The structural, magnetic and transport phase diagrams of the manganese perovskites (general formula: A1-xA'xMnO3) are characterised by a variety of exotic phenomena, including high-temperature polaronic behaviour, charge, orbital and magnetic ordering and colossal magnetoresistance (CMR). These properties can be “tuned” by changing the doping level x, the electronic bandwidth (through the average A-site ionic radius, <rA>), and the A-site disorder, and are believed to be a manifestation of the underlying competition between electron-lattice coupling and double exchange. Usually, at low temperatures, one of these two interactions is dominant, resulting in a homogeneous ground state, which is either a metallic ferromagnet or a charge-ordered insulator. We have recently found, however, that, for special points in the phase diagram (x ~ 0.3,,<rA> ~ 1.18 Å), the competition can be preserved down to low temperatures, resulting in an inhomogeneous ground state at the microscopic level. This unusual state is characterised by the coexistence of charge-ordered and metallic domains, which are intertwined over a variety of length-scales, and appear to show spin-glass-like dynamics. Upon application of an external field (magnetic field, pressure or even x-rays), the domains grow to macroscopic sizes, resulting in phase segregation. We speculate that the evolution of the local magnetic and crystal structures during this phase segregation process may parallel those occurring, at much higher temperatures, for compounds displaying CMR behaviour at the paramagnetic-toferromagnetic transition. Very recently, it has been suggested that the charge-ordered state, which is stable for higher values of the Mn oxidation state (x ≥ 0.5), may also be associated with modulated mesoscopic phase segregation, in the form of “stripes”. This hypothesis will be discussed in the light of recent x-ray synchrotron and neutron diffraction data on the crystallographic and magnetic modulation in La0.33Ca0.67MnO3

The electronic and magnetic properties of ATO3 and related perovskites (A is a mixture of lanthanide Ln3+ and alkaline earth M2+ cations; T is a transition metal) are very sensitive to the A site composition. The importance of doping effects controlled by the Ln3+/M2+ ratio is well-known, but the other lattice effects controlled by the sizes of these cations are less well understood. A simple approach making use of the mean (first moment) and the variance (second moment) in the A cation distribution has been applied to the metal-insulator transition temperature in colossal magnetoresistance AMnO3 perovskites and to the critical temperature in A2CuO4 and LnBa2Cu3O7-δ superconductors. Series of compositions prepared with a constant doping level and mean A cation radius show a linear decrease of the transition temperature Tt with the A cation size variance σ2. The rate of decrease -dTt/dσ2 is found to lie in the range 1,000-30,000 KÅ-2. The orthorhombic-tetra onal structural transition in the A2CuO4 materials is found to show a linear increase with σ2. A pair of quadratic relationships for the mean size and size variance effects are proposed to be the result of changing strain energies that give rise to these effects.

Substitutions in the pyrochlore-type oxide Tl2Mn2O7 have been carried out on both the Mn and the TI site, using high pressure synthesis at 4-6 GPa. We show that a continuous solid solution exists between Tl2Mn2O7, and Tl2Ru2O7, and that the replacement of a fraction of Mn by Ru dramatically changes the transport properties from metallic to semiconducting and induces magnetoresistance (MR) in this system. MRs up to 95% (vs. RO) have been observed. The Tl site, already known to accept In or Sc, can also be substituted by the larger cation Bi3+, which also induces a metal-insulator transition at low substitution level.

Orthorhombic perovskite-type (La0.1Ca0.9)(Mn1-xCox)O3 was synthesized in the range 0.00 ≤ x ≤ 0.08. The Rietveld analysis indicates that the (Mn, Co)-O(1 and 2) distances are independent of the composition (x). Measurements of the electrical resistivity (ρ) and the Seebeck coefficient (α) indicate that (La0.1Ca0.9)(Mn1-xCox)O3 is an n-type semiconductor at the low temperature. At the high temperature, (La0.1Ca0.9)(Mn1-xCox)O3 exhibits a metal-insulator transition in the range 0.0 ≤ x ≤ 0.04. The metal-insulator transition temperature (Tt) increases with increasing the Co3+ ion content, while dρ/dT in the metallic region decreases with increasing the Co3+ ion content. The variation of Ea and T+ is explained by the difference in the electronegativity between Mn and Co atoms. The variation of dρ/dT in the metallic region is explained by the increase in the collective o bond.

The effect of texture and orientation on the resistance (0T and 6T) and magnetoresistance (MR) of Lal-xCaxMnO3 (LCMO) and Ag2Te thin films on various substrates has been investigated. Similar features were found in the MR dependence of thin films of these compounds despite the fact that they belong to different types of MR materials. Epitaxial or strongly oriented films exhibit a peaked MR behavior while for polycrystalline films this curve does not have sharp features. Sign reversal effect of MR was found also in thin films of Ag2Te. MR changes from positive at transverse field-,current orientation to negative at parallel orientation.

The magnetic structures of Ca3LiRuO6 and Ca3NaRuO6 have been deduced from powder neutron diffraction data. In each case successive Ru5+ cations in the [001] chains of the Sr4PtO6-like structure are antiferromagnetically coupled, and nearest-neighbour Ru5+ cations in adjacent chains are also antiferromagnetically coupled.

