Profile analysis of powder neutron diffraction data: Its scope, limitations, and applications in solid state chemistry

The Pauling rules have been used for decades to rationalise the crystal structures of ionic compounds. Despite their importance, there has been no statistical assessment of the performances of these five empirical rules so far. Here, we rigorously and automatically test all five Pauling rules for a large data set of around 5000 known oxides. We discuss each Pauling rule separately, stressing their limits and range of application in terms of chemistries and structures. We conclude that only 13 % of the oxides simultaneously satisfy the last four rules, indicating a much lower predictive power than expected.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.202000829

The limited predictive power of the Pauling rules

The magnetic properties of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Na}\mathrm{Fe}{\mathrm{O}}_{2}$ are studied by neutron-diffraction and magnetization measurements. An ordered phase with spins aligned along the ${\stackrel{P\vec}{b}}_{\mathit{hex}}$ axis exists at low temperatures $(Tl4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. At intermediate temperatures $(4\phantom{\rule{0.3em}{0ex}}\mathrm{K}lTl11\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the system passes through an incommensurate ordered phase before transforming into a short-range-ordered state at higher temperatures that persists up to at least $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Although the short-range ordering does not persist to room temperature according to neutron diffraction, the magnetic susceptibility does not follow the Curie-Weiss behavior, even up to $320\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This rich magnetic behavior can be understood qualitatively as a competition between different magnetic exchange interactions that are similar in magnitude. The delicate balance between these interactions makes $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Na}\mathrm{Fe}{\mathrm{O}}_{2}$ a candidate for more detailed theoretical work to understand magnetic behavior in frustrated magnetic systems.

Magnetic structure and properties of the S=5/2 triangular antiferromagnet {alpha}-NaFeO{sub 2}

Coupling between noncollinear magnetic ordering and ferroelectricicty in magnetoelectric multiferroics has been extensively studied in the last decade. Delafossite family compounds with triangular lattice structure provide a great opportunity to study the coupling between spin and electric dipole in multiferroics due to the variety of magnetic phases with different symmetry. This review introduces the magnetic and ferroelectric phase transitions in delafossite ferrites, CuFe(1-x)X(x)O(2) (X = Al, Ga), AgFeO(2) and the related compound α-NaFeO(2). In CuFeO(2), the ferroelectric phase appears under a magnetic field or chemical substitution. The proper screw magnetic ordering with the magnetic point group 21', which has been determined by detailed analysis in neutron diffraction experiments, induces the ferroelectric polarization along the monoclinic b axis in CuFeO2. The cycloidal magnetic orderings are realized in AgFeO(2) and α-NaFeO(2), which are of the point group m1' allowing polarization in the ac plane. The emergence of ferroelectric polarization can be explained by both the extended inverse Dzyaloshinsky-Moriya effect and the d − p hybridization mechanism. These mechanisms are supported by experimental evidence in CuFe(1-x)Ga(x)O2. The polarized neutron diffraction experiment demonstrated one-to-one correspondence between ferroelectric polarization and spin helicity, S(i) × S(j). The incommensurate orbital ordering with 2 Q wave vector, observed by the soft x-ray resonant diffraction experiment, proved that the spin-orbit interaction ties spin and orbital orders to each other, playing a crucial role for the emergence of ferroelectricity in CuFe(1-x)Ga(x)O2.

http://iopscience.iop.org/article/10.1088/0953-8984/26/45/453202/pdf

Spin and orbital orderings behind multiferroicity in delafossite and related compounds.

β-NaFeO2 nanoparticles have been synthesized by sol–gel method and their morphological, structural and optical properties investigated. Transmission electron microscope study reveals that the size of the synthesis nanoparticles is 37 nm and they are possessing spherical symmetry. X-ray diffraction pattern shows the orthorhombic crystal structure of nanoparticles with space group Pn21a. UV–visible spectra of β-NaFeO2 divulges that these nanoparticles have direct band gap 2.35 eV. The observed Fourier transform infrared spectroscopy spectra confirms the presence of Fe–Na bonding at 1074 cm−1. The photoluminescence study of these nanoparticles shows that these nanoparticles possesses various transition in the visible spectrum.

Optical and Luminescence Properties of β-NaFeO 2 Nanoparticles

The magnetic structure of the layer type compound α-NaFeO2 has been determined. The magnetic ordering scheme is distinctly different from that in NaCrX2 (X = S, Se), in accordance with the fact that no two-dimensional behaviour has been observed. An estimation for the magnitude of the various exchange interactions has been made. All interactions are of the same order of magnitude.

B. van Laar

Papers

Magnetic properties and magnetic structure of α‐NaFeO2