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Register a new userStructural evolution of bismuth sodium titanate induced by A-site non-stoichiometry: Neutron powder diffraction studies
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We performed neutron powder diffraction measurements on (Bi$_{0.5}$Na$_{0.5+x}$)TiO$_3$ and (Bi$_{0.5+y}$Na$_{0.5}$)TiO$_3$ to study structural evolution induced by the non-stoichiometry. Despite the non-stoichiometry, the local structure ($r$$leq$ 3.5 {AA}) from the pair distribution function analysis is barely affected by the sodium deficit of up to -5 mol%. With increasing pair distance, however, the atomic pair correlations weaken due to the disorder caused by the sodium deficiency. Although the sodium and the bismuth share the same crystallographic site, their non-stoichiometry have rather opposite effects as revealed from a distinctive distortion of the Bragg peaks. In addition, Rietveld refinement demonstrates that the octahedral tilting is continually suppressed by the sodium deficit of up to -5 mol%. This is contrary to the effect of the bismuth deficiency, which enhances the octahedral tilting.
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We report on the crystallographic structure of the layered compound Pb3Mn7O15. Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic bulk measurements of this system, we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 K and 295 K. Our results show that the crystal structure of stoichiometric Pb3Mn7O15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.
The increasing scientific and technological interest in nanoparticles has raised the need for fast, efficient and precise characterization techniques. Powder diffraction is a very efficient experimental method, as it is straightforward and non-destructive. However, its use for extracting information regarding very small particles brings some common crystallographic approximations to and beyond their limits of validity. Powder pattern diffraction calculation methods are critically discussed, with special focus on spherical particles with log-normal distribution, with the target of determining size distribution parameters. A 20-nm CeO$_{2}$ sample is analyzed as example.
Lithium doped sodium niobate is an ecofriendly piezoelectric material that exhibits a variety of structural phase transitions with composition and temperature. We have investigated the phase stabilities of an important composition Li0.12Na0.88NbO3 (LNN12) using a combination of powder x-ray and neutron diffraction techniques in the temperature range 300 - 1100 K. Detailed Rietveld analyses of thermo-diffractograms show a variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric in nature. In the temperature range of 525 K to 675 K, unambiguous experimental evidence is shown for phase coexistence of orthorhombic paraelectric O1 phase (space group Cmcm) and orthorhombic ferroelectric O2 phase (space group Pmc21). The bp primitive lattice parameter of the ferroelectric orthorhombic phase (O2 phase) decreases, while the ap and cp primitive lattice parameters show normal increase with increase in temperature. Above 675 K, in the O1 phase, all lattice parameters come close to each other and increase continuously with increase of temperature, and around 925 K, ap parameter approaches bp parameter and thus the sample undergoes an orthorhombic to tetragonal phase transition. Further as temperature increases, the cp lattice parameter decreases, and finally approaches to ap parameter, and the sample transform into the cubic phase. The continuous change in the lattice parameters reveals that the successive phase transformations from orthorhombic O1 to high temperature tetragonal phase and finally to the cubic phase are not of a strong first order type in nature. We argue that application of chemical pressure as a result of Li substitution in NaNbO3 matrix, favours the freezing of zone centre phonons over the zone boundary phonons that are known to freeze in pure NaNbO3 as function of temperature.
The magnetic structure of the mixed antiferromagnet NdMn$_{0.8}$Fe$_{0.2}$O$_3$ was resolved. Neutron powder diffraction data definitively resolve the Mn-sublattice with a magnetic propagation vector ${bf k} = (000)$ and with the magnetic structure (A$_x$, F$_y$, G$_z$) for 1.6~K~$< T < T_N (approx 59$~K). The Nd-sublattice has a (0, f$_y$, 0) contribution in the same temperature interval. The Mn sublattice undergoes spin-reorientation transition at $T_1 approx 13$~K while the Nd magnetic moment keep ordered abruptly increases at this temperature. Powder X-ray diffraction shows a strong magnetoelastic effect at $T_N$ but no additional structural phase transitions from 2~K to 300~K. Density functional theory calculations confirm the magnetic structure of the undoped NdMnO$_3$ as part of our analysis. Taken together, these results show the magnetic structure of Mn-sublattice in NdMn$_{0.8}$Fe$_{0.2}$O$_3$ is a combination of the Mn and Fe parent compounds, but the magnetic ordering of Nd sublattice spans over broader temperature interval than in case of NdMnO$_3$ and NdFeO$_3$. This result is a consequence of the fact that the Nd ions do not order independently, but via polarization from Mn/Fe sublattice.
We have investigated the temperature evolution of the magnetic structures of HoFeO$_3$ by single crystal neutron diffraction. The three different magnetic structures found as a function of temperature for hfo are described by the magnetic groups Pb$$n$2_1$, Pbn$2_1$ and Pbn$2_1$ and are stable in the temperature ranges $approx$ 600-55~K, 55-37~K and 35$>T>2$~K respectively. In all three the fundamental coupling between the Fe sub-lattices remains the same and only their orientation and the degree of canting away from the ideal axial direction varies. The magnetic polarisation of the Ho sub-lattices in these two higher temperature regions, in which the major components of the Fe moment lie along $x$ and $y$, is very small. The canting of the moments from the axial directions is attributed to the antisymmetric interactions allowed by the crystal symmetry. They include contributions from single ion anisotropy as well as the Dzyaloshinski antisymmetric exchange. In the low temperature phase two further structural transitions are apparent in which the spontaneous magnetisation changes sign with respect to the underlying antiferromagnetic configuration. In this temperature range the antisymmetric exchange energy varies rapidly as the the Ho sub-lattices begin to order. So long as the ordered Ho moments are small the antisymmetric exchange is due only to Fe-Fe interactions, but as the degree of Ho order increases the Fe-Ho interactions take over whilst at the lowest temperatures, when the Ho moments approach saturation the Ho-Ho interactions dominate. The reversals of the spontaneous magnetisation found in this study suggest that in hfo the sums of the Fe-Fe and Ho-Ho antisymmetric interactions have the same sign as one another, but that of the Ho-Fe terms is opposite.
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