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1-xCaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.

We have hydrothermally synthesized the first reduced titanium phosphates with open-framework structures. The synthetic approach is quite unusual since elemental titanium is used. The full structural, thermal, magnetic, and spectroscopic characterization of a 1,3-diaminopropanetemplated compound, TiIIITiIV(PO4)(HPO4)2(H2O)2·(NH2CH2CH2CH2NH2)0.5, is presented. The frame of the compound is of the octahedral/tetrahedral type with octahedrally coordinated TiIII and TiIV atoms. The template of 1,3-diaminopropane occupies channels with dimensions that are comparable to those of many zeolites. Magnetic measurements are consistent with one d-electron per formula unit. TGA/DTA and IR data will be reported as well. This compound is most likely the first member of a larger class of reduced titanium phosphates. Some preliminary data suggests that similar reduced titanium phosphates with different templates and structures exist as well. All compounds are blue-purple due to Ti(III).

The hydrothermal crystallisation (130-180 °C) of three-dimensional open-framework gallium and aluminium oxyfluoro-phosphates with the ULM-3 and ULM-4 structures have been studied in situ for the first time. The in situ energy-dispersive X-ray diffraction method has allowed the formation of the crystalline products to be observed under hydrothermal conditions The integrated areas of the strongest Bragg reflections has allowed quantitative kinetic data to be extracted. The effect of temperature, phosphorus source, templating agent have been investigated. The nature of phosphorus source in the reaction mixture has been found to affect dramatically the course of reaction for certain combinations of amine and temperature. Previously unobserved transient crystalline phases have been seen during the production of ULM-3 gallium phosphates when P2O5 or polyphosphoric acid are used. The formation of these intermediates affects the kinetics of product growth. In the case of the aluminium ULM-3 materials reaction always proceeds via a crystalline intermediate whatever phosphorus source is used. The ULM-4 framework materials are found to always crystallise directly with no evidence for any intermediates. Kinetic data for each system have been modelled using standard solid-state chemistry expressions, and these calculations indicate the reactions to be diffusion controlled.

The structure and extraframework cations of dehydrated zeolites (Ba,K)-G,L (Si/Al=1) and (Ba,Li)-G,L (Si/Al=1) are characterized by powder neutron diffraction. The results provide direct evidence of the similar structure directing roles which barium and potassium play in the syntheses of zeolites with the LTL topology.

Dynamic redox processes in different complex copper oxides and oxide / mesoporous silica composites are characterized. In the spinel-type CuxMn3.xO4 the atomic and electronic structures are described with respect to the reversible process Cu2+ + Mn3+ ⇒ Cu3+ + Mn4+, the crucial dynamic equilibrium for the catalytic oxidation of CO to CO2 at ambient temperatures. A comparison with the perovskite-type LaCu1-xMnxO3-δ is presented. In the K2NiF4-type La2-xSrxCu1-yRuyO4-δ the electronic structure is characterized with respect to the equilibrium Cu2+ + Ru5+ ⇒ Cu3+ + Ru4+. Analogous redox processes are characterized in composite compounds made up of copper oxides inserted in mesoporous MCM-type silica. Due to their structural order in the nanoscopic range, the investigation of these materials required multiple, complementary characterization methods, i.e. X-ray diffractometry, thermal analysis, scanning and transmission electron microscopy, magnetic measurements, XPS, and particularly X-ray absorption spectroscopy (EXAFS / XANES) measurements.

Using multiple cycles of wet impregnation, drying and calcination procedures it was possible to form haematite nanoparticles within the pore system of mesoporous MCM-48 silica phase. A decoration/coating of the inner surface of the silica walls was indicated by a reduction of the BET surface area, the pore radius as well as HRTEM and EDX investigations. Existence of small, slightly disordered iron (III) oxide nanoparticles was proved by X-ray absorption spectroscopic measurements.

We report on the synthesis of lamellar mesostructured aluminophosphate composites which contain variable relative amounts of aluminium oxide species in the core regions of the lamellae; this is investigated quantitatively by Al K-edge XANES spectroscopy. Templating is achieved by the utilisation of monododecyl phosphate surfactant. If no phosphorous source (such as H3PO4) is used for the synthesis, the phosphate head groups of the surfactant become incorporated into the inorganic network to form similar lamellar aluminophosphate structures. Thus, the surfactant serves as both template and reactant.

The “1:2” family of A2+(B2+1/3B+5+2/3)O3 (e.g. B2+ = Mg, Ni, Zn; B5+ = Nb, Ta) mixed metal perovskites typically adopt a 1:2 layered ordered structure when A = Ba and Sr, and a 1:1 ordered doubled perovskite structure for A = Pb. To understand the stability of the cation order in these systems and explore how the different cation correlations affect their dielectric and ferroelectric properties, the response of several members of this family of perovskites to a series of chemical substitutions and thermal treatments have been explored. In this paper we present results for a system where the divalent A cation is replaced by a 1:1 mixture of Na+ and La3+. At all temperatures this substitution stabilizes 1:1 B-site ordering. Below ~ 950°C an additional ordering reaction occurs on the A sub-lattice with Na and La occupying alternate layers along the c axis. The formation of different orientational variants of the A-site ordered phase produces a twinned nano-domain structure which in turn perturbs the length-scale, but not the symmetry, of the order on the B-site lattice. The presence of (Na1/2La1/2) on the A-site is apparently effective in destabilizing the 1:2 layering of the B-site cations observed in alkaline earth systems, possibly through unfavorable local charge imbalances.

