Ordered Rock Salt Research Articles

Selective Synthesis and Crystal Chemistry of Candidate Rare-Earth Kitaev Materials: Honeycomb and Hyperhoneycomb Na2PrO3.

Rare-earth oxides have attracted interest as a platform for studying frustrated magnetism arising from bond-dependent anisotropic interactions. Ordered rock salt compounds Na2PrO3 crystallize in two polymorphs (α and β) comprising honeycomb and hyperhoneycomb lattices of octahedrally coordinated Pr4+ (4f1). Although possible realization of antiferromagnetic Kitaev interactions is anticipated for these phases on the basis of ab initio models, the air sensitivity of the two polymorphs has hampered reliable crystal growth and physical property measurements. Here, we have succeeded in preparing powder and single crystals of both α- and β-Na2PrO3 using modified synthetic procedures. Revised crystal structures for both polymorphs are obtained from refinement of untwinned single-crystal X-ray diffraction data.

Synthesis, crystal structure of Ca6Sb2O11 from experiments and DFT-electron structure calculation

This article reports a new perovskite compound Ca6Sb2O11. The X-ray powder diffraction technique was used to analyze its crystal structure. The results show that Ca6Sb2O11 has a tetragonal structure with the I4/m space group, and the lattice parameters are a = b = 5.658 (1) Å, c = 7.984 (1) Å, α = β = γ = 90°. It exhibits a typical A4B'2B″2X12-x anoxic multi-perovskite structure with rock-salt ordering of the B-site cations. The compound's chemical composition and valence states were characterized through scanning electron microscopy-energy dispersion spectrum (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The presence of oxygen vacancies was confirmed by electron paramagnetic resonance (EPR) spectroscopy. A chemical and structure transition temperature and the full melting temperature of Ca6Sb2O11 were measured, with values of 1515 ± 3 K and 1720 ± 3 K, respectively. The density functional theory (DFT)-electron structure calculation results show that Ca6Sb2O11 is an indirect wide-band gap (∼4.47 eV) semiconductor material, which agrees well with the experimental value (∼4.38 eV) obtained through diffuse reflection spectroscopy (DRS). Moreover, the top of the valence band and bottom of the conduction band are mainly contributed to by O-2p and Ca-3d states, respectively.

Structural mode coupling in perovskite oxides using hypothesis-driven active learning

Finding the ground-state structure with minimum energy is paramount to designing any material. In ABO3-type perovskite oxides with Pnma symmetry, the lowest energy phase is driven by an inherent trilinear coupling between the two primary order parameters such as rotation and tilt with antiferroelectric displacement of the A-site cations as established via hybrid improper ferroelectric mechanism. Conventionally, finding the relevant mode coupling driving phase transition requires performing first-principles calculations which is computationally time-consuming as well as expensive. It involves following an intuitive iterative hit and trial method of (a) adding two or multiple mode vectors, followed by (b) evaluating which combination would lead to the ground-state energy. In this study, we show how a hypothesis-driven active learning framework can identify suitable mode couplings within the Landau free energy expansion with minimal information on amplitudes of modes for a series of double perovskite oxides with A-site layered, columnar and rocksalt ordering. This scheme is expected to be applicable universally for understanding atomistic mechanisms derived from various structural mode couplings behind functionalities, for e.g. polarization, magnetization and metal–insulator transitions.

Ambient pressure synthesis and structure and magnetic properties of a new A- and B-site ordered multinary quadruple perovskite.

