Cooperative Jahn-Teller Distortion Research Articles

High-Pressure Induced Continuous Structural Evolution of Kagome Antiferromagnet MgMn3(OH)6Cl2: A Structural Analogue to Quantum Spin Liquid Herbertsmithite.

MgMn3(OH)6Cl2 serves readily as the classical Heisenberg kagome antiferromagnet lattice spin frustration material, due to its similarity to herbertsmithite in composition and crystal structure. In this work, nanosheets of MgMn3(OH)6Cl2 are synthesized through a solid-phase reaction. Low-temperature magnetic measurements revealed two antiferromagnetic transitions, occurring at ∼8 and 55 K, respectively. Utilizing high-pressure synchrotron radiation X-ray diffraction techniques, the topological structural evolution of MgMn3(OH)6Cl2 under pressures up to 24.8 GPa was investigated. The sample undergoes a second-order structural phase transition from the rhombohedral phase to the monoclinic phase at pressures exceeding 7.8 GPa. Accompanying the disappearance of the Fano-like line shape in the high-pressure Raman spectra were the emergence of new Raman active modes and discontinuities in the variations of Raman shifts in the high-frequency region. The phase transition to a structure with lower symmetry was attributed to the pressure-induced enhancement of cooperative Jahn-Teller distortion, which is caused by the mutual substitution of Mn2+ ions from the kagome layer and Mg2+ ions from the triangular interlayer. High-pressure ultraviolet-visible absorption measurements support the structural evolution. This research provides a robust experimental approach and physical insights for further exploration of classical geometrical frustration materials with kagome lattice.

Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO2 Cathodes

AbstractThe O‐type layered nickel oxides suffer from undesired cooperative Jahn–Teller distortion stemming from Ni3+ ions and undergo multiple biphasic structural transformations during the insertion/extraction of large Na+ ions, posing a significant challenge to stabilize the structural integrity. We present here a systematic investigation of the impact of substituting 5 % divalent (Mg2+) or trivalent (Al3+ or Co3+) ions for Ni3+ to alleviate Na+ion ordering and perturb the Jahn–Teller effect to enhance structural stability. We gauge a fundamental understanding of the Mg−O and Na−O or Mg−O−Na bonding interactions, noting that the ionicity of the Mg−O bond deshields the electronic cloud of oxygen from Na+ ions. Furthermore, calculations of the Van Vleck distortion modes reveal a relaxation of NiO6 octahedra from Jahn–Teller distortion and a reduced electron density at the interlayer with Mg2+ substitution. Long‐range (operandoX‐ray diffraction) and short‐range (magic angle spinning nuclear magnetic resonance) structural analyses provide insights into reduced ordering, allowing a stable continuous solid solution. Overall, Mg‐substitution results in a high‐capacity retention of ~96 % even after 100 cycles, showcasing the potential of this strategy for overcoming the structural instabilities and enhancing the performance of sodium‐ion batteries.

Mitigating Sodium Ordering for Enhanced Solid Solution Behavior in Layered NaNiO2 Cathodes.

The O-type layered nickel oxides suffer from undesired cooperative Jahn-Teller distortion stemming from Ni3+ ions and undergo multiple biphasic structural transformations during the insertion/extraction of large Na+ ions, posing a significant challenge to stabilize the structural integrity. We present here a systematic investigation of the impact of substituting 5 % divalent (Mg2+) or trivalent (Al3+ or Co3+) ions for Ni3+ to alleviate Na+ion ordering and perturb the Jahn-Teller effect to enhance structural stability. We gauge a fundamental understanding of the Mg-O and Na-O or Mg-O-Na bonding interactions, noting that the ionicity of the Mg-O bond deshields the electronic cloud of oxygen from Na+ ions. Furthermore, calculations of the Van Vleck distortion modes reveal a relaxation of NiO6 octahedra from Jahn-Teller distortion and a reduced electron density at the interlayer with Mg2+ substitution. Long-range (operando X-ray diffraction) and short-range (magic angle spinning nuclear magnetic resonance) structural analyses provide insights into reduced ordering, allowing a stable continuous solid solution. Overall, Mg-substitution results in a high-capacity retention of ~96 % even after 100 cycles, showcasing the potential of this strategy for overcoming the structural instabilities and enhancing the performance of sodium-ion batteries.

