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We have determined why hydrothermal synthesis is not an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, which is more appropriate for obtaining nanoparticles, Sr is partially incorporated inside an intermediate (K,Sr) x MnO2·nH2O layered phase that prevents the formation of 4H-nanoparticles. Numerous experiments have been performed, screening the parameters involved in such processes (precursors, pH, temperature, etc.) in order to understand their influence on the shape, size, and purity of the 4H-final product. On analyzing the experimental results obtained by XRD, TEM, TGA, and EDS, among others, some peculiarities of the synthetic procedure were detected, including the appearance of side phases, SrMn3O6-x nanoribbons or the above-mentioned birnessite-type (K,Sr) x MnO2·nH2O phase, and the insertion of K+ within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed. [ABSTRACT FROM AUTHOR]

Stoichiometric4H-SrMnO3.0nanoparticles have been synthesizedfrom thermal decomposition of a new molecular heterometallic precursor[SrMn(edta)(H2O)5]·3/2H2O whose crystal structure has been solved by singlecrystal X-ray diffraction. From this precursor, highly homogeneous4H-SrMnO3.0nanoparticles, with average particle size of70 nm, are obtained. The agglomeration of these nanoparticles maintainsthe sheet-assembling morphology of the metal–organic compound.Local structural information, provided by atomically resolved microscopytechniques, shows that 4H-SrMnO3.0nanoparticles exhibitthe same general structural features as the bulk material, althoughstructural disorder, due to edge dislocations, is observed. The nanometricparticle size enables a topotactic reduction process at low temperaturestabilizing a metastable 4H-SrMnO2.82phase. The oxygendeficiency is accommodated through extra cubic layers breaking the...chch... 4H-sequence. These defect areas are Mn3+rich,as evidenced by high energy resolution EELS data. Magnetic characterizationof nano-SrMnO3.0shows significant variations with respectto the bulk material. Besides the dominant antiferromagnetic interactions,a weak ferromagnetic contribution as well as exchange bias and a glassy-likecomponent are present. After the reduction process, the stabilizationof Mn3+in the 4H-structure gives rise to magnetic anomaliesin the 40–60 K temperature range. The origin of such magneticfeatures is discussed. [ABSTRACT FROM AUTHOR]

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr2NaNb5O15 (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at TC = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at 240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects. [ABSTRACT FROM AUTHOR]

A transmission electron microscopy (TEM) study of the complex NaNb1-xTaxO3 (0.4 ≤ x ≤ 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb1-xTaxO3 oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d0 octahedrally coordinated cations, they show a propensity to second-order Jahn-Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium-tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice. [ABSTRACT FROM AUTHOR]

The influence of particle size in both the structure and thermochromic behavior of 4H-SrMnO3 related perovskite is described. Microsized SrMnO3 suffers a structural transition from hexagonal (P63/mmc) to orthorhombic (C2221) symmetry at temperature close to 340 K. The orthorhombic distortion is due to the tilting of the corner-sharing Mn2O9 units building the 4H structural type. When temperature decreases, the distortion becomes sharper reaching its maximal degree at ∼125 K. These structural changes promote the modification of the electronic structure of orthorhombic SrMnO3 phase originating the observed color change. nano-SrMnO3 adopts the ideal 4H hexagonal structure at room temperature, the orthorhombic distortion being only detected at temperature below 170 K. A decrease in the orthorhombic distortion degree, compared to that observed in the microsample, may be the reason why a color change is not observed at low temperature (77 K).

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.

A transmission electron microscopy (TEM) study of the complex NaNb(1-x)Ta(x)O(3) (0.4 < or = x < or = 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb(1-x)Ta(x)O(3) oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d(0) octahedrally coordinated cations, they show a propensity to second-order Jahn-Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium-tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice.