Double Perovskite System Research Articles

Structure evolution upon chemical and physical pressure in (Sr1−xBax)2FeSbO6

Here we demonstrate the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P63/mmc structure upon the isovalent larger-for-smaller A-site cation substitution in the B-site ordered double-perovskite system (Sr1−xBax)2FeSbO6. This is the same transformation sequence previously observed up to Fm-3m upon heating the parent Sr2FeSbO6 phase to high temperatures. High-pressure treatment, on the other hand, transforms the hexagonal P63/mmc structure of the other end member Ba2FeSbO6 back to the cubic Fm-3m structure. Hence we may conclude that chemical pressure, physical pressure and decreasing temperature all work towards the same direction in the (Sr1−xBax)2FeSbO6 system. Also shown is that with increasing Ba-for-Sr substitution level, i.e. with decreasing chemical pressure effect, the degree-of-order among the B-site cations, Fe and Sb, decreases.

Structural and electrical characterizations of cerium (Ce3+)-doped double perovskite system Sr2NiMoO6−δ

The double perovskite system Sr2−xCexNiMoO6−δ (SCNM) with 0.01 ≤ x ≤ 0.05 was synthesized by the citrate–nitrate auto-combustion synthesis route. Thermal studies were carried out by simultaneous differential scanning calorimetry and thermal gravimetry. Phase constitution was analyzed by powder X-ray diffraction (XRD). Rietveld refinement showed that the major phase exists in tetragonal form with space group I4/m. Microstructural investigations revealed the formation of uniform grains. The electrical conductivity studied by impedance spectroscopy in the temperature range 300–600 °C was found to follow a thermally activated process. The sample with x = 0.01 showed the highest conductivity with lowest activation energy. The electrical conductivity of the system was discussed in terms of identified impurity phases and charge density \(\left[{{\text{Mo}}_{{{\text{Mo}}^{6 +}}}^{5 +} {}^{\prime}} \right]\). The variation of electrical conductivity with composition was explained on the basis of X-ray photoelectron spectroscopy and XRD studies.

Improper Ferroelectric Contributions in the Double Perovskite Pb2Mn0.6Co0.4WO6 System with a Collinear Magnetic Structure.

The physical characterization and the extended crystallographic study of the double perovskite system Pb2Mn0.6Co0.4WO6 indicate an improper ferroelectric contribution to the polarization induced by the magnetic ordering. In the paramagnetic phase, the compound displays a centrosymmetric orthorhombic double perovskite structure with the Pmcn1' symmetry. The structure is strongly distorted by the lead stereoactivity. Magnetization measurements show two magnetic transitions at 188 and 9 K, but the time-of-flight neutron diffraction data provide evidence for a long-range magnetic ordering only below the second transition. Quantitative structure refinements combined with a comprehensive symmetry analysis indicate the Pm'c21' magnetic space group to be the adequate symmetry to describe the structural distortions and spin ordering in the ground state of the system. The symmetry implies a coexistence of a spontaneous ferromagnetic moment and a ferroelectric polarization along the orthogonal b- and c-axes, respectively, in the long-range ordered structure. Macroscopic measurements confirm the presence of the spontaneous polarization also below the first transition at 188 K, where only short-range magnetic correlations are evidenced by diffuse scattering in neutron diffraction.

Giant spontaneous exchange bias effect in Sm1.5Ca0.5CoMnO6 perovskite

We report here on the enormously large value of the spontaneous exchange bias (SEB) of ~5.1 kOe in the Sm1.5Ca0.5CoMnO6 bulk double perovskite system after zero-field cooling from an unmagnetized state. Depending on the path used for measuring the magnetic hysteresis loop, P-type or N-type, the EB field varies significantly. Dc and ac magnetization reveals a super-spin-glass-like (SSG) state below 19 K. The EB effect has been observed below the glassy transition temperature. The conventional exchange bias (CEB) as well as the SEB increase with a decrease in temperature and show monotonic variation. All these interesting features can be explained through the multimagnetic phase of double perovskite and the onset of unidirectional anisotropy driven by interface exchange coupling between the SSG, turned isothermally into a field-induced superferromagnetic shell and antiferromagnetic core.