Two-dimensional ferromagnetism induced by Jahn-Teller distortion and orbital order

As the dimension of the system decreases, the quantum confinement effect and electronic correlation interaction inside the material will be correspondingly enhanced, often resulting in some novel physical properties. Recently, the freestanding perovskite oxide films down to the monolayer limit have been successfully prepared and can be transferred to any desired substrate, providing a great opportunity to explore the functional properties of two-dimensional perovskites. In perovskite materials, Jahn-Teller distortion and orbital order often cause a variety of correlated electronic behaviors. However, unlike van der Waals materials that retain their structural and chemical bonding characteristics when reduced to the monolayer limit, perovskite materials may undergo structural reconstruction when reduced to two dimensions. Therefore, whether Jahn-Teller distortion and related effects exist in the perovskite monolayer limit, and whether two-dimensional perovskite can exhibit some new properties different from its bulk phase, have become urgent issues to be solved. In this work, perovskite fluoride KCuF<sub>3</sub> and its monolayer have been comparatively studied by means of first-principles calculation, symmetry analysis, and Monte Carlo simulation methods, to reveal the change in lattice dynamics, structural, electronic, and magnetic properties caused by dimensionality reduction in perovskites. The results show that the cooperative Jahn-Teller distortion and the in-plane staggered orbital order occurring in the KCuF<sub>3</sub> bulk can be retained to the monolayer limit. However, unlike the bulk phase, the Jahn-Teller distortion mode appears as a soft mode of the prototype phase in the monolayer, and the insulating property of the monolayer does not rely on the emergence of the Jahn-Teller distortion, but is related to the enhancement of the electronic correlation effect. The staggered orbital order causes the nearest-neighbor exchange interaction to be ferromagnetic, resulting in the monolayer being a two-dimensional ferromagnetic insulator, different from the antiferromagnetic phase in the bulk. Monte Carlo simulations predict that the Curie temperature of the monolayer is about 5 K, which is much lower than the Néel temperature of the bulk phase, indicating that the disappearance of interlayer coupling leads to a significant reduction in the magnetic phase transition temperature. This work provides guidance and reference for the study of two-dimensional perovskite materials and the design of perovskite-based two-dimensional ferromagnets.

