Layered Manganates Research Articles

Effect and fate of Ni during aging and thermal-induced phyllomanganate-to-tectomanganate transformation

Phyllomanganates are ubiquitous in a variety of environments and commonly enriched in transition metal elements, such as Ni. The effect of such foreign metal cations on phyllomanganate transformation is widely documented under aqueous conditions together with the induced modification of Ni geochemical behavior. A similar knowledge is lacking however on phyllomanganate transformation and on the induced fate of associated metal elements that may occur under dry conditions, that prevail in deserts and arid areas increasingly exposed to severe droughts or wildfires. The present study shows that crystallinity, morphology, Mn oxidation state, and Ni binding mechanisms are essentially unaffected when aging hexagonal birnessite (Mn oxidation state ∼ 3.90 and Ni/Mn molar ratios of 0.00 and 0.13) in the dry state at room temperature for up to 8 years. In contrast, heating aged Ni-doped birnessite to 25–200 °C results in an increased proportion of edge-sharing Ni-Ni(Mn) pairs with increasing temperature induced by the migration of interlayer Ni to birnessite octahedral layers and/or by an increased sharing of coordination oxygens by interlayer Ni/Mn from adjacent layers. Further heating to 400 °C does not change this proportion, with birnessite layer structure being retained. Transformation of Ni-doped birnessite to cryptomelane is complete at 500 °C, while that of Ni-free birnessite is achieved at 400 °C, suggesting that Ni doping increases birnessite thermal stability. Birnessite-to-cryptomelane transformation comes with a strong increase of Mn oxidation state, whereas this parameter remains unchanged in heated birnessite samples. Ni incorporation in the cryptomelane framework, reduces its release during reductive acid dissolution by a factor of 396 ± 15 compared to initial birnessite. These results shed light on mineral transformation affecting layered manganates under dry conditions and on the fate of associated transition metal elements.

Layered manganates from soft-templates: preparation, characterization and enhanced dye demethylation capabilities

A soft-template method for the synthesis of layered manganate (KxMnO2, birnessites) compounds with a relatively good control at the nanoscale regime on the number of octahedral layers (thickness) is described. The soft-template method is based on the stabilization of reverse micelles at moderate temperatures and the optimization of parameters to modulate the size of the templates while still preserving their stability during the formation of layered manganates. Degradation experiments of a model system (methylene blue) using the birnessites here reported have shown efficient demethylation in the absence of oxidants for aqueous suspensions made of non-dried birnessites containing the fewest number of layers. The observed strong thickness and aggregation dependence (i.e. dependence on exposed surfaces) for demethylation explains why particulate birnessites have only been considered for dye sequestration and not as an efficient demethylation agent in the absence of oxidants. Finally, when compared to electrodeposited thin films, the higher loading capabilities of the colloidal suspensions here prepared have made it possible to demethylate larger amounts of methylene blue at a similar time.

Kinetic analysis of cation exchange in birnessite using time-resolved synchrotron X-ray diffraction

In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates of cation exchange, R (Å 3 min −1), were determined from changes in unit-cell volume to follow these rate laws: R = 1.75[ K + ( aq ) ] 0.56, R = 41.1[ Cs + ( aq ) ] 1.10, R = 1.15[ Ba 2 + ( aq ) ] 0.50. Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs ≫ K > Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.

Preparation of protonic layered manganates and their intercalation behavior

The Na + extraction of α-NaMnO 2 upon acid leaching and subsequent intercalation of hexylamine were examined. The acid treatments using 0.01–1 M HCl solutions produced a protonic layered manganate H x MnO 2· yH 2O ( x=0.1–0.3, y∼0.6). The oxidation state of Mn was dependent on the HCl concentration. The change in oxidation state was attributed to the competition of redox extraction and ion-exchange reaction of Na ions: the redox extraction involved disproportionation, 2 Mn III(s)→Mn IV(s)+Mn 2+(aq). The disproportionation reaction deteriorated the crystallinity of a protonic form of α-NaMnO 2. On the other hand, the slow acid treatment and the pre-oxidized starting materials remarkably improved crystallinity of the protonic products of α-NaMnO 2. These protonic manganates underwent intercalation with hexylamine, inducing interlayer expansion to 1.8 or 2.1 nm. Their intercalation compounds showed that crystallinity was dependent on their original protonic phases.