Exchange bias effect in ferromagnetic LaSrCoMnO6 double perovskite: Consequence of spin glass-like ordering at low temperature

We report here the observation of exchange bias (EB) effect and spin glass – like ordering in LaSrCoMnO6 double perovskite system. The detail investigation of time dependent magnetic relaxation dynamics reveals the underlying spin glass – like ordering in this double perovskite at low temperature. Exchange bias anisotropy effect has been observed in this double perovskite system and it can be tuned by the strength of the cooling magnetic field. The spin configurationally relaxation model has been used for analysis of the observed magnetic training effect. A proposed phenomenological core–shell type model has been attributed to an exchange coupling between the spin-glass like shell (surrounding) and antiferromagnetic core of the double perovskite mainly on the basis of uncompensated surface spins. In view of spintronics application of perovskites, the present observation of exchange bias effect in this double perovskite may have great technological importance.

Influence of Ni/Mo ratio on structural and electrical properties of double perovskite system Sr2Ni1+x Mo1−x O6−δ

Technologically important double perovskite system Sr2Ni1+x Mo1−x O6−δ with x = 0.00, 0.05, 0.10, and 0.15 was prepared by solution combustion method. The structural and the Rietveld analysis of compositions revealed the formation of double perovskite tetragonal phase Sr2NiMoO6 with space group I4/m as a major phase. SrMoO4 and NiO were also observed as minor phases. Microstructural studies depicted the formation of uniform grains for all the samples. The average grain size was found to lie between the ranges of 1–4 μm. XPS analysis of the synthesized compositions showed the decreasing ratio of Mo5+ to Mo6+ ions in the system with increasing Ni content, which played an important role in the conduction mechanism. The thermal expansion coefficient (TEC) of all compositions indicated that it is more compatible to the TEC of standard electrolytes. The electrical conductivity for all the compositions was studied using impedance spectroscopy in the temperature range 200–600 °C. Composition with x = 0.05 showed better electrical conductivity with good catalytic activity.

Electronic structure and magnetic properties of an iron‐ and tantalum‐based double perovskite

AbstractBased on density functional theory, the dynamic stability, electronic structure, and magnetic properties of a LiNbO3 ‐type double perovskite (A2BB′O6), Zn2 FeTaO6, have been studied. Results show that the experimental structure is dynamic stabilized with a spontaneous polarization of 57C cm−2 along the c‐axis. By using GGA+U method, the system is predicted to be a charge transfer type insulator with 4.3 magnetic moment on Fe ion. The easy axis is in the ab‐plane. Calculations on exchange integrals indicate that the magnetic coupling between Fe ions of the Fe–O–Ta–O–Fe bridge is very weak. However, the AFM coupling of the Fe–O–Fe bridge is much stronger, which suggests a potential way to design magnetoelectric materials in ferrimagnetic phase with 3d and 5d ions at B‐ and ‐site in the LiNbO3 double perovskite system.

Structural and electric evidence of ferrielectric state in Pb₂MnWO₆ double perovskite system.

In this paper we describe the new ferri-electric compound Pb2MnWO6 (PMW), a double perovskite that can be considered as a novel structural prototype showing complex nuclear structure and interesting electric properties. According to single-crystal synchrotron data, PMW crystallizes in the noncentrosymmetric polar group Pmc21, in which the two symmetry-independent lead atoms give rise to a ferrielectric arrangement. The accurate crystallographic characterization indicates the presence of a complex distortion of the perovskite lattice driven by the local instability induced by the 6s(2) lone pair of the lead atoms. These peculiar structural features are confirmed by the complete electrical characterization of the system. Dielectric and transport measurements indicate an insulating character of the sample, while pyroelectric measurements point out a ferrielectric state characterized by different contributions. The magnetic transition at 45 K is accompanied by a magnetostrictive effect indicating a probable spin-lattice coupling. The characterizations carried out on PMW, showing the evidence of a coexistence of antiferromagnetism and ferrielectricity at low temperature, could lead to the definition of a new class of multiferroic materials.

