., 2014; Chen et al., 2013) [1,2]. The magnetic transitions observed in the compound around 160 and 60 ​K are attributed to short-range orders. The static disorder at the B-sublattice introduced by non-magnetic Ti and Sb precludes the possibility of long-range magnetic order in the compound. Lower saturation magnetization presumably supports antiferromagnetic ordering of A-site Cu2+ and B-site Fe3+ spins interrupted by the B-site disorder restricting it to a short-range phenomenon. The ferrimagnetic quadruple perovskite also show reasonably high dielectric constant in the low-frequency region and a low dielectric loss at room temperature. The stabilization of LaCu3Fe2TiSbO12 under ambient pressure despite having a fairly large global instability index (GII, calculated with SPuDS) of 0.276 v.u. is interesting and may suggest accessibility of many new quadruple perovskites at atmospheric pressure.

Magnetic structures and spin reorientation in the B-site disordered perovskite PrFe0.5Cr0.5O3

Through the detailed studies utilizing x-ray and neutron powder diffraction and magnetometry measurements, we report the magnetic phase transitions in the B-site disorder perovskite oxide PrFe0.5Cr0.5O3. The Fe3+/Cr3+ sublattice was observed to form a long-range magnetic order with the GxFz configuration below TN = 270 K from neutron powder diffraction. It then undergoes the second-order phase transition to FxGz spin configuration below 210 K and the transition ends at ~170 K. On the other hand, long range magnetic ordering with FxCy configuration at Pr3+ sublattice can be identified at base temperature. The magnetic phase diagram is derived with spontaneous spin ordering and reorientation at A- and B-sublattices.

Study of disorder in pulsed laser deposited double perovskite oxides by first-principle structure prediction

Double perovskite oxides, with generalized formula A2BB^{prime}O6, attract wide interest due to their multiferroic and charge transfer properties. They offer a wide range of potential applications such as spintronics and electrically tunable devices. However, great practical limitations are encountered, since a spontaneous order of the B-site cations is notoriously hard to achieve. In this joint experimental-theoretical work, we focused on the characterization of double perovskites La2TiFeO6 and La2VCuO6 films grown by pulsed laser deposition and interpretation of the observed B-site disorder and partial charge transfer between the B-site ions. A random structure sampling method was used to show that several phases compete due to their corresponding configurational entropy. In order to capture a representative picture of the most relevant competing microstates in realistic experimental conditions, this search included the potential formation of non-stoichiometric phases as well, which could also be directly related to the observed partial charge transfer. We optimized the information encapsulated in the potential energy landscape, captured via structure sampling, by evaluating both enthalpic and entropic terms. These terms were employed as a metric for the competition of different phases. This approach, applied herein specifically to La2TiFeO6, highlights the presence of highly entropic phases above the ground state which can explain the disorder observed frequently in the broader class of double perovskite oxides.

Magnetic ordering in the Ising antiferromagnetic pyrochlore Nd2ScNbO7

The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behaviour, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd2ScNbO7 is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T N = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in only 14(2)% of the neodymium ions through spectroscopy and is consistent with total scattering measurements as well as the magnitude of the dynamic moment 0.26(2) μ B . The remaining magnetic species order completely into the all-in all-out Ising antiferromagnetic structure. This can be seen as a result of local symmetry breaking due disordered Sc+3 and Nb+5 ions about the A-site. From this work, it has been established that B-site disorder restores the dipole-like behaviour of the Nd+3 ions compared to the Nd2B2O7 parent series.

Contrasting effects of mismatch strain on the magnetic behavior of undoped and doped BaFeO3-δ thin films

We investigate the effects of biaxial compressive and tensile strain on the magnetic and structural properties of BaFeO3-δ (BFO), BaFe0.9Ti0.1O3-δ (BFTO) and BaFe0.9Ru0.1O3-δ (BFRO) thin films fabricated on SrTiO3 (001) (STO) and MgO (001). The magnetization measurements show a long range ferromagnetic ordering in compressive strained BFO thin film while an antiferromagnetic ordering with weak ferromagnetism appears in tensile strained BFO thin film. This behavior is ascribed to the difference in oxygen stoichiometry under the two strain states in the BFO thin films. Ti4+ (3d0) and Ru4+ (4d5) ion substitutions at B-site cause an increase in unit cell volume irrespective of the strain type resulting in relaxation of tensile strain and enhancement of compressive strain. This B-site disorder leads to collapse of long range ferromagnetic ordering in BFTO and BFRO thin films grown on STO but an increased ferromagnetic ordering comes along in BFTO and BFTRO thin films on MgO. The contrasting results portray that tensile strain suppresses the effect of cation disorder evident under compressive strain, on the magnetic and structural properties of BFO, BFTO and BFRO thin films.

Effects of (K0.5Na0.5)(Nb0.96Sb0.04)O3 on microstructure and electrical properties of BNT-based piezoelectric ceramics

In this work, (1-x)Bi0.5Na0.5TiO3-x(K0.5Na0.5)(Nb0.96Sb0.04)O3 [abbreviated as (1-x)BNT-xKNNS] lead-free piezoelectric ceramics were successfully prepared by the conventional solid-state reaction method, and effects of (K0.5Na0.5)(Nb0.96Sb0.04)O3 [abbreviated as KNNS] on microstructure structure and electrical properties of the ceramics were systematically discussed. Results of phase characterizations showed that KNNS led to phase transition from orthorhombic (O) phase to monoclinic (M)-O phase coexistence. However, KNNS had small effects on grain growth of the ceramics according to scanning electron microscope (SEM) analysis. The existence of KNNS resulted in unique dielectric relaxor behavior of such ceramics, i.e. vanished depolarization temperature (Td) and insensitive relative dielectric constants (εr) to frequencies when T < Ts. When T ≥ Ts, obvious frequency dispersion of εr resulted from thermally-activated B-site disorder. Furthermore, electrical properties and energy storage properties were studied and the ceramics with x = 0.02 possessed optimized dielectric properties of εr ~ 773 (@100 kHz) and dielectric loss (tanδ) ~ 0.02 over wide temperature range.