Quadruple perovskites with high magnetic transition temperatures are an interesting class of compounds but are synthesized typically under high pressure. Ambient pressure synthesis of new multinary quadruple perovskites having a high global instability index (GII) and transition temperature can be interesting for future exploration of high-TC oxides. A new A- and B-site ordered multinary quadruple perovskite, LaCu3Fe2RuSbO12, is synthesized by conventional solid-state reactions at ambient pressure. Rietveld structure refinement revealed that the compound crystallizes in the Pn3̄ space group with a lattice parameter of 7.4556(4) Å. The compound showed complete 1 : 3 ordering of La and Cu at the A-site and 1 : 1 rock-salt ordering of Fe with Ru/Sb at the B-site. The compound is also probed with scanning and transmission electron microscopy and XPS to investigate the chemical composition, microstructure, lattice and oxidation states of the elements. Magnetic studies revealed antiferromagnetic (AFM) correlations with magnetic ordering transitions at ∼170 and 40 K. Furthermore, the M-H hysteretic behavior at 100 and 5 K indicated ferrimagnetism due to short-range AFM interactions among Fe3+(3d5) and Ru4+(4d4) spins involving Cu2+(↑)-Fe3+(↓)-Ru4+(↑) triads. The specific heat data reaffirmed the magnetic signatures while electrical transport showed semiconducting behavior with variable range hopping. The details of synthesis and structural and compositional studies along with the magnetic and electrical transport properties of LaCu3Fe2RuSbO12 are reported in this paper.

On the atomic ordering in the B-sublattice of high-entropy perovskites

Atomic ordering in perovskites has been the subject of intensive research for decades. With the growing interest in high-entropy perovskites (HEP), the issue of atomic ordering is now reaching a new level of significance. Many multi-cationic perovskites have already been synthesized, and some of them exhibit atomic ordering. Therefore, the problems of estimating the phase transition temperature of atomic ordering, determining the type of emerging atomic order, and the distribution of cations in ordered sublattices need to be addressed. In this work, based on the unstrained elastic cation-anion bonds model, we develop a Monte Carlo calculation scheme of atomic order-disorder phase transitions in the B-sublattice of perovskites with arbitrary composition and apply it to the case of HEP. We find that HEP’s may experience rock-salt type atomic ordering, while preserving considerable configurational entropy and essentially becoming ordered disordered perovskites. For many HEP’s, whose synthesis has been already reported in literature or which await their synthesis, we estimate the phase transition temperature Tod, reduced cubic lattice parameters, and the composition of the ordered sublattices. Finally, we derive a simple relation for the estimation of Tod from normalized standard deviation of the unstrained cation-anion bond lengths of atoms in the B-sublattice, which can be easily calculated.

Functional properties of A-site cation ordered phases derived from La[formula omitted]MnNiO[formula omitted] double perovskites

Starting from a room temperature parent La2MnNiO6 double perovskite as a prototype example, we ventured ferroelectricity and metal–insulator transition in the AA′BB′O6 double perovskites. Using first-principles density functional calculations guided by symmetry, we have shown that by appropriate A-site substitution i.e., by forming AA′ combinations, La2MnNiO6 within Mn and Ni in rock-salt ordering can be transformed into both ferroelectric insulating or polar metallic phase. We found that the charge difference between the cations and the radius mismatch between A and A′ are the key factors for deciding a particular ordering. The layered ordering becomes more stable when both charge difference and radius mismatch are large. The energy gain is more pronounced when the charge difference at the A-site is large. Further, we have identified and discussed the key structural distortions responsible for driving functional properties into the system. Our proposed mechanism is valid for other similar A2BB′O6 systems within 3d-3d double perovskites architecture with B and B′ sublattices in the rock-salt ordering.

Structural transformation induced twinning for enhanced conversion reaction of vacancy-ordered metal oxides with Li ions

Metal oxide anode materials based on conversion reaction usually deliver a capacity much higher than that of commercial graphite anodes in lithium-ion batteries (LIBs), and the porous forms of the materials can effectively alleviate volume change associated with (de)lithiation. In this work, tetragonal γ-Fe2O3, which is a vacancy-ordered superstructure containing large vacancy clusters, is studied as the representative of intrinsic nanoporous metal oxide anode materials of LIBs. γ-Fe2O3 exhibits a reversible capacity higher than the theoretical value in initial cycles, but a steady capacity same as that of α-Fe2O3. In lithiation, γ-Fe2O3 first transforms irreversibly to an ordered rock salt Li1-xFe1+xO2 (LiTiO2 type), and the Li1-xFe1+xO2 is converted reversibly into Li2O and Fe0 upon further lithiation: γ-Fe2O3+Li→Li1-xFe1+xO2+Li ↔ Fe0+Li2O. The alternate stacking of dense tetrahedral layers and porous octahedral layers in γ-Fe2O3 enables the simultaneous formation of Li1-xFe1+xO2 phases with different Li contents in a single particle and triggers structure twinning, accounting for fast reaction and likely high capacity in initial cycles. The structural evolution disclosed in γ-Fe2O3 not only updates the understanding of conversion reactions of vacancy-ordered metal oxides, but also offers an innovative approach for the fabrication of twinning structures in metal oxides including cathode materials.