Investigation of the physico-chemical properties of Sr2FeNb1−x W x O6 (0.0 ≤ x ≤ 0.1) for visible-light photocatalytic water-splitting applications

Incorporation of the pentavalent W-ion has been investigated in Sr2FeNb1−x W x O6 for 0.0 ≤ x < 0.15. The synthesized double perovskite system was used to study its structural and optical properties. The optical, as well as structural, properties were found to change with the varying Nb:W stoichiometric ratio. The Sr2FeNb1−x W x O6 samples with 0.0 ≤ x 0.1, the Sr2FeNb1−x W x O6 showed no change in the optical properties but crystal structure was drastically affected, exhibiting impurity phases and a disordered structure, thus yielding a degraded photocatalytic behavior. The higher absorption coefficient in the Sr2FeNb0.99W0.01O6 and a higher electron-density acquired by n-type lattice doping seems to be responsible for the better efficient charge separation in Sr2FeNb1−x W x O6 (0.01 ≤ x ≤ 0.20) and consequently the higher photocatalytic activity in co-catalyst loaded photocatalysts.

Coupled Nd andB′spin ordering in the double perovskites Nd2NaB′O6(B′=Ru, Os)

We present a neutron powder-diffraction study of the monoclinic double perovskite systems Nd${}_{2}$Na${B}^{\ensuremath{'}}$O${}_{6}$ (${B}^{\ensuremath{'}}=\text{Ru}$, Os), with magnetic atoms occupying both the $A$ and ${B}^{\ensuremath{'}}$ sites of the ${A}_{2}B{B}^{\ensuremath{'}}{\text{O}}_{6}$ structure. Our measurements reveal coupled spin ordering between the Nd and ${B}^{\ensuremath{'}}$ atoms with magnetic transition temperatures of 14.3(1) K for Nd${}_{2}$NaRuO${}_{6}$ and 16.2(2) K for Nd${}_{2}$NaOsO${}_{6}$. There is a type-I antiferromagnetic structure associated with the Ru and Os sublattices, with the ferromagnetic planes stacked along the $c$ axis and [110] direction respectively, while the Nd sublattices exhibit complex, canted antiferromagnetism with different spin arrangements in each system.

Enhanced low field magnetoresistance in Sr2Fe1−xAgxMoO6 double perovskite system

Polycrystalline samples of Sr2Fe1−xAgxMoO6 (x=0, 0.05, 0.10 and 0.20) have been prepared by solid state sintering. Single phase nature of the samples has been confirmed by the XRD analysis for the samples up to an Ag content of 0.10. SEM analysis shows that Ag enhances the grain growth and coarsening of the system. Though the saturation magnetisation and Curie temperature decreases, the Fe–Mo ordering as well as spin polarization enhances with respect to Ag substitution. All the samples except for the x=0.20, shows the LFMR hysteresis nature at 20K indicative of the tunnelling magneto-resistance behaviour. The magneto-resistance values were found to increase with respect to Ag content up to a limit of x=0.10. The enhancement of LFMR behaviour and the tunnelling magnet-resistance is explained in terms of the formation of the non magnetic patches like Mo–O–Ag–O–Mo–O within the grain which acts as insulating tunnel barrier between the ferromagnetic regions of Fe–O–Mo–O–Fe–O due to the substitution of Ag in place of Fe. The striking feature of our finding is that the LFMR nature of the double perovskite increases with respect to the enhancement in the grain size which is contrary to the reported results wherein LFMR is found to increase with respect to a decrease in the grain size. So it is proposed that the LFMR nature and magnetoresistance of double perovskite systems in general and Sr2FeMoO6 in particular depend on intra-grain properties also.

ELECTRONIC STRUCTURE AND MAGNETISM IN Ir-BASED DOUBLE-PEROVSKITE Sr2CeIrO6

The electronic structure and magnetism of Sr 2 CeIrO 6, an Ir -based double perovskite system has been investigated using first-principles calculations. We found that a strong spin-orbit coupling dictate the electronic and magnetic properties of this system. A small value of U along with SOC could open up a gap at the Fermi level, offering the possibility of novel J eff = ½ Mott quantum state. Our calculations revealed that the magnetic ground state is antiferromagnetic which is in agreement with the magnetization data and provide the value of spin and orbital moment for this system which is in agreement with the other isostructural Ir -based compound.

