Influence of Alkaline-Earth-Metal Substitutions on the Bismuth Ruthenate Structure: Bi2-xA'xRu2O6O'1-y (A' = Mg, Ca, Sr).

Solid solutions with the formula of Bi2-xA'xRu2O7-y (A' = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) have been synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group Fd3̅m with complex A/A' cation coordination environments. The Bi cation is found to be off-center from the ideal position because of a lone-pair distortion, while the positions of the substituted A' cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 Å. Interestingly, this is one of only two known pyrochlores with Mg2+ located in an 8-coordinated site. The average Ru oxidation state for each substitution is found to increase, and charge compensates for the lower divalent A' substitution. The solid solutions show temperature-independent resistance across the series with small changes in magnitude that scale with the amount of substitution, while displaying Pauli paramagnetic behavior throughout.

Os4+ Instability in the Pyrochlore Structure: Tl2-xBixOs2O7-y.

Osmium-containing oxides are rare due to the difficulty in stabilizing complex structures with a fixed stoichiometry and metastability of the phases. Bismuth-substituted thallium osmate pyrochlore samples, Tl2-xBixOs2O7-y, were synthesized using solid-state reactions where the solubility limit was found to be approximately x = 1.4. Members of this solid solution were characterized by their structural, electronic, magnetic, and thermal properties to understand the influence of Bi3+ substitution on the ground state. The Os-containing pyrochlores crystallize in the ideal cubic pyrochlore structure (Fd3m), and the lattice parameter a was found to slightly increase as a function of Bi content. A possible interplay between structure and cation valence states was explored using both neutron powder diffraction and X-ray absorption spectroscopy, suggesting that a combination of Os4+/Os5+ and Tl1+/Tl3+ mixed valency throughout the solid solution allows for the stabilization of the pyrochlore structure. The system is metallic for the entire solid solution and predominantly exhibits temperature-independent paramagnetism. Specific heat measurements show an enhanced Sommerfeld coefficient, a possible flat-band signature. This system gave insight into the bonding preferences of Os, indicating a dependence on high oxidation states and mixed valence for the stability of complex structures.

Hibonite Blue: A New Class of Intense Inorganic Blue Colorants.

Commercially available spinel cobalt blue (CoAl2O4) utilizes a significant amount of carcinogenic Co2+, which makes its synthesis more hazardous and environmentally harmful. Considerable effort has been put into developing more environmentally benign and robust blue pigments to replace cobalt blue. A new class of blue pigments with tunable hue were prepared. The solid solution series, CaAl12-2x Co x Ti x O19 (0 < x ≤ 1), crystallizes in a hexagonal mineral hibonite (CaM12O19) structure with five distinct crystallographic sites for M cations (M = Al, Co, and Ti). The origin of intense blue color is attributed to a synergistic effect of allowed d-d transitions involving the chromophore Co2+ in both tetrahedral and trigonal bipyramidal crystal fields. Compared with commercial cobalt blue, these tunable hibonite blues possess a reddish hue that intensifies the blue color as observed in Y(In,Mn)O3 (YInMn) blues, with a significant reduction of Co2+ concentration from 33% to as low as 4% by mass. A significant advantage of hibonite blues over cobalt blue is the substantial reduction in carcinogenic cobalt content while enhancing the color properties at a reduced cost for raw materials.

Local Moment Instability of Os in Honeycomb Li2.15Os0.85O3.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, ß = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 µB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)-Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

Bi2-xCaxIr2O6+y Pyrochlore Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3.

Pyrochlore phases Bi2-xCaxIr2O6Oy' with x from 0.0 to 1.0 have been evaluated based on Rietveld analysis of neutron diffraction data, electrical resistivity and thermopower data from 3 to 756 K, and magnetic susceptibility data from 3 to 298 K. The average Ir oxidation state is less than 4+ at low x, above 4+ for high x, and is very close to 4+ at x = 0.5. All samples show metallic properties with an electrical resistivity of ∼10-3 Ω·cm at room temperature. For low x, the sign of the Seebeck coefficient is negative at low temperature but becomes positive at high temperature. For high x, the sign of the Seebeck coefficient is positive from 3 to 756 K. Magnetic measurements indicate no magnetic ordering down to 3 K for all values of x. All Bi is in its ideal position for all values of x, but much of the Ca is strongly displaced from the ideal A cation site. This displacement of Ca apparently only occurs when there is an adjacent vacancy at the O' site.

