NEW TRANSITION METAL OXIDES BY TOPOTACTIC ION EXCHANGE AND THEIR MAGNETIC PROPERTIES

Abstract

Transition metal oxides continue to attract research attention owing to their various novel and exciting physical and chemical properties. The oxides may adopt structures ranging from rock salt, spinel to perovskites and its variants. Although a large variety of transition metal oxides are reported in the literature, a lot of possibility remains to be explored with new chemical compositions and structures that are not achievable by conventional high temperature synthetic route. Although, the routine solid-state synthesis is very convenient and widely employed to synthesize many functional oxides, but the range of compositions accessible are limited by the competition between thermodynamic and kinetic factors. The key limiting factor in solid-state synthesis is often the requirement for diffusion of individual atoms/ions that necessitates high temperature reaction condition and thus produces thermodynamically stable products or phases. On the other hand, low temperature syntheses methods enable one to access the so called metastable phases, while retaining part or all of the structural and bonding features of the precursor molecules/motifs. In this way, a thermodynamically stable precursor, typically made by a high temperature solid-state reaction, can be converted into a related metastable structure, which is not accessible by direct synthesis, through soft-chemical methods. These kinds of reactions, initially developed by French chemists, were called “chimie douce”, most commonly translated as “soft chemistry” to signify the gentle conversion of one structure into another with part or complete retention of chemical bonding and structural features of the precursor. Soft-chemical reactions have since been widely explored for the synthesis of layered oxides and include ion exchange, intercalation, reductive deintercalation, layer expansion, grafting, exfoliation, pillaring, and substitution reactions. The utilization of topotactic reactions has increased in recent years allowing access to a series of new compounds that are structurally varied and can exhibit significantly different properties, including those of fundamental and technological interest. Topochemical manipulations include a number of methods, such as, substitution or ion exchange, intercalation and deintercalation, to name a few. The ion exchange simply involves the substitution or replacement of one type of cations or anions by another. While most of these types of reactions involve a one-to-one exchange of monovalent ions, in some instances aliovalent species may likewise take part in exchange (e.g. one divalent cation replacing two monovalent ii cations) and in rare cases the co-exchange of cation/ anion combinations can also occur. Another topochemical method, namely intercalation occurs when species are inserted into a host compound. These reactions can involve reductive or oxidative processes that result in either the introduction of cationic or anionic species, respectively, or the insertion of neutral molecules (e.g. H2O). In the deintercalation process, cations or anions can be removed by oxidative or reductive means and these can be executed either chemically or electrochemically. Ion exchange is one of the most popular soft-chemistry routes in preparing novel metastable phases of layered oxides and utilizes general chemistry principles of charge density and acid-base chemistry. Generally, an ion exchange reaction refers to a class of chemical reactions between two materials that involve an exchange of one or more ionic components. Certain inorganic crystalline materials can react with aqueous salt solutions to selectively remove one ionic component and replace it with other ions from the solution with the end result being new materials. The ion exchange process, based on topotactic reactions can result in a material crystallographically analogous to the parent phase but with a new chemical composition. Thus, it is a simple and versatile method for exploring new compositions of materials having a certain crystal framework and is used widely in various fields. Ion exchange has been mostly limited to the layered oxides with the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) compounds, such as, NaLaTiO4, RbLaNb2O7, RbLa2Ti2NbO10, RbSrNb2O6F, K2SrTa2O7 and Rb2La2Ti3O10. The DJ phases were the first reported examples of ion exchangeable layered perovskites, and the initial ion exchange reactions involved the replacement of larger interlayer cations such as Cs+, Rb+, and K+ with smaller cations, such as Na+, Li+, NH4+, and Tl+ using molten nitrate salts (Tm ~ 300 ºC) as the ion exchange medium. DJ phases containing small interlayer cations, such as, Li+ or Na+ are often difficult to synthesize as phase-pure materials at high reaction temperatures (> 1000 ºC) where the three dimensional perovskite phases are usually more stable. RP phases also undergo similar ion exchange reactions. Like the DJ phases, the interlayer cations of RP phases (typically Na+, K+, and sometimes Rb+) can be replaced by smaller alkali cations, such as, Na+, Li+, NH4+, and Ag+ using molten salt ion exchange reactions. RP phases are especially amenable to divalent ion exchange because two interlayer monovalent alkali cations can be replaced with one divalent cation to transform a RP phase into a DJ phase. Hyeon et al. reported MIILa2Ti3O10 (M = Co, Cu, Zn) by exchanging 2Na+ iii for M2+ using molten nitrates, chlorides, or a eutectic mixture. Mallouk et