Electronic properties and crystal structures of double-perovskites, Ba2BiIIIBiVO6,Ba2PrBiO6, and Ba2PrSbO6: First-principles study

In recent experiments, a significant band gap widening was observed when Sb was substituted for Bi in the double-perovskite Ba2PrBiO6. In this work, we study a series of double-perovskites, Ba2BiIIIBiVO6, Ba2PrBiO6, and Ba2PrSbO6 using the first-principles density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional to investigate the substitution effect on the structural and electronic properties. We find that the two topmost valence bands are disappeared on the substitution of PrIII for BiIII, and the two bottommost conduction bands are disappeared on the substitution of SbV for BiV, causing the significant band gap widening. Further, our calculation suggests that the Ba2PrPrBiO6 is a possible candidate as a source of the PrIV signal observed in the experiment. We find that the B-site disordering atomic configuration, Ba2B″VB′IIIO6, are restored to those of the original structures. On the other hand, our results also suggest the importance of the partial B-site disorder to explain the experimentally observed band gaps.

Magneto-dielectric effect in relaxor superparaelectric Tb2CoMnO6 film

We report magneto-dielectric properties of partially B-site ordered monoclinic Tb2CoMnO6 double perovskite thin film epitaxially grown by metalorganic aerosol deposition technique. Transmission electron microscopy and electron energy loss spectroscopy mapping shows the presence and distribution of both Co2+ and Co3+ ions in the film, evidencing a partial B-site disorder, which was further confirmed by the observation of reduced saturation magnetization at low temperatures. The ferromagnetic Curie temperature, TC=110 K, is slightly higher as compared to the bulk value (100 K) probably due to an in plane tensile strain. Remarkably, a short range ordering of spins at T*~190 K>>TC was established and assigned to the B-site disorder in the film. Two different dielectric relaxation peaks have been observed; they merge at the same temperature T* of short range spin correlations. Moreover, an unexpected high temperature dipolar relaxor-glass-like transition at T~T* was observed, at which a coupling to short range magnetic correlations results in a 4% magneto-dielectric coupling.

Brillouin scattering studies of ordered Pb(Sc1/2Nb1/2)O3 crystal with vacancies

Pb(Sc1/2Nb1/2)O3 (PSN) with a perovskite structure is a well-known ferroelectric material. The rapidly quenched PSN (PSN-D) shows a B-site disordered structure, and exhibits the noncanonical relaxor behavior with Tmax=around 351 K. The well-annealed PSN (PSN-O) exhibits a B-site 1:1 ordered structure, and undergoes a normal ferroelectric phase transition at TC= about 346 K, while, the introduction of vacancies may lead to the broadening of dielectric peak with frequency dispersion down to low temperatures, typical of relaxor nature. It was reported that as-sintered ordered PSN (PSN-OV) with Pb/oxygen vacancies exhibits relaxor ferroelectric behavior with TC= about 292 K. In this work, the elastic anomaly of a longitudinal acoustic (LA) mode and the central peak of a PSN-OV crystal were studied by Brillouin scattering. The temperature dependences of the frequency shift and the width of the LA mode along the [100] direction were investigated. A broad elastic anomaly and a broad slowing down of the relaxation time were observed at the ferroelectric phase transition. The elastic anomaly of PSN-OV is compared with those of PSN-D and PSN-O crystals reported in our previous studies. It is found that the relaxor nature of PSN-OV caused by vacancies is stronger than that of PSN-D caused by the B-site disorder.

Exchange bias and spin glass states driven by antisite disorder in the double perovskite compound LaSrCoFeO6

Antisite (B-site) disorder in double perovskite lattice is responsible for various magnetic phenomena such as exchange bias, spin glass, memory effect, colossal magnetoresistance, etc. By controlling the antisite disorder in the antiferromagnetic double perovskite ${\mathrm{LaSrCoFeO}}_{6}$, we achieve intrinsic exchange bias effect with a large exchange bias field ($\ensuremath{\sim}1.2$ kOe) and giant coercive field ($\ensuremath{\sim}12.8$ kOe). Further, we find that the effect of such antisite disorder induces a spin-glass state in ${\mathrm{LaSrCoFeO}}_{6}$. Multiple signatures of slow dynamics were confirmed by frequency-dependent peak shift, slow spin relaxation, and memory effect over a wide temperature regime ($5\ensuremath{-}80$ K). The AC susceptibility data near the spin-glass temperature ($\ensuremath{\sim}72.1\ifmmode\pm\else\textpm\fi{}0.6$ K) are best fit by a critical slowing down model described by a dynamical exponent $\mathrm{z}\ensuremath{\nu}=7.5\ifmmode\pm\else\textpm\fi{}0.5$ and ${\ensuremath{\tau}}_{0}=1.05\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ s. The origin of exchange bias and spin glass are briefly discussed.