P'2-Na2/3[Mn1-X Sc x ]O2 Electrode in Sodium-Batteries

In recent years, Na-ion batteries (SIB) are expected to be an alternative to Li-ion batteries. One of the cathode materials for SIB is transition metal layered oxides Na x TMO2 (TM = transition metal). In order to improve their capacity, efforts have been made to utilize the redox process(es) of not only TM ions but also oxide ions (oxygen redox). In addition, partial replacement of TMs with electrochemically inactive elements has been investigated to enhance cycle stability. Our group reported P2-type Na x MnMeO2 (Me = Li, Mg) exhibit higher capacity than existing materials via using additional redox of oxide ions (1, 2). Another example delivering high capacity reported in our group is P’2-type Na2/3MnO2, where the average oxidation states of Mn is about +3.37 and a co-operative Jahn-Teller distortion (CJTD) of Mn3+ (t2g 3-eg 1) is shown (3). In this study, we synthesized P’2-Na2/3Mn1-x Sc x O2 and studied the effect of Sc substitution on the electrochemical properties of P’2-Na2/3MnO2. P’2-Na2/3Mn1-x Sc x O2 (x = 0, 0.08, 0.11) samples were synthesized using Na2CO3, Mn2O3, and Sc2O3 as starting materials, based on the synthesis method of the non-doped P’2-Na2/3MnO2 as we previously reported (3). For galvanostatic cycling tests, a working electrode consisted of active materials, acetylene black, and PVdF in 80:10:10 wt.% on Al foil. Na metal was used as a counter electrode, and an electrolyte used was 1.0 mol dm-3 NaPF6 PC solution. XRD patterns of P’2-Na2/3Mn1-x Sc x O2 are shown in Fig. 1 (a). Major diffraction peaks can be indexed as orthorhombic Na2/3MnO2 with space group of Cmcm, confirming P’2-type layered oxides. Note that non-substituted Na2/3MnO2 shows a superstructure peak at 23.9º in 2θ attributed to in-plane superstructure (3). At x = 0.11, impurity of Sc2O3 is observed, indicating that Sc is substituted for Mn up to about 8% in P’2-Na2/3MnO2. Sc3+ ions which has a large ionic radius than Mn3+ ions and no Jahn-Teller distortion, resulted in the enlarged lattice constant a (Figure 1 (a), inset) and the reduced lattice distortion (4). Fig. 1 (b) shows the discharge capacity and the coulombic efficiency of P’2-Na2/3Mn1-x Sc x O2 Electrode in Na cell. The capacity retention after 50 cycles for the non-substituted x = 0 (non-sub) and Sc-substituted x = 0.08 (Sc-sub) in the voltage range of 1.5 – 4.4 V was 66.4% and 85.4%, respectively. In-situ XRD data can support better cycling life. The Sc-sub exhibited better rate characteristics than the non-sub, indicating that there is an optimal amount of Sc to improve the electrochemical properties. Looking at the initial charge/discharge curves at different cutoff voltages (Fig. 1 (c)), setting the upper cutoff voltage beyond 3.47 V, causes a significant polarization due to the phase transition from P’2 to OP4. From the cycle stability tests at different upper cutoff voltages (Fig. 1 (d)), the upper cutoff limit of 4.0 V demonstrates notable capacity fade than that of 4.4 V. One possible reasons should be changing the surface chemistry of electrode by applying higher voltage than 4.0 V. We will further discuss the effects of Sc substitution and charge/discharge voltage range on the electrochemical properties.References Yabuuchi, S. Komaba, et al., J. Mater. Chem., 2, 16851 (2014).Yabuuchi, S. Komaba, et al., Adv Energy Mater., 4, 1301453 (2014).Kumakura, S. Komaba, et al., Angew. Chem. Int. Ed., 55, 12760 (2016).S. Kumakura, S. Komaba, et al., Chem Mater., 29, 8958 (2017). Figure 1

Pseudo-dielectric functions, band-to-band transitions, and dielectric-related factors in a single-crystal LiTi2O4 thin film

This article is devoted to studying the optical properties of a novel single-crystal LiTi2O4 thin film. A high-quality (011)-oriented LiTi2O4 single-crystal thin film was prepared on a MgAl2O4 (011) substrate by pulsed laser deposition. The weak in-plane optical anisotropy is discovered by Mueller-matrix spectroscopic ellipsometry, which can be attributed to the cooperative Jahn-Teller distortion in the Ti–O octahedrons. The pseudo-dielectric functions were determined in the photon energy of 1.24–5.06 eV using the ellipsometric spectra. The optical conductivity, plasma frequency, effective mass, scattering time and other dielectric-related factors are also discussed. The critical points obtained in the pseudo-dielectric functions are originated from the charge-transfer transitions of the Ti related pairs. The results demonstrate that the enhanced effective mass is responsible for the high optical transparency of LiTi2O4 rather than the large high frequency dielectric constant ε∞.

Copper Ferrite Nanoparticles Synthesized Using Anion-Exchange Resin: Influence of Synthesis Parameters on the Cubic Phase Stability

Copper ferrite is of great interest to researchers as a material with unique magnetic, optical, catalytic, and structural properties. In particular, the magnetic properties of this material are structurally sensitive and can be tuned by changing the distribution of Cu and Fe cations in octahedral and tetrahedral positions by controlling the synthesis parameters. In this study, we propose a new, simple, and convenient method for the synthesis of copper ferrite nanoparticles using a strongly basic anion-exchange resin in the OH form. The effect and possible mechanism of polysaccharide addition on the elemental composition, yield, and particle size of CuFe2O4 are investigated and discussed. It is shown that anion-exchange resin precipitation leads to a mixture of unstable cubic (c-CuFe2O4) phases at standard temperature and stable tetragonal (t-CuFe2O4) phases. The effect of reaction conditions on the stability of c-CuFe2O4 is studied by temperature-dependent XRD measurements and discussed in terms of cation distribution, cooperative Jahn-Teller distortion, and Cu2+ and oxygen vacancies in the copper ferrite lattice. The observed differences in the values of the saturation magnetization and coercivity of the prepared samples are explained in terms of variations in the particle size and structural properties of copper ferrite.