Insights into Cation Ordering of Double Perovskite Oxides from Machine Learning and Causal Relations

This work investigates the origins of cation ordering of double perovskites using first-principles theory computations combined with machine learning (ML) and causal relations. We have considered various oxidation states of A, A', B, and B' from the family of transition metal ions to construct a diverse compositional space. A conventional framework employing traditional ML classification algorithms such as Random Forest (RF) coupled with appropriate features including geometry-driven and key structural modes leads to highly accurate prediction (~98%) of A-site cation ordering. We have evaluated the accuracy of ML models by entailing analyses of decision paths, assignments of probabilistic confidence bound, and finally introducing a direct non-Gaussian acyclic structural equation model to investigate causality. Our study suggests that the structural modes are the most important features for classifying layered, columnar and rock-salt ordering. For clear layered ordering, the charge difference between the A and A' is the most important feature which in turn depends on the B, B' charge separation. Based on the outputs from ML models, we have designed functional forms with these features to derive energy differences forming clear layered ordering. The trilinear coupling between tilt, rotation, and A-site antiferroelectric displacement in Landau free-energy expansion becomes the necessary condition behind the formation of A-site cation ordering.

Synthesis and Characterization of LaNi0.5Ti0.5O3 and La2NiTiO6 Double Perovskite Nanoparticles.

In the present work, LaNi0.5Ti0.5O3 and La2NiTiO6 nanoparticles were synthesized by the modified Pechini method. LaNi0.5Ti0.5O3 was calcined at 1073 K for 17 h or 100 h, while La2NiTiO6 was calcined at 1273 K for 135 h. The double perovskite calcined at 1073 K for 17 h presented orthorhombic symmetry with Pbnm space group, mean particle size was 31.9 ± 1 nm, random ordering of Ni2+ and Ti4+ cations, Néel temperature close to 15 K, and magnetic moment of 1.29 . By increasing the calcination time, this material showed the same symmetry and space group, a mean particle size of 50.7 ± 2 nm, short-range ordering of Ni2+ and Ti4+ cations, Néel temperature around 12 K, and magnetic moment of 0.96 . La2NiTiO6 presented a monoclinic crystal structure, with P21/n space group, mean particle size of 80.0 ± 5 nm, rock salt ordering of Ni2+ and Ti4+, Néel temperature of approximately 23 K, and magnetic moment of 2.75 .

(Invited) Operando X-Ray Absorption Fine Structure Studies of Polymer Electrolyte Membrane Electrolysis and Alkaline Water Electrolysis