Synthesis and Characterization of Sr2Cu(W1–xMox)O6: A Quasi-Two-Dimensional Magnetic System

Here, we report the synthesis of the double-perovskite system Sr2Cu(W1–xMox)O6, with x = 0.0–1.0, where the Mo-containing members have not been synthesized before. This system provides an example of a quasi-two-dimensional S = 1/2 square lattice, with the possibility of magnetic frustration. The compounds were synthesized under ambient pressure for 0 ≤ x ≤ 0.6 and under high pressure (HP) for 0.7 ≤ x ≤ 1.0. A small percentage of impurities, primarily Sr(W,Mo)O4, was present in all samples and increased for x ≥ 0.5. Tetragonal distortion of the compounds decreases with increasing x, and correspondingly, a first-order tetragonal-cubic cooperative Jahn–Teller ordering of the 3d hole on the CuII into dx2–y2 orbitals below a TJT ≈ 920 °C in Sr2CuWO6 is lowered to a TJT ≈ 500 °C in Sr2CuMoO6. The samples were all insulators, but the band gap was found to decrease with increasing Mo content due to the weaker Mo 4d–O 2p hybridization compared to that of W 5d–O 2p. Ordering of the CuII holes into dx2–y2 orbitals creates magnetic interactions between spins S = 1/2 with antiferromagnetic 180° Cu–O–(W,Mo)–O–Cu parameters (−J2) and ferromagnetic 90° Cu–O–(W,Mo)–O–Cu parameters (+J1) in the (001) planes. Magnetic susceptibility showed no long-range magnetic order in any sample down to 5 K.

Comparative Study for Simulation of EELS Core Loss for Transition Metals in Double Perovskite Systems

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

An investigation of the Fe and Mo oxidation states in Sr2Fe2−xMoxO6 (0.25 ⩽ x ⩽ 1.0) double perovskites by X-ray absorption spectroscopy

Sr2FeMoO6 double perovskite systems are widely studied because of their interesting and technologically relevant physical properties. Sr2FeMoO6 is just a single composition in the Sr2Fe2−xMoxO6 solid-solution, and it is important to understand how the composition impacts the transition-metal valence states. Variations in the lattice parameters of these materials were studied using powder X-ray diffraction and it was found that a large change in the lattice constant occurs between Sr2Fe1.50Mo0.50O6 and Sr2Fe1.35Mo0.65O6 that likely coincides with a transition from a cubic to a tetragonal unit cell, in agreement with previous studies. Fe K- and Mo K-edge X-ray absorption near-edge spectra were also collected to investigate how the oxidation state and coordination environment change with composition. When the Mo content is low, Fe adopts a 3+ oxidation state and Mo adopts a 6+ oxidation state. As the Mo content is increased, the Fe and Mo cations are both partially reduced, resulting in a mixture of Fe3+ and Fe2+ and Mo5+ and Mo6+. The reduction of the metal centers apparently drives the change in unit cell. The influence of preparation method on the oxidation states of Fe and Mo was also investigated by annealing the materials under vacuum. The results reported here show that the oxidation states of Fe and Mo are strongly impacted by both composition and preparation method, which may account for the wide variety of oxidation state and magnetic properties that have been reported previously.

Ab initio study of the electronic and magnetic properties of Sr2−xLaxFe1+y/2Mo1−y/2O6 double perovskites

Using the first-principles full potential linearized augmented plane wave method, the electronic structure of Sr2−xLaxFe1+y/2Mo1−y/2O6 (SLFMO) double-perovskite systems is investigated for x = 1/2 and 1 and for y =+ 1 and − 1. Substituting Sr atoms by La atoms allows one to tune the electrons added into the minority spin band and enhances the half-metal feature—according to the rigid band shift model—even if the magnetization decreases with increasing La concentration. By taking into account the chemical disorder on the Fe and Mo sites, resulting from the introduction of La as shown experimentally, it is shown (i) that a supplemental magnetization reduction occurs because the magnetic moment on the Fe antisite is opposite to the one on the Fe regular sites and (ii) that the half-metal feature is preserved in SLFMO for x = 1/2 and 1 contrary to the case for SFMO. Finally, because surface or interface Fe deficiency should have a more limited impact on the spin polarization than for SFMO, SLFMO/SrTiO3 multilayers are investigated in order to confirm this prediction by determining the spin polarization at the interface, which is found to remain high.