From Serendipity to Rational Design: Tuning the Blue Trigonal Bipyramidal Mn3+ Chromophore to Violet and Purple through Application of Chemical Pressure.

We recently reported that an allowed d-d transition of trigonal bipyramidal (TBP) Mn3+ is responsible for the bright blue color in the YIn1-xMnxO3 solid solution. The crystal field splitting between a'(dz2) and e'(dx2-y2, dxy) energy levels is very sensitive to the apical Mn-O distance. We therefore applied chemical pressure to compress the apical Mn-O distance in YIn1-xMnxO3, move the allowed d-d transition to higher energy, and thereby tune the color from blue to violet/purple. This was accomplished by substituting smaller cations such as Ti4+/Zn2+ and Al3+ onto the TBP In/Mn site, which yielded novel violet/purple phases. The general formula is YIn1-x-2y-zMnxTiyZnyAlzO3 (x = 0.005-0.2, y = 0.1-0.4, and z ≤ 0.1), where the color darkens with the increasing amount of Mn. Higher y or small additions of Al provide a more reddish hue to the resulting purple colors. Substituting other rare earth cations for Y has little impact on color. Crystal structure analysis by neutron powder diffraction confirms a shorter apical Mn-O distance compared with that in the blue YIn1-xMnxO3. Magnetic susceptibility measurements verify the 3+ oxidation state for Mn. Diffuse reflection spectra were obtained over the wavelength region 200-2500 nm. All samples show excellent near-infrared reflectance comparable to that of commercial TiO2, making them ideal for cool pigment applications such as energy efficient roofs of buildings and cars where reducing solar heat to save energy is desired. In a comparison with commercial purple pigments, such as Co3(PO4)2, our pigments are much more thermally stable and chemically inert, and are neither toxic nor carcinogenic.

Influence of Structural Disorder on Hollandites A(x)Ru4O8 (A(+) = K, Rb, Rb(1-x)Na(x)).

Structural disorder can play an important role in the electrical properties of correlated materials. In this work we examine the average and local disorder in hollandites A(x)Ru4O8 (A(+) = K, Rb, Rb(1-x)Na(x)) through neutron total scattering techniques. Samples with A(+) = Rb, Rb(1-x)Na(x) exhibit the largest amount of local disorder as evidenced by higher atomic displacement parameters, and as a result, a weakened temperature dependence of the resistivity is observed upon cooling as compared to K(x)Ru4O8. All samples exhibit anisotropic resistivity that is dominated by metallic conductivity at lower temperatures, and this is corroborated by Pauli paramagnetic behavior throughout the measured temperature regime.

Structure and Properties of Ir-Containing Oxides with Large Spin-Orbit Coupling: Ba2In(2-x)Ir(x)O(5+δ).

In this work, the solid solution series Ba2In(2-x)Ir(x)O5+δ (x = 0-1.4, 2) was synthesized, and its structural, magnetic, and charge-transport properties were measured. With increasing Ir content, three transitions in the room-temperature structure were observed: orthorhombic to tetragonal to cubic to a monoclinic distortion of a hexagonal BaTiO3 structure. Neutron diffraction shows Ba2In(1.6)Ir(0.4)O5.4 to be cubic and Ba2InIrO6 to be monoclinic, the latter contrary to previously published X-ray diffraction refinements. Magnetization measurements show Curie-Weiss behavior for x = 0.2-0.6, which arises from nearly 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid-state methods under ambient conditions. The electrical resistivity measurements show all the compounds studied are semiconducting and that resistivity decreases with increasing Ir content, suggesting the proximity to a metal-insulator transition. A sign reversal in the high-temperature Seebeck coefficient is observed indicating both electron and hole charge transport.

Magnetic properties and electronic structure of manganese-based blue pigments: a high-frequency and -field EPR study.