Possible absence of a Jahn-Teller distortion critical thickness in geometrically designed LaMnO3 films

We investigated the Jahn-Teller phonon modes in ${\mathrm{LaMnO}}_{3}$ film grown on an orthorhombic (110)-oriented ${\mathrm{GdScO}}_{3}$ substrate using Raman spectroscopy, which enabled us to verify the presence of the cooperative Jahn-Teller distortion in the ultrathin limit. A characteristic ${B}_{1g}$ phonon mode with preserved symmetry was observed down to the two-unit-cell thickness. This finding suggests the absence of a film-thickness-driven critical transition of the cooperative Jahn-Teller distortion and the corresponding orbital order when the two-dimensional Jahn-Teller-distortion plane is designed to be parallel to the substrate by misfit strain.

Effect of Cu Substitution in P′2- and P2-Type Sodium Manganese-Based Oxides

Layered manganese oxides have been considerably studied toward sustainable sodium-ion battery materials. There has been a focus on the specific roles of Cu as a member of substituents in sodium manganese oxides. Herein, the effects of Cu substitution for Mn in P′2- and P2-type Na0.67[CuxMn1–x]O2 (0 ≤ x ≤ 0.33) are systematically investigated, focusing on the cooperative Jahn–Teller distortion of Mn3+ and Cu2+ ions. P2-type Na0.67[Cu0.2Mn0.8]O2 is selected to gain further insights on oxygen redox along with P2-type Na0.67[Cu0.1Mg0.1Mn0.8]O2 and P2-type Na0.67[Cu0.1Zn0.1Mn0.8]O2. Comparative studies among the three materials reveal that the type of substituent(s) for Mn has no impact on the activation of oxygen redox. In contrast, the nature and the concentration of substituted element(s) influence structural evolution during charge/discharge. P2-type Na0.67[Cu0.2Mn0.8O2] maintains a P2 phase without any secondary phase at the bulk and local scales at the end of charge to 4.5 V, leading to suppressed voltage hysteresis. At the end of discharge to 1.5 V, the presence of higher Cu2+ in P2-type Na0.67[Cu0.2Mn0.8]O2 results in a distorted P′2 phase with a distortion degree of 10.6(1)%. This can induce a large lattice stress related to the lattice mismatch between P2 and P′2, which jeopardizes cyclability.

Ground-state structure, orbital ordering and metal-insulator transition in double-perovskite PrBaMn2O6

In recent years, A-site ordered half-doped double-perovskite manganites $\rm RBaMn_2O_6$ (R=rare earth) have attracted much attention due to their remarkable physical properties and a prospect of application as magnetoresistance, multiferroic, and oxygen storage materials. The nature of the ground state in ${\rm RBaMn_2O_6}$ as well as sequence of phase transitions taking place at cooling are not yet well understood due to complexity in both experimental and theoretical studies. Here we address the origin of the ground-state structure in PrBaMn$_2$O$_6$ as well as its electronic and magnetic properties. Utilizing GGA+U approach and specially designed strategy to perform structural optimization, we show that the system has two competing AFM-A and AFM-CE magnetic structures with very close energies. The AFM-A structure is a metal, while AFM-CE is an insulator and the transition to the insulating state is accompanied by the charge Mn$^{3+}$/Mn$^{4+}$, and orbital $3x^2-r^2$/$3y^2-r^2$ orderings. This orbital ordering results in strong cooperative Jahn-Teller (JT) distortions, which lower the crystal symmetry. Our findings give a key to understanding contradictions in available experimental data on ${\rm PrBaMn_2O_6}$ and opens up the prospects to theoretical refinements of ground-state structures in other ${\rm RBaMn_2O_6}$ compounds.