With increasing global environmental problems, hydrogen, produced by renewable energies, has been considered a sustainable and clean fuel for next-generation energy sources[. Among the many proposed methods to produce hydrogen, the most renewable method is water electrolysis. However, the four electrons/protons needed for the oxygen evolution reaction (OER) in water electrolysis result in sluggish kinetics and low efficiency, representing the biggest obstacle for widespread applications. Polymer electrolysis membrane (PEM) electrolysis is expected to be commercialized due to its high current density, compact system, and high gas purity. Nevertheless, catalyst corrosion in acidic environments and high maintenance costs limit their general application. In contrast, alkaline water electrolysis (AWE) has been considered an alternative method for hydrogen production because earth-abundant transition-metal oxides (TM = Fe, Co, and Ni) can be used as catalysts, and certain states of TM cations accelerate the OER. Clarification of the electronic and local structure of catalysts during water electrolysis is important for understanding the activity and durability of catalysts. We have developed an operando X-ray Absorption Fine Structure (XAS) method that can be used to measure at high current densities under actual water electrolysis conditions. In this presentation, we will present examples of catalyst development using the established operando XAS method.Previous studies have focused on Ni-based oxides, including Ni(OH)2 and doped NiOOH, due to their low material cost, easily mediated structure, and high OER activity originating from various dopants, making them promising catalysts to replace noble metal oxides. Compared to Co-containing perovskite-type oxides, relatively few studies correlating the electronic states of Ni-based oxides with their OER activities have been reported. Recent studies on nickel oxides have elaborated the doping effect of lithium on catalyst activity, such as in LiNiO2 where lithium doping changed the local electronic structure of nickel, improving OER activity. LiNiO2 has a nominal oxidation state of Ni3+ and exhibits excellent OER activity considering the eg state descriptor. Moreover, because the energy levels of the Ni 3d orbitals of LiNiO2 are close to those of the O 2p orbitals, they can strongly hybridize. Therefore, LiNiO2 offers a suitable model to study the relationship between electronic structure and OER activity. However, for practical catalysts, the durability is also important and the deterioration mechanism of the LiNiO2 catalyst has not yet been clarified. LiNiO2, having an ordered rock salt structure, is a well-known cathode material for lithium-ion batteries, and exhibits extremely high lithium-ion mobility. Thus, catalyst degradation may be caused by de-intercalation of lithium ions during anodic polarization. In the Li-ion battery field, many studies have shown that the electrochemical properties of LiNiO2 cathodes are extremely dependent on the degree of cation mixing, primarily due to the presence of Ni2+ at Li+ and Ni3+ sites because of the similarity in their ionic radii. Cation mixing is a disadvantage for cathode materials because the presence of Ni2+ at Li+ sites hinders Li+ diffusion. Inspired by this, hindrance of the lithium diffusion path by cation mixing of Li and Ni is expected to be an effective solution when LiNiO2 is applied as an OER catalyst to improve stability. Herein, the cation mixing effect on OER activity and stability was studied using LixNi2-xO2. A combination of transmission electron microscopy (TEM), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) investigations were conducted to probe the electronic structure of the materials. The valence changes of Ni during OER were further examined using operando XAS using a home-made flow-type cell. The degradation process was also investigated in detail using operando XAS.

Synthesis, characterization and multiferroic properties of the doubly-ordered polar perovskite Na[formula omitted]NiWO[formula omitted] ([formula omitted] = Er, Tm, and Lu)

The doubly-ordered perovskite NaLnNiWO6 (Ln = Lanthanide) have been prepared at high-pressure and high-temperature conditions. Analysis of powder X-ray diffraction data reveals that these compounds crystallize in a polar monoclinic (P21) structure. In addition to the commonly observed rock salt ordering of B-site cations, an unusual layered order of A-site cations occurs in these compounds. Highly charged second-order Jahn–Teller (SOJT) active cation (W6+) at the B-site stabilizes the layered ordering and the overall crystal structure. Magnetization and heat capacity measurements confirm the antiferromagnetic order of these compounds in the temperature (TN) range 18–21 K. Dielectric anomaly, and dc-bias signal at the magnetic ordering temperature (TN) reveal the multiferroic properties of these compounds.

B-Site Columnar-Ordered Halide Double Perovskites: Theoretical Design and Experimental Verification.