al. used aqueous ion exchange to form AIIEu2Ti3O10 (A = Ca, Sr) and MIIEu2Ti3O10 (M = Ni, Cu, Zn). Gopalakrishnan et al. used a higher temperature metathesis reaction to form AIILa2Ti3O10 (A = Sr, Ba, Pb). Additionally, protons can also replace the interlayer alkali cations, which opens up the interlayer gallery to subsequent acid/base reactions including the intercalation of long chain amines. Wiley et al. have described an extension of ion exchange reactions in the triple-layered RP series to obtain Li0.3Ni0.85La2Ti3O10 in aqueous solution. While the topotactic ion exchange is common in oxides with the layered perovskite or in other tunnel and framework structures, but are rare in the 3D close packed structures based on all edge and/or corner shared polyhedra. An interesting type of exchange, namely, ‘intra-site exchange’, where perovskite A-site cations were exchanged with the interlayer A-cations within a compound, was demonstrated in a layered perovskite. A cubooctahedral A-site ion exchange in NaTaO3 is the only report involving a three-dimensional perovskite structure with all octahedral corner connectivity. In the present investigation, topotactic ion exchange have been explored in three-dimensional structures based purely on corner/edge connected octahedral network in addition to a ribbon type and a framework titanate, also based on edge/corner shared TiO6 octahedra. The study has enabled transformation of a number of non-magnetic oxides into magnetic ones in addition to altering its optical property to a significant extent. Chapter 1 gives a brief outline of ion exchange and other soft-chemistry reactions in layered and other oxides, in addition to various other soft-chemical manipulations that these compounds undergo. Syntheses of precursor oxides, their ion exchange, characterization and properties of the resulting compounds are described in subsequent chapters. Chapter 2 describes the synthetic methodologies and the details of all the characterization techniques used in the present study. The precursor oxides were synthesized by solid-state reactions starting from binary metal oxides and carbonates. The ion exchange reactions were carried out in aqueous metal nitrate or metal chloride solutions. The progress of the reaction and formation of final products were monitored by powder X-ray diffraction (P-XRD) and elemental analysis by Energy Dispersive X-ray Spectroscopy (EDX). Final morphological and compositional characterizations were carried out by Field Emission-Scanning Electron Microscopy (FE-SEM), Transmission-Electron Microcopy (TEM) and EDX analysis. Further, iv the optical properties were studied by UV-vis diffuse reflectance spectroscopy (UV-vis DRS). Solid-state NMR technique was used as an additional probe to estimate the amount of exchanged Li by comparing the integrated intensities of the parent and its ion exchanged counterpart. X-Ray Photoelectron Spectroscopy (XPS) was used to ascertain the oxidation states of the redox active metals in the compounds. Magnetic measurements of the synthesized compounds were carried out by SQUID magnetometer and VSM. The details of P-XRD, FE-SEM, EDX, TEM, UV-vis DRS, ssNMR, XPS, SQUID/VSM instruments and characterization techniques are discussed in this chapter. Chapter 3 describes divalent iron-exchange in a three-dimensional close-packed trirutile oxide with all octahedral coordination. For the first time, the transition metal ion exchange in α-LiNbWO6 and characterization of the resulting trirutile oxide is reported here. The ion exchange with FeII has been achieved by refluxing α-LiNbWO6 in an aqueous solution of FeCl2.4H2O under an argon atmosphere at 60 ºC for 4 days. The close resemblance of the P-XRD pattern of the ion exchanged sample with that of the parent substantiated the topotactic nature of the exchange. An ICP-OES analysis of the Fe-exchanged LiNbWO6 revealed the exchange of 90% of the Li, leaving behind 10% Li in the sample. Considering 90% exchange of Li, the ion-exchanged compound is formulated as Li0.1Fe0.45NbWO6. The FE-SEM analysis indicated retention of the particle morphology upon exchange and the EDX data confirmed the elemental ratio for Fe, Nb, and W in corroboration with the ICP-OES derived composition. The lattice fringes with 0.932 nm separation in the HR-TEM image conformed to the quasi-ordered trirutile phase akin to the periodicity of the (Li/Fe)O6 octahedron connected to an NbO6–WO6 bioctahedral unit along the c direction. The corresponding SAED pattern is consistent with the tetragonal crystal system and in agreement with the P-XRD data. While the 7Li MAS NMR spectra of LiNbWO6 clearly revealed the presence of only one type of Li site, the presence of a 7Li peak in the Fe exchanged sample with the same chemical shift as that of the parent is supportive of the retention of a small amount of Li without any change to its coordination environment. The XPS data indicated Fe2p binding energies of 723.0 and 709.7 eV for spin–orbit coupled Fe2p1/2 and Fe2p3/2 states, respectively, corresponding largely to the presence of Fe2+ in Li0.1Fe0.45NbWO6. As evidenced in the UV–vis DRS, Fe exchanged has clearly resulted in extension of the optical absorption edge deep into the visible region as compared to its parent, only a near-UV absorber. This amounts to a significant reduction of the indirect band gap of Li0.1Fe0.45NbWO6 to 1.71 eV from 3.01 eV of v the parent, as estimated from the Tauc plots. The magnetic susceptibility data below 200 K indicates Curie-like paramagnetism, which arises due to the disorder of