Magnetoresistance anisotropy in ErB12 antiferromagnetic metal: The evidence for dynamic charge stripes

The nature of charge transport and magnetization anisotropy was investigated in the ErB12 antiferromagnetic metal with both cooperative Jahn-Teller distortions of the fcc crystal structure and electronic instability (dynamic charge stripes). A butterfly pattern of the H-φ magnetic phase diagram in the (1 1 0) plane was reconstructed, including a large number of different magnetic phases separated from each other by radial and circular boundaries. It is argued that fluctuations of electron density are responsible for the suppression of the indirect Ruderman-Kittel-Kasuya-Yoshida (RKKY) exchange between the nearest neighbored Er3+ ions located along the 〈110〉 directions producing the magnetic phase diversity in ErB12.

LiMnO2 cathode stabilized by interfacial orbital ordering for sustainable lithium-ion batteries

Global lithium-ion battery deployments stand poised to grow substantially in the coming years, but it will be necessary to include sustainability considerations in the design of electrode materials. The current cathode chemistry relies heavily on cobalt, which, due to its scarcity and the environmental abuse and violation of human rights during its mining, must be replaced by abundant and environmentally friendly elements such as redox-active manganese. LiMnO2 is a strong contender for sustainable cathodes but cycles poorly because the Jahn–Teller distorted Mn3+ ions destabilize the lattice framework. Here, we report a LiMnO2 cathode design with interwoven spinel and layered domains. At the interface between these two domains, the Mn dz2 orbitals are oriented perpendicular to each other, giving rise to interfacial orbital ordering, which suppresses the otherwise cooperative Jahn–Teller distortion and Mn dissolution. As a result, the heterostructured cathode delivers enhanced structural and electrochemical cycling stability. This work provides a new strategy for interface engineering, possibly stimulating more research on Mn-rich cathode materials for sustainable lithium-ion batteries. The field of battery chemistry must embrace abundant elements such as Mn for improved sustainability. Here the authors engineer the orientation of Mn 3d orbitals, resulting in excellent performance in LiMnO2 cathodes.

Local polarization in oxygen-deficient LaMnO3 induced by charge localization in the Jahn-Teller distorted structure

The functional properties of transition metal perovskite oxides are known to result from a complex interplay of magnetism, polarization, strain, and stoichiometry. Here, we show that for materials with a cooperative Jahn-Teller distortion, such as LaMnO$_3$ (LMO), the orbital order can also couple to the defect chemistry and induce novel material properties. At low temperatures, LMO exhibits a strong Jahn-Teller distortion that splits the $e_g$ orbitals of the high-spin Mn$^{3+}$ ions and leads to alternating long, short, and intermediate Mn--O bonds. Our DFT+$U$ calculations show that, as a result of this orbital order, the charge localization in LMO upon oxygen vacancy formation differs from other manganites, like SrMnO$_3$, where the two extra electrons reduce the two Mn sites adjacent to the vacancy. In LMO, relaxations around the defect depend on which type of Mn--O bond is broken, affecting the $d$-orbital energies and leading to asymmetric and hence polar excess-electron localization with respect to the vacancy. Moreover, we show that the Mn--O bond lengths, orbital order and consequently the charge localization and polarity are tunable via strain.

Cooperative Jahn-Teller effect in NdMn1-xFexO3+δ (0 ≤ x ≤ 0.2)

NdMn1-xFexO2.90(5) (0 ≤ x ≤ 0.2) compounds undergo first order reversible orthorhombic-orthorhombic structural phase transition from a high temperature O phase to low temperature O′ phase after heat treatment in Ar. This transition takes place in wide temperature range between the Jahn Teller (JT) temperature TJT and the onset of transition to orbital ordered state at T∗. Iron shifts the transition to lower temperatures TJT = 1230(5) K, 1190(5) K, 1160(5) K and T∗ = 1035(5) K, 985(5) K, 950(5) K for x = 0; 0.1; and 0.2. Both O phase and O’ phase coexist in short temperature range above T∗ and cooperative Jahn Teller distortions (CJTDs) persist even in O phase for x = 0 and 0.1 in vicinity of T∗.Oxygen is being built into crystal lattice for heat treatment in air at elevated temperatures. As a consequence the composition changes to NdMn1-xFexO3, the CJTDs vanish from O phase. The increase of oxygen in samples results in the reduction of anomalies at the phase transition and their shift to lower temperatures (TJT = 1106(5) K and T∗ = 1045(5) K for x = 0). The transformation from O phase to O’ phase is not connected with strong anomaly anymore for x = 0.1 and 0.2. The CJTDs are present below 750 K on all samples, which are temperatures from about 350 K to 250 K smaller in comparison with Ar atmosphere. Subsequent thermal cycling in air leads to phase separation into two phases for compounds with higher concentration of iron.Analysis of our results revealed that the O to O′ phase transitions are much stronger for samples in oxygen deficient state where oxygen vacancy clusters can be created at relevant temperature. We suppose that oxygen vacancy clusters are responsible for appearance of strong anomalies at the O to O′ phase transition and for easier formation of CJTDs over the volume of samples. Conditions for formation of CJTDs as well number of JT active ions in the sample are main factors which affect the O to O’ phase transition.