Halide double perovskites A2B(I)B(III)X6, in which monovalent B(I) and trivalent B(III) cations are arranged in the B-sites of the perovskite structure with a rock-salt ordering, have attracted substantial interest in the field of optoelectronics. However, the rock-salt ordering generally leads to low electronic dimensionality, with relatively large bandgaps and large carrier effective masses. In this work, we demonstrate, by density functional theory (DFT) calculations, that the electronic dimensionality and thus the electronic properties of halide double perovskites can be effectively modulated by manipulating the arrangement of the B-site cations. Through symmetry analysis and DFT calculations, we propose a family of halide double perovskites A2B(I)B(II)X5 where the B-site cations adopt a columnar-ordered arrangement. Among the considered compounds, Cs2AgPdCl5, Cs2AgPdBr5, and Cs2AgPtCl5 were successfully synthesized as the first examples of the B-site columnar-ordered halide double perovskites. These compounds exhibit small bandgaps of 1.33-1.77 eV that are suitable for visible light absorption, small carrier effective masses along the octahedra chains, and good thermal and air stability. Our work provides a prototype double perovskite structure to incorporate cations in +1 and +2 oxidation states, which may significantly expand the large family of the halide double perovskites and offer a platform to explore prospective optoelectronic semiconductors.

Structural, Magnetic, and Electrical Properties of Doubly Ordered Perovskites NaLnNiWO6 (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb)

We report the structural, magnetic, and electrical properties of a new family of doubly ordered perovskites NaLnNiWO6 (Ln = La, Pr, Nd, Sm, Eu, Gd, and Tb), synthesized at high-pressure (4.5 GPa) and high-temperature (1000 °C) conditions. These materials crystallize in the monoclinic structure with the polar space group P21 as revealed by neutron diffraction (Ln = La), X-ray diffraction, and second-harmonic-generation (SHG) studies. These compounds exhibit a combined layered ordering of A-site cations and rock-salt ordering of B-site cations with octahedral tilting, which breaks the inversion symmetry and results in a polar structure. The magnetic and heat capacity measurements reveal the antiferromagnetic order of Ni2+ ion moments in the temperature range of 23–30 K, depending on the Ln ion. Analysis of neutron powder diffraction data for the La compound reveals that the Ni2+ spins couple antiferromagnetically along the a- and c-directions and ferromagnetically along the b-axis, consistent with the propagation vector, k = (1/2, 0, 1/2). In contrast to the compounds with smaller lanthanide ions (Ln = Y, Dy, Ho, and Yb), these compounds do not exhibit measurable electric polarization at TN but show a dielectric anomaly.

Effect of B-Site Order-Disorder in the Structure and Magnetism of the New Perovskite Family La2MnB'O6 with B' = Ti, Zr, and Hf.

In this work, we report the synthesis as well as the structural and magnetic characterization of the three perovskites La2MnB'O6 (B' = Ti, Zr, and Hf). Interestingly, only La2MnTiO6 crystallizes in the monoclinic double perovskite space group P21/n, with a complete rocksalt order of the B-site cations, whereas La2MnZrO6 and La2MnHfO6 crystallize in the orthorhombic simple perovskite space group Pbnm, with complete disorder in the B site. Moreover, the magnetic susceptibility at low temperatures shows clear antiferromagnetic transitions below 10 K for the three compounds, but only the Ti-based perovskite has long-range magnetic ordering. The latter compound has an antiferromagnetic type-II structure described by the PS-1 magnetic space group, while the other two have a spin-glass behavior below the transition temperature due to both spin disorder and competing superexchange interactions in the systems. This is the first time that two of the three studied compounds were synthesized (B' = Zr and Hf) and the first time that the whole series is described in thorough detail using symmetry-adapted refinements and magnetic crystallography.

(Invited) Ions, Electrons, and Phonons: On the Movement of Charge through Solids

Li-ion batteries are ubiquitous; powering our cell phones, tablets, and laptop computers. Yet, we’ve almost exclusively relied on materials that adopt the same ordered rock salt structure type for over 20 years. What makes these materials so special and why is it so hard to move beyond them? This talk will present work our group has done to create a deeper understanding of the fundamental materials criterion required to move Li-ions through densely packed solids and outline new design principles for the development of next-generation intercalation hosts. Special attention will be paid to how changes to ionocovalent character of the chemical bonding can influence the mobility of not only the alkali ions, but also the redox-active transition metal centers.