Multistep topotactic routes are exploited to introduce mixed-valency into mixed-metal oxides at low temperatures (≤ 350°C). A new set of single- and triple-layered perovskites, Na1-x-yCax/2LaTiO4 and Na2-x+yCax/2La2Ti3O10, respectively, has been prepared by a combination of ion exchange and reductive intercalation. The single-layer series are metastable compounds. The magnetic and electronic behavior of the triple-layer titanate is consistent with Anderson localization effects, while those of the single-layer materials are more complex; samples demonstrate “Hurd-like” conductivity and an unusual magnetic response. The details of the synthesis and characterization of these materials are presented and their magnetic and electronic behavior discussed.

New perovskite-type oxide (Tl1/2Na1/2)TiO3 was synthesized by a cubic-anvil-type high pressure apparatus at 5.5 GPa and 1273 K for 30 sec. The perovskite-phase was found to have a tetragonal symmetry with space group P4/mmm. The lattice parameters a and c are 0.3811 nm and 0.3826 nm, respectively. An insulating skin covered the sample, but real resistivity is smaller and shows a metallic behavior.

Rate controlled sintering (RCS) was used to sinter nano-sized BaTiO3 particles. A distinct shift in temperature slope above 1150-1200°C during RCS implied a change in sintering mechanism, and a corresponding exaggerated grain growth (grain size ~40-50 μm). By suppression of the exaggerated grain growth by firing a specimen packed in BaCO3, these phenomena were shown to be the result of a shift in phase equilibria from the volatilization of BaO from BaTiO3; the eutectic liquid which formed permitted rapid grain growth by a dissolution and precipitation process.

The effect of MnO on the electrical properties of Nb-doped SrTiO3 was investigated to understand the electronic states of grains and grain boundaries. Grain size and dielectric constant were decreased with increasing Mn-doped content from 0 to 0.07 mol%. However, it was observed that the non-linearity coefficient of Nb-doped SrTiO3 was changed from 5 to 43 with Mn-doped content increasing from 0 to 0.03 mol%. Current-voltage (I-V) characteristics, capacitance-voltage (C-V) measurement, complex plane analysis and impedance spectroscopy were performed to obtain the information about the electrical properties of grains and grain boundaries. The potential barrier height increased and the donor concentration decreased with increasing Mn-doped content. This phenomena, that the electrical properties of Nb-doped SrTiO3 were changed with the addition of Mn, would be determined by the effect which Mn-2-3 substituted for Ti sites. It was observed that the breakdown voltage of single grain boundary was changed from 3.8 V to 8.9 V when measured by micro-electrode method.

Doping of calcium tantalate, Ca2Ta2O7, with small amounts (<5 mole%) of Ca2Nb2O7 results in a 50% increase in the dielectric constant, K (from 23 to 35 at 1 MHz), and a zero temperature coefficient of K is achieved at about 18 mole% of Ca2Nb2O7. This opens up the possibility of applications in microwave communication devices. Crystallographic information needed to understand the structure/property relationships in this system is lacking. We have prepared samples of calcium tantalates doped with different elements including Nb, Sm, Ti, Nd and Zr, and have structurally characterised them using X-ray diffraction and transmission electron microscopy. The diffraction patterns of the tantalates all display strong subcell reflections characteristic of the trigonal weberite structure together with complex distributions of superlattice reflections that depend on the nature and concentration of the dopant and on the reaction temperature. The diffraction patterns were indexed using related trigonal or pseudo-trigonal unit cells with a ~7.3 Å, c ~ n × 6 Å. Polytypes with n = 3, 4, 5, 6 and 7 were prepared as single crystals by the flux method. Single crystal X-ray data was used in conjunction with high resolution electron microscope images to solve the structures. Details are given for a 6M polytype (M=monoclinic). A characteristic structural feature of the polytypes is a fluorite-type cation array which is periodically twinned on (111)f to give mixed cubic-hexagonal stacking of the cation layers. The structure relations with the pyrochlore and weberite structures are discussed.

A new perovskite was recovered from the high pressure-high temperature treatment of the α-TlSbO3 form of Na2SnTeO6 at 7 GPa and 950 °C for 30 minutes. Synchrotron x-ray powder diffraction data show the space group is P21/n with a=5.40361 (5), b=5.46152(5), c=7.69288(7) Å and ß=90.034(3)°. Using disk samples of both polymorphs, the dielectric properties were measured as a function of temperature. At ambient conditions, the perovskite form has a more than 1.5 fold enhancement in dielectric constant compared to the α-TlSbO3 form while the molar volume and the molecular polarizability decrease.