Manganese Spinel: Double the Capacity of Manganese Spinel for Lithium‐Ion Storage by Suppression of Cooperative Jahn–Teller Distortion (Adv. Energy Mater. 34/2020)

In article number 2000363 Jiaxin Zheng, Feng Pan and co-workers demonstrate that the capacity of lithium-ion manganese spinel can be doubled by the suppression of the cooperative Jahn-Teller effect. The basic suppression mechanism can be described by the image. When the dragon boat teammates work in a cooperative way just like the cooperative Jahn-Teller effect, the phase transition can be severe and result in the deterioration of the electrochemical performance. However, once the cooperative work is disturbed, like the non-coordination in the dragon boat, the phase transition can be suppressed, which makes it possible for the spinel to access double the capacity.

Double the Capacity of Manganese Spinel for Lithium‐Ion Storage by Suppression of Cooperative Jahn–Teller Distortion

AbstractThe relatively low capacity and capacity fade of spinel LiMn2O4 (LMO) limit its application as a cathode material for lithium‐ion batteries. Extending the potential window of LMO below 3 V to access double capacity would be fantastic but hard to be realized, as it will lead to fast capacity loss due to the serious Jahn–Teller distortion. Here using experiments combined with extensive ab initio calculations, it is proved that there is a cooperative effect among individual Jahn–Teller distortions of Mn3+O6 octahedrons in LMO, named as cooperative Jahn–Teller distortion (CJTD) in the text, which is the difficulty to access the capacity beyond one lithium intercalation. It is further proposed that the cationic disordering (excess Li at Mn sites and Li/Mn exchange) can intrinsically suppress the CJTD of Mn3+O6 octahedrons. The cationic disordering can break the symmetry of Mn3+ arrangements to disrupt the correlation of distortions arising from individual JT centers and prevent the Mn3+O bonds distorting along one direction. Interestingly, with the suppressed CJTD, the original octahedral vacancies in spinel LMO are activated and can serve as extra Li‐ion storage sites to access the double capacity with good reversible cycling stability in microsized LMO.

New Universality Class Associated with Jahn–Teller Distortion and Double Exchange

The scaling of the magnetic heat capacity in the two manganites La0.85Ag0.15MnO3 and Sm0.55Sr0.45MnO has given the critical exponents α = –0.23 and ν = 0.7433 of the heat capacity and correlation radius of the magnetic order parameter, respectively, which do not belong to any known universality class. These results cannot be attributed to chemical inhomogeneities and/or structural imperfections because the samples are of a high quality. Thus, unusual critical exponents can be associated not only with the chemical disorder and/or structural defects but also with the collective behavior of the lattice. An analogy has been revealed between the effects of the magnetic field and doping on ternary oxides of transition metals: the magnetic field affecting lattice distortions through the orientation of t2g orbitals acts as chemical doping. It seems that scaling relations are more stable than critical exponents in them. The synchronism of lattice distortions and ferromagnetism leads to a novel criticality, but their desynchronization induced by magnetostructural disorder results in the violation of scaling relations between isothermal and isomagnetic exponents. Although double-exchange systems demonstrate novel criticality, they satisfy scaling relations until the magnetic behavior is synchronized with the coherent lattice behavior in the form of cooperative Jahn–Teller distortions. Breaking of double exchange bonds leads to the formation of metamagnetic clusters with magnetic dipole–dipole interaction between them, which desynchronizes lattice distortions and ferromagnetism, resulting in the violation of scaling relations. The proposed new universality class includes diverse materials such as manganites, cobaltites, crystalline Fe–Pt and amorphous Fe–Mn alloys, and high-Tc superconductors. Unusual criticality in double-exchange systems is due to an unusual semiclassical nature of double-exchange ferromagnetism caused by real exchange, i.e., electron current through Mn3+–O–Mn4+ chains with the conservation of the spin rather than by virtual exchange as in a usual ferromagnet. Double-exchange ferromagnetism arises only because to freely itinerate, electrons orient the magnetic moments of Mn cations in a single direction.