A-Site and B-Site Cation Ordering Induces Polar and Multiferroic Behavior in the Perovskite NaLnNiWO6 (Ln = Y, Dy, Ho, and Yb)

Octahedral distortion in ABO3 perovskite materials is ubiquitous because of the ionic size mismatch between A and B cations, leading to various kinds of crystal symmetry. However, such a distortion always results in centrosymmetric structures except for the covalent bond formation because of the second-order Jahn–Teller (SOJT) effect that occurs with d0 or lone-pair cations. Here, we report that an unusual combination of the layered A-site cation ordering and B-site rock salt ordering in NaYNiWO6 prepared under high-pressure and high-temperature conditions results not only in a polar (P21) structure, as revealed by the neutron diffraction analysis, but exhibits multiferroic properties below the magnetic ordering of Ni2+ ions (TN = 21 K). Analysis of the neutron diffraction data at 20 K reveals an incommensurate sinusoidal spin ordering with the propagation vector, ki = (0.471(2), 0, 0.491(4)), and a commensurate collinear spin structure with kc = (0.5, 0, 0.5) below 18 K. X-ray diffraction data confirm the polar structure in Dy, Ho, and Yb compounds. All four compounds exhibit a switchable change in electric polarization (ΔP) at the magnetic ordering temperatures, demonstrating coupling between ferroelectricity and magnetism.

Synthesis, Crystal Structure, and Cooperative 3d–5d Magnetism in Rock Salt Type Li4NiOsO6 and Li3Ni2OsO6

Two new transition metal oxides with the nominal chemical compositions of Li4NiOsO6 and Li3Ni2OsO6 were successfully synthesized. Both compounds crystallize in an ordered rock salt structure type in the monoclinic C2/m space group. The crystal structures were determined using both synchrotron X-ray and time-of-flight neutron, powder diffraction data. In both phases, Ni2+ ions are present while oxidation states of osmium are +6 and +5 in Li4NiOsO6 and Li3Ni2OsO6, respectively. Ni2+ ions in the hypothetical fully ordered phase form a honeycomb arrangement in the ab crystallographic plane and these hexagons are centered by osmium ions. The magnetic layers are separated along the c axis by the octahedra, which are centered by Li+ (or Li+/Ni2+, depending on the chemical compositions). Crystal structure refinements reveal that there is some degree of mixed occupancy in cationic positions. Temperature dependent magnetic susceptibility data for both phases show ferrimagnetic transitions with predominant antiferromagnetic (AFM) interactions among 3d electrons of nickel and 5d electrons of osmium. Iso-thermal magnetization loops as a function of the applied magnetic field below the transition temperatures confirm the ferrimagnetic nature in magnetic transitions. Temperature dependent heat capacity data, however, did not exhibit any anomaly in either phase, indicating the absence of long-range magnetic ordering. The lack of long-range order for both Os5+ and Os6+-based compounds was also confirmed by low temperature neutron diffraction data down to 10 K. Temperature dependent AC magnetic susceptibility data in various frequencies for both samples indicate that Li4NiOsO6 exhibits spin-glass-like behavior, while the transition temperature for Li3Ni2OsO6 is nearly frequency independent.

First-principles soft-mode lattice dynamics of PbZr0.5Ti0.5O3 and shortcomings of the virtual crystal approximation