A variety of new oxide-based materials based on hexagonal phase of YInO3 have been recently described. In some of these materials, the In(III) ions are substituted by Mn(III), which finds itself in a trigonal-bipyramidal (TBP) coordination environment. While YInO3 is colorless and YMnO3 is black, mixed systems YIn1-xMnxO3 (0.02 < x < 0.25) display intense blue color and have been proposed as novel blue pigments. Since the Mn(III) ion is paramagnetic, its presence imparts distinct magnetic properties to the whole class of materials. These properties were investigated by electron paramagnetic resonance (EPR) in its high-frequency and -field version (HFEPR), a technique ideally suited for transition metal ions such as Mn(III) that, in contrast to, for example, Mn(II), are difficult to study by EPR at (conventional) low frequency and field. YIn1-xMnxO3 with 0.02 < x < 0.2 exhibited high-quality HFEPR spectra up to room temperature that could be interpreted as arising from isolated S = 2 paramagnets. A simple ligand-field model, based on the structure and optical spectra, explains the spin Hamiltonian parameters provided by HFEPR, which were D = +3.0 cm(-1), E = 0; g⊥ = 1.99, g∥ = 2.0. This study demonstrates the general applicability of a combined spectroscopic and classical theoretical approach to understanding the electronic structure of novel materials containing paramagnetic dopants. Moreover, HFEPR complements optical and other experimental methods as being a sensitive probe of dopant level.

True composition and structure of hexagonal "YAlO3", actually Y3Al3O8CO3.

The discovery of a brilliant-blue color upon the introduction of Mn(3+) to the trigonal-bipyramidal (TBP) sites in YInO(3) has led to a search for other hosts for Mn(3+) in TBP coordination. An obvious choice would be YAlO(3). This compound, which has only been prepared through a citrate precursor route, has long been considered isostructural with YInO(3). However, Mn(3+) substitutions into YAlO(3) have failed to produce a product with the anticipated color. We find that the hexagonal structure for YAlO(3) with Al in TBP coordination proposed in 1963 cannot be correct based on its unit cell dimensions and bond-valence sums. Our studies indicate instead that all, or nearly all, of the Al in this compound has a coordination number (CN) of 6. Upon heating in air, this compound transforms to YAlO(3), with the perovskite structure liberating CO(2). The compound long assumed to be a hexagonal form of YAlO(3) is actually an oxycarbonate with the ideal composition Y(3)Al(3)O(8)CO(3). The structure of this compound has been characterized by powder neutron and X-ray diffraction data obtained as a function of temperature, magic-angle-spinning (27)Al NMR, Fourier transform infrared, and transmission electron microscopy. Refinement of neutron diffraction data indicates a composition of Y(3)Al(3)O(8)CO(3). We find that the hexagonal structures of YGaO(3) and YFeO(3) from the citrate route are also stabilized by small amounts of carbonate. Surprisingly, Y(3)Al(3)O(8)CO(3) forms a complete solid solution with YBO(3) having tetrahedral borate groups. Other unlikely solid solutions were prepared in the YAlO(3)-YMnO(3), YAlO(3)-YFeO(3), YAlO(3)-YBO(3), YBO(3)-YMnO(3), YBO(3)-YFeO(3), and YBO(3)-YGaO(3) systems.

Structural investigation of the substituted pyrochlore AgSbO3 through total scattering techniques.

Polycrystalline samples of the pyrochlore series Ag(1-x)M(n)(x)SbO(3+x[(n-1)/2]) (M = Na, K, and Tl) have been structurally analyzed through total scattering techniques. The upper limits of x obtained were 0.05 for Na, 0.16 for K, and 0.17 for Tl. The Ag(+) cation occupies a site with inversion symmetry on a 3-fold axis. When the smaller Na(+) cation substitutes for Ag(+), it is displaced by about 0.6 Å perpendicular to the 3-fold axis to achieve some shorter Na-O bond distances. When the larger Tl(+) cation substitutes for Ag(+), it is displaced by about 1.14 Å along the 3-fold axis and achieves an environment typical of a lone pair cation. Some of the Tl(3+) from the precursor remains unreduced, leading to a formula of Ag(0.772(1))Tl(+)(0.13(2))Tl(3+)(0.036(1))SbO(3.036(1)). The position of the K(+) dopant was effectively modeled assuming that K(+) occupied the same site as Ag(+). The expansion of the lattice caused by substitution of the larger K(+) and Tl(+) cations results in longer Ag-O bond lengths, which would reduce the overlap of the Ag 4d and O 2p orbitals that compose the valence band maximum. Substitution of the smaller Na(+) results in a decrease in the Ag-O bond distance, thus increasing the overlap of the Ag 4d and O 2p orbitals. This will have a direct influence on the band composition and observed properties of this material of interest.