Effects of Cr-doping on the Jahn-Teller, the orthorhombic to rhombohedral, and the magnetic transitions in LaMn1-xCrxO3 compounds

We report on the electrical resistivity, the specific heat, and differential thermal analysis of LaMn1-xCrxO3 (x = 0.00–0.35) polycrystalline compounds at T > 300 K, as well as on their magnetic properties in the temperature range from 80 K to 280 K. The transport and the thermal results show that the isovalent substitution of Mn3+ by Cr3+ decreases the cooperative Jahn-Teller distortion and consequently affects orbital ordering in the whole concentration range. It is found that Cr drastically reduces the entropy change associated to the Jahn-Teller transition to negligible values for x ≥ 0.20. The orthorhombic to rhombohedral transition is also displaced to lower temperatures by Cr-doping, however exhibiting non-zero associated enthalpy changes for all x. A ferromagnetic contribution develops in orbitally disordered alloys with x ≥ 0.20. The paramagnetic to ferromagnetic transition temperatures show a similar variation upon increasing x as for the LaMn1-xGaxO3 system, implying a common origin for the ferromagnetic interaction in both systems.

Jahn-Teller and geometric frustration effects on the structural and magnetic ground states of substituted spinels (Ni,A)Cr2O4 (A = Mn/Cu)

Comparative study of the effects of substitution (25%) of Jahn-Teller (JT) active Cu2+ and non JT active Mn2+ cations, at the Ni2+ site, on the geometric frustration (GF) and cooperative JT distortion (CJTD) in multiferroic NiCr2O4 has been carried out with the help of low-temperature magnetization, x-ray diffraction and time of flight (TOF) neutron powder diffraction (NPD) measurements. On Mn2+ substitution the CJTD, which causes elongation of the NiO4 tetrahedra in I41/amd phase of NiCr2O4, gets suppressed at room-temperature (RT) and stabilizes the Fd-3m phase in (Ni0.75Mn0.25)Cr2O4. On the contrary the Cu2+ substitution inverts the effect of CJTD and the tetrahedra get flattened at RT, which in turn oppositely distorts the I41/amd lattice in Ni0.75Cu0.25Cr2O4. The frustration index decreases on substitution. Intriguingly, the tetragonal to orthorhombic phase transition and the antiferromagnetic ordering, which are clearly evident in the pristine NiCr2O4 and the Cu2+ substituted sample respectively at 30 K and 70 K, have not been observed for the Mn2+ substituted sample. This has been attributed to the change in Cr3+- Cr3+ direct magnetic-interaction on structural transition. However, the long-range ferrimagnetic order remains intact for all the three compositions i.e. NiCr2O4, Ni0.75Mn0.25Cr2O4 and Ni0.75Cu0.25Cr2O4 occurring at 75 K, 70 K and 95 K, respectively. The Mn2+ substituted compound shows short-range magnetic correlation at low-temperatures, as evident from the diffuse and broad hump in the intensity-vs-d spacing plot of TOF NPD data. This short-range magnetic order has been attributed to the known spiral magnetic order.

Unraveling the Critical Role of Ti Substitution in P2-NaxLiyMn1–yO2 Cathodes for Highly Reversible Oxygen Redox Chemistry

Monovalent Li-substitution has been proven to be an effective strategy to resolve the pivotal problems confronted with P2-type layered Mn oxides, such as cooperative Jahn–Teller distortions of Mn3+...