Magnetic properties of the ordered and disordered double perovskite Sr2Fe1+xMo1−xO6 (−1 ≤ x ≤ 1/3)

We have studied the effect of cationic disorder on the spin polarization of the double perovskite system Sr2Fe1+xMo1−xO6 with −1 ≤ x ≤ 1/3. The composition x = 0 corresponds to the well-known double-perovskite Sr2FeMoO6, which is expected to have complete spin polarization, however all samples present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. We consider an electronic model within the renormalized perturbation expansion Green’s functions, consisting in a correlated electron picture with localized Fe-ions and itinerant electrons interacting with the local spins via a double-exchange type mechanism. Our results show the influence of disorder on the density of states and the ground-state properties, particularly on the spin polarization over the whole range of x.

Identifying the contribution of band filling effects in the double perovskite system [formula omitted

This work investigates various factors affecting the Curie temperature in double perovskite A 2FeMoO 6 systems and the contribution of band filling effects for various parameters. The measurements were conducted on Sr 0.4Ba 1.6FeMoO 6 samples prepared at different sintering temperature and a series of trivalent and divalent substituted Sr 0.4Ba 1.6− x A x FeMoO 6 samples (A=La 3+, Sr 2+). A careful investigation allowed delineating the contributions of pristine band filling. The contribution of band filling effects to the Curie temperature increase is observed to be 1.5 K for each percent of trivalent substitution (about 60% of total increase in the present study). For an equal decrease of the A site cation radii, the changes observed in the lattice parameter for trivalent and divalent doped stems exhibited differences up to 33%. The increase in unit cell length (the a value) associated with band filling is thus 9.35×10 −4 Å for each percent of trivalent doping. It is also observed that a moderate level of impure phases and disorder defects plays a minor role in determining the Curie temperature of such a system.

B Cation Ordered Double Perovskite Ba2CoMo0.5Nb0.5O6-δ As a Potential SOFC Cathode

This paper reports the evaluation of the cubic B site cation-ordered double perovskite system Ba2Co2−x(Mo1/2Nb1/2)xO6, resulting in the single phase composition Ba2CoMo0.5Nb0.5O6−δ (BCMN) with a mixed Co charge state stabilized by a combination of Mo and Nb doping as a new mixed conductor with potential SOFC cathode applications. X-ray, neutron, and electron diffraction show that hexagonal intergrowths found in multiple phase samples at lower Mo/Nb contents are suppressed in BCMN, which has large domains of rock-salt ordered B site cations with separate Co and Mo/Nb sites. Conductivity measurement and impedance spectroscopy investigation shows that BCMN has a considerably reduced dc conductivity compared with materials such as BSCF but exhibits comparable electrochemical properties to some existing cathode materials in symmetrical cell measurements, and shows higher structural stability and reduced reactivity with the Ce0.8Sm0.2O2−δ (SDC) electrolyte. The role of Mo in dioxygen activation is proposed to offset the reduction that the d0 Mo(VI) and Nb(V) cations on the B site produce in the electronic and ionic conductivity.

Magnetic and magneto-transport properties of [formula omitted

The variation in structural, magnetic and magneto-transport properties of the double perovskite system (Ba 0.8Sr 0.2) 2− x Nd x FeMoO 6 {0.0< X<0.5} induced by Nd 3+ doping (electron doping) has been studied and compared. The samples were prepared by standard solid state reaction method in a reducing atmosphere. The parent compound showed a saturation magnetic moment value of 3.75 μ B/f.u. at an applied field of 0.5 T and a change in magnetoresistance value up to 26% (77 K, 0.8 T). The Rietveld refinement of the X-ray diffraction data showed a continuous decrease in lattice parameters and Fe–Mo ordering with increasing Nd 3+ doping. The Curie temperature was found to increase with Nd 3+ doping (3 K per % of Nd) while the saturation magnetic moment values and magnetoresistance values were found to decrease. The observed variations in magnetic and magneto-transport properties of the system are explained on the basis of increasing antisite disorder defects and band filling effects induced by electron doping. We have observed the dominant role of band filling in determining the low field magnetoresistance of these systems.