Staged fluorine insertion into manganese oxides with Ruddlesden–Popper structures: LaSrMnO4F and La1.2Sr1.8Mn2O7F

The synthesis, structures and magnetic properties of two fluorine-intercalated layered manganates, LaSrMnO4F and La1.2Sr1.8Mn2O7F, are reported. Rietveld structure refinement based on time of flight neutron powder diffraction data indicates that both compounds are tetragonal (space group P4/nmm) with cell parameters a = 3.77696(7) Å, c = 14.1026(8) Å (LaSrMnO4F) and a = 3.8103(1) Å, c = 21.7220(2) Å (La1.2Sr1.8Mn2O7F). Fluorine occupies interstitial sites in only one of the two rock salt layers available in the unit cell, leading to novel staged intercalation structures. The interstitial fluorine increases the spacing between the two adjacent rock salt layers and thereby causes a significant reduction in the Mn–O apical bonds directed towards the intercalated regions. The Mn valences of both compounds are close to 4+, and no long range magnetic order was observed in the title compounds down to 5 K.

Comment on `The metal-insulator transition and ferromagnetism in theelectron-doped layered manganates La2.3-xYxCa0.7Mn2O7(x = 0.0,0.3,0.5)'

We show that the report of Raychaudhuri et al (1998 J. Phys.: Condens. Matter 10 L191), that the samples of nominal compositionsLa2.3-xYxCa0.7Mn2O7 (x = 0.0,0.3,0.5) are electron doped layered manganates,is incorrect. Rietveld profile refinement of the x-ray diffraction data of thesample La2.3Ca0.7Mn2O7 (x = 0.0) synthesized by us following the samesynthesis process as reported by Raychaudhuri et al clearly shows that itforms a multiphase mixture comprising hole doped perovskite manganates andlanthanum oxide.

Properties of the electron-doped layered manganates La2-2xCa1+2xMn2O7 (0.6~x~1.0)

The ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Ca}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(0.6<~x<~1.0)$ series has been successfully synthesized using a citrate gel technique in order to study their structure and properties as a function of x in the ${\mathrm{Mn}}^{4+}$-rich region of the phase diagram. Rietveld refinement of powder x-ray diffraction data, combined with electron microscopy, shows that the phases are of high purity and adopt the $n=2$ Ruddlesden-Popper structure comprised of perovskite bilayers separated by rocksalt layers. The lattice parameter c reaches a minimum at $x\ensuremath{\sim}0.7$ and the increase with x is attributed to elongation of the apical Mn-O bonds due to preferential A-site occupancy and/or splitting of the ${t}_{2g}$ levels of ${\mathrm{Mn}}^{4+}.$ X-ray absorption spectroscopy measurements confirm that the Mn-valence variation in these series tracks formal valence expectations. In the region $0.6<~x<~0.8$ the magnetic susceptibility manifests a peak or shoulder at \ensuremath{\sim}280 K due to charge and orbital ordering, and antiferromagnetic order develops at lower temperatures ${(T}_{N}\ensuremath{\sim}150--200\mathrm{K})$ with quasi-two-dimensional antiferromagnetic fluctuation effects being evidenced above ${T}_{N}.$ The magnetic properties change significantly at $0.825<~x<~1.0:$ at higher temperatures two-dimensional magnetic coupling is observed and at \ensuremath{\sim}115 K the system spontaneously orders antiferromagnetically, but with a ferromagnetic moment. The transport results indicate insulating behavior at all compositions, but with an enhanced localization upon charge/orbital ordering in the $x=0.7$ material.