Fe at the 2c sites and the presence of random vacancies that keeps the Fe2+ sites isolated from other neighboring Fe2+ spins in the lattice. The paramagnetic moment corroborates well with the spin-only moment for high-spin Fe2+. Chapter 4 deals with the synthesis, characterization and magnetic properties of a new tri- α-PbO2 type oxide, Li0.08Fe0.46SbWO6. The compound is synthesized for the first time by ion exchange reaction of LiSbWO6 at 60 ºC for 4 days under an argon atmosphere. The phase purity of the resulting compound and the topotactic nature of exchange are ascertained by P-XRD analysis. The morphological homogeneity upon ion exchange and the elemental composition are determined using FE-SEM and EDX studies. The UV-vis DRS data of the iron-antimony tungstate also showed a considerable reduction of the visible band gap to 2.06 eV from ~ 3.02 eV of the parent lithium-antimony tungstate. The reduction in the band gap is attributed to the formation of valence band (VB) states, primarily constituted by the overlap of Fe3d and O2p orbitals, which would be situated at higher energies in Li0.08Fe0.46SbWO6 as compared to the VB states of the parent LiSbWO6. This upshift of VB edge and a consequent decrease in the band gap is ascribed to the extended optical absorption of Li0.08Fe0.46SbWO6. The Fe2p binding energies of 723.2 and 709.6 eV confirmed the presence of Fe2+. The observed lattice fringes in the HR-TEM image with a separation of 0.495 nm is in agreement with the periodicity in the c-direction. The SAED pattern is consistent with the orthorhombic crystal system and the indexed spots are in agreement with the P-XRD data. The ZFC magnetization data show antiferromagnetic transition at ~ 20 K. Unlike Li0.1Fe0.45NbWO6, where isolated Fe2+ are present, Fe(II) dimers can exist in Li0.08Fe0.46SbWO6 due to the presence of edge-shared chains of LiO6 octahedra running along the c-direction in the tri-α-PbO2 structure. This probably is responsible for the low-temperature antiferromagnetic transition in the compound. The high temperature paramagnetic moment, however, matches well with the spin-only moment of high spin Fe2+. In Chapter 5, the synthesis, characterization and magnetic properties of transition metal exchanged ribbon-type layered titanates, Na2(1-x)MxTi3O7 (M = Mn, Fe, Co and Ni), are reported. The compounds are synthesized by ion exchange of Na2Ti3O7 at 60 ºC for 48 hours. The close similarity of the P-XRD patterns of the resulting compounds to that with the parent and a color vi change over from white to orange-brown/brown/grey-green/light green confirmed the exchange of transition metals in a topotactic fashion. The morphological homogeneity and elemental composition is established with FE-SEM and EDX studies for all the compounds. The HR-TEM showed clear lattice fringes with interfringe spacing of 0.390 nm, which is consistent with the b-parameter of the monoclinic unit cell. The SAED pattern is also indexable in the monoclinic space group and consistent with the P-XRD data. UV-vis DRS study confirmed the presence of visible band gaps for all the compounds ranging from 1.99 - 2.86 eV. The Mn compound, Na0.02Mn0.99Ti3O7, shows antiferromagnetic transition at 32 K, while the rise in susceptibility at ~ 8 K is ascribed to a ferrimagnetic correlation between the Mn2+ and Mn3+ spins that may arise due to spin canting. Similar antiferromagnetic transition is also observed in the Ni compound but the low temperature feature is absent. The M-H data for Mn and Ni at low temperatures (5 and 30 K for Mn and 5 K for Ni) showed hysteretic behavior which is a signature of ferrimagnetism possibly arising out of the spin canting. On the contrary, the corresponding Fe and Co compounds showed paramagnetic behaviors throughout the entire temperature range (5 – 300 K). Chapter 6 describes the synthesis, characterization and magnetic properties of a new framework titanate, Na0.02Fe0.99Ti6O13. The compound is prepared by ion exchange reaction between Na2Ti6O13 and FeCl2. 4H2O in aqueous medium at 60 ºC under continuous stirring for 4 days. The P-XRD pattern indicated that the ion-exchanged product retained the layered framework structure of the parent. The morphology and elemental ratios were characterized by FE-SEM and EDX analysis, respectively. As prepared Na2Ti6O13 and the ion-exchanged compound showed plate-like morphology, often seen and expected in layered type oxides. The elemental ratios obtained from EDX analysis, both on spot and area basis are in good agreement with the nominal compositions. UV-vis DRS data revealed Na0.02Fe0.99Ti6O13 as a visible light active semiconductor with a band gap of 2.05 eV. The magnetization data indicates paramagnetic character of the compound. Chapter 7 provides overall conclusions and future prospects of the present investigation. Ion exchange method have been exploited to synthesize a series of metastable transition metal oxides with close-packed three-dimensional trirutile and tri α-PbO2 type structures, in addition to a ribbon-type and a framework layered structure. These results reveal that simple topotactic reactions can be applied for the construction of metal-anion arrays for a number of transition vii metals and structures. The effectiveness of this approach is noteworthy as none of them could be prepared by a direct solid-state reaction method. It is expected that the continued development of such synthetic strategies will eventually result in a general set of methodologies for the conscious design and preparation of intricate structural arrangements.