A comparative study between PbTiO$_3$, PbZrO$_3$, and the solid solution PbZr$_{0.5}$Ti$_{0.5}$O$_3$ is performed on the soft mode lattice dynamics within the first Brillouin Zone. We consider the six unique B-site orderings for PbZr$_{0.5}$Ti$_{0.5}$O$_3$ representable within the 2$\times$2$\times$2 primitive perovskite supercell as well as the virtual crystal approximation (VCA) to extract the phonon dispersion relations of a high-symmetry cubic-constrained form using density functional perturbation theory. We find that the most unstable modes in the rock-salt ordered structure and the VCA, like pure PbZrO$_3$, are antiferrodistortive (AFD) whilst lower symmetry arrangements are dominated by $\Gamma$-point ferroelectric (FE) instabilities like pure PbTiO$_3$. Despite similarities in the phonon dispersion relations between the rock-salt ordered supercell and the VCA, the character of modes at high symmetry points are found to be different. In particular, the a$^{0}$a$^{0}$c$^{-}$ & a$^{0}$a$^{0}$c$^{+}$ AFD instabilities of the rock-salt ordering are replaced with a$^{-}$b$^{-}$c$^{-}$ & a$^{+}$b$^{+}$c$^{+}$ instabilities within the VCA. Such a rotation pattern is not seen in any of the supercell-based calculations thus serving as a quantitative example of the inability of the method to represent accurately local structural distortions. Single modes are found exhibiting dual order parameters. At the zone centre, some arrangements show mixed FE & antipolar soft modes (due to Pb motion tansverse to the polar axis) and at long wavelengths all arrangements have soft modes of a mixed antipolar & AFD character. These are described with direct analysis of the eigendisplacements.

Expanding the Doubly Cation Ordered AA′BB′O6 Perovskite Family: Structural Complexity in NaLaInNbO6 and NaLaInTaO6

Two new perovskite compounds, NaLaInNbO6 and NaLaInTaO6, have been synthesized. Both compounds have a rock-salt ordering of the In/Nb or In/Ta cations and a layered ordering of the Na/La. They are unusual among this family of doubly cation ordered perovskites for having a +3/+5 combination of B/B' oxidation states instead of a divalent cation and W6+. Synchrotron powder diffraction and electron diffraction data show both compounds have tetragonal √2ap × √2ap × 2ap unit cells. The octahedral tilt system of these compounds is complicated and seems to vary depending on the length scale considered. Bond valence considerations demonstrate that none of the tilt systems compatible with a tetragonal unit cell produce a stable structure. It is proposed that additional oxygen displacements are occurring on a local scale. The XRD data show that the B-site ordering is nearly complete in both cases, while the A-site cations show a lower degree of order. The tendency of the B'-cation to undergo a second order Jahn-Teller distortion is identified as having an influence over the degree of A-site ordering. Transmission electron microscopy shows fragmentation into nanosized domains with perpendicular orientations of the layered A-site cation ordering. Such structures may be polar over short length scales which could lead to interesting dielectric properties such as relaxor behavior. The prospects for finding new doubly cation ordered perovskites are also discussed.

Spin ordering and physical properties of NaPrFeWO6 and NaSmFeWO6 with polar double perovskite structure

NaPrFeWO6 and NaSmFeWO6 have been synthesized as polycrystalline powders by means of a sol-gel route. Both samples adopt a noncentrosymmetric structure derived from the simple perovskite ABO3 and characterized by a layered ordering of A-site atoms (Na and rare earth) and a rock-salt ordering of B-site atoms (Fe and W) as deduced from x-ray and neutron powder diffraction techniques. Magnetization measurements revealed antiferromagnetic ordering below TN ≈23 K in both compounds but weak spontaneous magnetization is observed in the hysteresis loops of the samples above and below TN revealing a parasitic ferromagnetic component that comes from minor ferromagnetic impurities. The magnetic structure of NaPrFeWO6 is formed by two antiferromagnetic sublattices (rare earth and Fe) that follow the same propagation vector k = (½, 0, ½). The magnetic structure is purely collinear with the moments oriented along the b-axis. It presents competitive magnetic interactions and reduced ordered moments at low temperatures. The rare earth moments in the zig-zag chains running along a (parallel to the ab plane) are ordered following a sequence typical of an E-type arrangement (up-up-down-down). Each Fe2+ atom has four nearest Pr3+ neighbors and it is coupled antiferromagnetically to three of them and ferromagnetically to the fourth one. Strong neutron absorption from Sm atoms hinders obtaining reliable neutron patterns of NaSmFeWO6 for Rietveld analysis but the comparison of neutron patterns above and below TN detected the same magnetic peaks indicating a similar magnetic structure. Dielectric measurements are compatible with a paraelectric system with a strong Maxwell-Wagner contribution from depletion layers and electrical properties are dominated by leakage currents.