Intense turquoise and green colors in brownmillerite-type oxides based on Mn5+ in Ba2In(2-x)Mn(x)O(5+x).

Brownmillerite-type oxides Ba(2)In(2-x)Mn(x)O(5+x) (x = 0.1-0.7) have been prepared and characterized. Magnetic measurements indicate that manganese in as-prepared samples is substituting predominantly as Mn(5+) for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Electron paramagnetic resonance measurements indicate that this Mn(5+) is present in a highly distorted tetrahedral environment. Neutron diffraction structure refinements show that Mn(5+) occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ≥ 0.3 samples, neutron refinements show that the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x = 0.1) to green (x = 0.2, 0.3) or to dark green (x ≥ 0.4). Under reducing conditions, Mn(5+) is reduced to Mn(3+), and Ba(2)In(2-x)Mn(x)O(5+x) phases become black Ba(2)In(2-x)Mn(x)O(5) phases still with the brownmillerite structure.

New layered compounds with honeycomb ordering: Li3Ni2BiO6, Li3NiM'BiO6 (M' = Mg, Cu, Zn), and the delafossite Ag3Ni2BiO6.

The new layered compound Li(3)Ni(2)BiO(6) has been prepared by a solid-state reaction. It crystallizes in the monoclinic C2/m space group; its lamellar structure is characterized by a honeycomb ordering between Ni(2+) and Bi(5+) within the slabs, while Li(+) ions occupy octahedral sites in the interslab space. Stacking defects weakly alter the XRD pattern. By substitution of half of the nickel ions, the new phases Li(3)NiM'BiO(6) (M' = Mg, Cu, Zn) isostructural with Li(3)Ni(2)BiO(6) have been synthesized under similar conditions. All these compounds demonstrate paramagnetic behavior at high temperature, and Li(3)Ni(2)BiO(6) exhibits an antiferromagnetic ordering at 5.5 K. By topotactic molten salt ionic exchange, the new delafossite compound Ag(3)Ni(2)BiO(6) has been also obtained and characterized.

Undulating layers in a new rhodate network: structure of Bi(1.4)CuORh5O10.

A new rhodate, Bi(1.4)CuRh(5)O(11), with an hitherto unknown channel structure containing undulating layers of RhO(6) octahedra sharing corners and edges has been discovered and its structure refined from single crystal X-ray diffraction data. The channels contain Bi(3+), Cu(2+), and some O strongly bound to Cu. The Cu coordination is distorted square planar. Mixed Rh(3+)/Rh(4+) valency leads to significant electrical conductivity.

CsTe2O(6-x): novel mixed-valence tellurium oxides with framework-deficient pyrochlore-related structure.

Structures of CsTe2O(6-x) phases were investigated by single-crystal X-ray diffraction and neutron powder diffraction. Stoichiometric CsTe2O6 is a mixed-valence Cs2Te4⁺Te36⁺O12 compound with a rhombohedral pyrochlore-type structure where there is complete order of Te4⁺ and Te6⁺. On heating, this compound develops significant electrical conductivity. As CsTe2O6 becomes oxygen deficient above 600 °C, the rhombohedral pyrochlore-type structure is replaced by a cubic pyrochlore-type structure with disordered Te4⁺/Te6⁺ and oxygen vacancies. However, for CsTe2O(6-x) phases prepared at 500 °C, the observed pyrochlore-type structure has symmetry. The Te4⁺ and O vacancies are all on chains running along the b axis, and the maximum value of x observed is about 0.3. At still higher values of x a new compound was discovered with a structure related to that reported for Rb4Te34⁺Te56⁺O23.