Cr doping in the La1.2Sr1.8Mn2O7 system

The effect of doping Cr in the Mn site of the La1.2Sr1.8Mn2O7 system has been studied. Addition of Cr modifies the transport and magnetic properties of the parent phase. With increasing Cr, the insulator-metal transition observed in the parent phase is suppressed and insulating behaviour is induced. In the range of doping studied (25%), the compositions show ferromagnetic behaviour with the Curie temperature decreasing with increasing Cr. The unit cell volume also shows a decrease. However, the magnetoresistance ratio is not significantly affected. We compare these results with an earlier study of doping Cr in the three-dimensional LaMnO3 structure, where it was seen that the ferromagnetic and magnetoresistance characteristics are sensitive to Cr doping. The results of the present study suggest that the layered manganates are more accommodative to doping compared to the three-dimensional perovskites.

Charge-Ordering in Manganates

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln1-xAxMnO3 (Ln = rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulating properties. This phenomenon is to be contrasted with the ferromagnetic metallic behavior that occurs when double-exchange between the Mn3+ and Mn4+ ions predominates. Two distinct types of charge-ordering can be delineated. In one, a ferromagnetic metallic (FMM) state transforms to the charge-ordered (CO) state on cooling. In the other scenario, the CO state is found in the paramagnetic ground state and there is no ferromagnetism down to the lowest temperatures. Magnetic fields transform the CO state to the FMM state, when the average radius of the A-site cations is sufficiently large (〈rA〉 > 1.17 Å). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also found only when 〈rA〉 ≳ 1.17 Å. The effect of the size of the A-cations on the Mn−O−Mn angle is not enough to explain the observed variations of the charge-ordering temperature as well as the ferromagnetic Curie temperature Tc. An explanation based on a competition between the Mn and A-cation orbitals for σ-bonding with the oxygen ρσ orbitals is considered to account for the large changes in Tc and hence the true bandwidth, with 〈rA〉. Effects of radiation, electric field, and other factors on the CO state are discussed along with charge-ordering in other manganate systems. Complex phase transitions, accompanied by changes in electronic and magnetic properties, occur in manganates with critical values of 〈rA〉 or bandwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-δ.

Pressure dependence of resistivity and magnetoresistivity in layered manganates(La,Sr)3Mn2O7

Preliminary results on the pressure dependence of resistivity and magnetoresistivity in layered manganates La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x=0.3, 0.4, 0.5) are reported. Layered manganates show remarkable differences in their electron transport behavior under pressure due to the inherent structural as well as electronic and magnetic anisotropy compared to the isotropic perovskite counterparts. The pressure-induced effects and coupled magnetic field and pressure effects bring forth subtle features which enable us to distinguish the nature of electronic transitions in these layered manganates. {copyright} {ital 1998} {ital The American Physical Society}

Electrochemical lithium insertion into layered manganates

Various dehydrated phyllomanganates A xMnO ≈2 containing A = Na, K or NH 4 as interlayer cations A were intercalated with lithium using cells A xMnO ≈2-graphite |LiclO 4-PC|Li. Potentiostatic discharge at low scanning speeds showed two main reaction peaks at ca. 3V and below 2 V. The first one corresponds to reduction to a disordered Mn 3+ phase ( Δx ≈ 0.6) and was found reversible for A = Na and K. The capacity of sodium phyllomanganate at the third cycle is 188 mAh/kg, i.e. ca. 560 Wh/kg. No marked difference was found as a function of A or of the degree of order of the starting compound.

Influence of water on structure and ionic conduction of phyllomanganates(IV)

The variation of cell parameters and ac conductivity with hygrometry and temperature were measured on powders of the layered manganates Na 4Mn 14O 27,zH 2O (I), KMn 5O 10,zH 2O (II) and Mn 14O 25,zH 2O (III). They all lose quantitatively water below ≈120°C (reversibly for I and II) with lattice contraction in the direction normal to the layers. DTG shows that samples kept in air contain adsorbed and structural (interlayer) water. The conductivity is strongly dependent on the water partial pressure below 80°C; the highest σ value at 25°C is 1.2 10 −4ohms −1cm −1 for II in 78% humidity. Above 100°C, the conductivity is probably electronic with activation energies 0.32−0.40 eV, in agreement with their mixed-valence character. III (prepared by acid treatment of I) is no better ionic conductor than I, confirming structural models showing that Na cations are not replaced by H 3O + in III.