- Polymorphism in NaSbO3: Structure and Bonding in Metal Oxides
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A new polymorph of NaSbO3 has been synthesized at 10.5 GPa and 1150 °C in a uniaxial split sphere anvil type press (USSA-2000) and recovered back to ambient conditions. The high-pressure form of NaSbO 3 adopts an orthorhombically distorted perovskite structure, isostructural with CaTiO3, GdFeO3, and NaTaO3. The space group is Pnma, and the unit cell dimensions are a = 5.43835(6) A, b = 7.66195-(8) A, c = 5.38201 (5) A. It is a white insulator with an optical band gap of 3.4 eV. This compound represents the first ternary perovskite prepared containing Sb5+ on the octahedral site. The octahedral tilting distortion in this compound is much larger than expected from ionic radii considerations. The distortion is driven by a second-order Jahn-Teller distortion originating on oxygen that can be traced back to strong Sb-O covalent bonding. A conflict arises between the strong covalent bonding interactions at oxygen that favor a large octahedral tilting distortion and the repulsive Na-O interactions that oppose excessive octahedral tilting. This conflict destabilizes the perovskite topology, thereby stabilizing the ilmenite polymorph under ambient conditions. Analysis of ionic and covalent bonding explains why ASbO3 and ABiO3 compositions frequently adopt structures that violate Pauling's rules.
- Mizoguchi, Hiroshi,Woodward, Patrick M.,Byeon, Song-Ho,Pariset, John B.
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- New series of honeycomb ordered oxides, Na3M2SbO6 (M(ii) = Mn, Fe, (Mn, Fe), (Mn, Co)): Synthesis, structure and magnetic properties
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New layered oxides, Na3M2SbO6 (M(ii) = Mn, Fe) have been synthesized by solid state reactions under inert conditions. Rietveld refinements of the powder X-ray diffraction measurements confirmed the structures in the C2/m space group. The layered structure consists of honeycomb slabs with ordered M2+ (Mn or Fe) and Sb5+ cations, separated by Na+ ions in the interlayer octahedral sites. X-ray photoelectron spectroscopy measurements further substantiated the presence of Mn2+, Fe2+ and Sb5+ ions. A paramagnetic behavior has been exhibited by Na3Mn2SbO6 in the range of 300-100 K with negative Weiss constant (θ = -144 K). Zero field cooled and field cooled values diverged below 50 K and without the indication of a long-range antiferromagnetic order down to 2 K. On the other hand, Na3Fe2SbO6 has been found to display two different transitions in the magnetic susceptibility measurements. A distinct cusp appeared around T ～ 120 K followed by the divergence between the zero field cooled and the field cooled values. The observation of antiferromagnetic ordering (TN ～ 7.5 K) suggested the magnetic frustration behavior arising out of the placement of Fe2+ ions in the triangular lattice of the honeycomb layers. Partial substitution of Mn2+ ions by Fe2+ and Co2+ ions has resulted in isostructural Na3MnFeSbO6 and Na3MnCoSbO6 under similar experimental conditions. Magnetic properties have been significantly modified by the coexistence of Mn2+ and Fe2+ ions in Na3MnFeSbO6. Co2+ substitution resulted in paramagnetic behavior with θ ～ -8.4 K confirming thereby the competing antiferromagnetic interactions. The scope of the present work has been explored by carrying out the ion-exchange experiments of the sodium analogues using molten salt (LiNO3) to synthesize new Li3M2SbO6 (M = Mn, Fe) oxides.
- Yadav, Dileep Kumar,Sethi, Aanchal,Shalu,Uma
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