New oxides showing an intense orange color based on Fe3+ in trigonal-bipyramidal coordination.

Hexagonal YIn(1-x)Fe(x)O(3) phases have been prepared and characterized. The coordination for the In/Fe site in this structure is trigonal-bipyramidal. The colors of the phases change from yellow to orange to dark red with increasing Fe content. Magnetic measurements confirm high-spin Fe(3+) for all phases.

Structural studies and electrical properties of Cs/Al/Te/O phases with the pyrochlore structure.

A series of polycrystalline and single crystal cesium aluminum tellurates with the pyrochlore structure have been prepared and characterized. The variations in cell edge for the Cs/Al/Te/O phases range from 10.06 Å for the Al rich limit to 10.14 Å for the Te rich limit. Rietveld structural analyses based on both X-ray and neutron diffraction data were performed on 5 different compositions. Single crystals of 3 compositions were prepared and studied by X-ray diffraction. The anharmonic component of the thermal motion for Cs was small but became significant on replacing Cs with Rb. A maximum in the electrical conductivity of about 0.1 S/cm is found in the middle of this range close to the ideal composition of CsAl(1/3)Te(5/3)O(6). The conductivity is attributed to filled Te 5s states associated with Te(4+) lying just below the conduction band based on empty Te 5s states associated with Te(6+). The relatively large Te(4+) ion is compressed by the lattice, and as this compression increases the filled 5s states approach the conduction band and thereby increases conductivity.

New oxides showing an intense blue color based on Mn3+ in trigonal-bipyramidal coordination.

Substitution of Mn(3+) into the trigonal-bipyramidal sites of oxides with YbFe(2)O(4)-related structures produces an intense blue color because of an allowed d-d transition. This has been demonstrated utilizing a variety of hosts including ScAlMgO(4), ScGaMgO(4), LuGaMgO(4), ScGaZnO(4), LuGaZnO(4), and LuGaO(3)(ZnO)(2). The hue of the blue color can be controlled by the choice of the host.

Mn3+ in trigonal bipyramidal coordination: a new blue chromophore.

We show that trivalent manganese, Mn(3+), imparts an intense blue color to oxides when it is introduced at dilution in trigonal bipyramidal coordination. Our optical measurements and first-principles density functional theory calculations indicate that the blue color results from an intense absorption in the red/green region. This absorption is due in turn to a symmetry-allowed optical transition between the valence-band maximum, composed of Mn 3d(x(2)-y(2),xy) states strongly hybridized with O 2p(x,y) states, and the narrow Mn 3d(z(2))-based conduction-band minimum. We begin by demonstrating and explaining the effect using a well-defined prototype system: the hexagonal YMnO(3)-YInO(3) solid solution. We then show that the behavior is a general feature of diluted Mn(3+) in this coordination environment.

Tin(II) doped anatase (TiO2) nanoparticles: a potential route to "greener" yellow pigments.

During our exploration of compounds in the Sn(II)-Ti(IV)-O system, we discovered that hydrolysis of titanium alkoxide solution in the presence of Sn(II) salts resulted in stable deep-yellow colored anatase nanoparticles. The samples were characterized by X-ray powder diffraction, electron microprobe, thermal analysis, transmission electron microscopy, and (119)Sn Mössbauer spectroscopy. Mössbauer data of the yellow colored samples showed the presence of both Sn(II) and Sn(IV) in a distorted environment as expected in the anatase structure. It is suggested that the cationic charge imbalance is compensated by oxygen vacancies and/or hydroxyl groups as evidenced by Mössbauer data which show two types of Sn(II) environments. When heated in air to 300 degrees C the samples changed color to completely white and (119)Sn Mössbauer data of these samples showed only the presence of Sn(IV). These observations indicate that the origin of the yellow color in our Sn doped anatase nanoparticles arises from filled Sn 5s states just above the O 2p band, thus decreasing the band gap. The Sn(II) doped anatase TiO(2) nanoparticles reported here can potentially lead to environmentally benign yellow pigments. The simplistic nature of the synthetic procedure could easily be adapted to large-scale industrial manufacture.