Synchrotron Radiation Powder X-ray Diffraction Research Articles

Ion-intercalation mechanism and structural relaxation in layered iron phosphate Na3Fe3(PO4)4 cathodes

Layered Na3Fe3(PO4)4 can function as a positive electrode for both Li- and Na-ion batteries and may hold advantages from both classical layered and phosphate-based electrode materials. Using a combination of ex-situ and operando synchrotron radiation powder X-ray diffraction, void space analysis, and Mössbauer spectroscopy, we herein investigate the structural evolution of the Na3Fe3(PO4)4 framework during Li- and Na-ion intercalation. We show that during discharge, Li- and Na-intercalation into Na3Fe3(PO4)4 occurs via a solid solution reaction wherein Na-ions appear to be preferentially intercalated into the intralayer sites. The intercalation causes an expansion of the unit cell volume, however at open circuit conditions after ion-intercalation (i.e., after battery discharge), Na3+xFe3(PO4)4 and LixNa3Fe3(PO4)4 undergo a structural relaxation, wherein the unit volume contracts below that of the pristine material. Rietveld refinement suggests that the ions intercalated into the intra-layer sites diffuse to the sites in the inter-layer space during the relaxation. This behavior brings new perspectives to understanding structural relaxation and deviations between structural evolution observed under dynamic and static conditions.

Two types of cubic components coexisting in the paraelectric phase of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 revealed by synchrotron radiation X-ray diffraction

The crystal structures of relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) have been investigated using synchrotron radiation X-ray powder diffraction. Two different types of cubic components coexist in the paraelectric phase at 600 K. The first is Cubic-I, in which the Pb ion is isotropically off-centered from the corner of the perovskite-type unit cell. The other, Cubic-II, has the Pb ion preferentially off-centered in the <111> directions from the corner. The volume fractions of Cubic-I and Cubic-II are approximately 83% and 17%, respectively. Previous studies have shown that only approximately 20% of PMN transitions to a rhombohedral structure at 100 K. This observation suggests a close relationship between Cubic-II and the rhombohedral structure at low temperatures. The intrinsic structural inhomogeneity observed in the paraelectric phase, such as variations in the disordering behavior of Pb ions, is potentially linked to the relaxor characteristics of PMN.

Local Structural Investigation of Near-Infrared-Reflective Black Ca2(Mn,Ti)O4 Pigments Using Synchrotron Radiation X-Ray and Density Functional Theory Calculations.

Layered perovskite black Ca2(Mn,Ti)O4 ceramics were studied by using synchrotron radiation X-rays (SRX) and density functional theory (DFT) calculations to investigate the valence states of the cations, average/local structures, and electronic states. The crystallographic data were obtained by the Rietveld refinement of the obtained synchrotron radiation X-ray powder diffraction patterns. X-ray absorption near edge structure (XANES) spectroscopy measurements revealed that the ratios of Mn4+ and Ti4+ were about 82 and 95%, respectively, in all samples, and the Mn/Ti valences were not affected by the introduction of Ti4+. In addition, two pre-edge peaks were observed in the Mn-XANES spectra, but their peak positions and intensities were affected by doping with Ti4+, indicating that the symmetry of the MnO6 octahedra was changed. A comparison of the atomic distances estimated from the Rietveld analysis and radial distribution function (RDF) revealed that there were large differences between the M-M distances (M = Mn, Ti). Therefore, XANES simulations were carried out to obtain models of the local structure. The experimental and theoretical data indicate that the Mn atoms were configured in a zigzagging arrangement, and the distortion of the MnO6 octahedra increased with the increase in the degree of Ti4+ doping. The origin of the changes to the pre-edge peaks was not only the crystal field strength around Mn but also the symmetry of the MnO6 octahedra.

Anharmonicity effects and thermal expansion of thermoelectric (M,M’,M”)NiSn (M,M’,M”=Ti, Zr, Hf) half-Heusler alloys

High-resolution synchrotron radiation X-ray powder diffraction was used for the structural characterization of half-Heusler alloys with compositions MNiSn, M0.5M′0.5NiSn, and M0.5M′0.25M”0.25NiSn ((M, M′, M”) = Ti, Zr, Hf). These alloys were synthesized by arc melting and subsequent thermal annealing. Rietveld refinements of the room temperature patterns revealed that the unit cell parameter is larger for ZrNiSn and smaller for TiNiSn, in accordance with the respective atomic radii. The site occupation of each atomic species exhibited similar trends to those reported in the literature. The values of the linear coefficient of thermal expansion (α) for all the investigated phases within the alloys are < 10−5 K−1. In addition, the value of the Debye temperature (θD) is estimated by analyzing the temperature dependence of the isotropic atomic displacement parameters. In the case of these half-Heusler alloys, it is observed that a lower θD corresponds to a lower α. The good agreement between the temperature dependence of the Debye temperature and the calculated values using the Thirring-Stern expansion enables us to demonstrate the significant role of anharmonicity: the lighter the compound, the lower the anharmonicity. Furthermore, we also discuss the implications of anharmonicity on the thermal conductivity of this family of half-Heusler compounds at room temperature.

Time-resolved in-situ X-ray diffraction and crystal structure analysis of porous coordination polymer CPL-1 in CO2 adsorption

The phase transition from the degassed phase to the gas adsorption phase in a copper porous coordination polymer with pillared-layer structure CPL-1 was observed using time-resolved in-situ synchrotron radiation X-ray powder diffraction. The time evolution of the gas adsorption phase and the structural change of the framework were simultaneously observed. As regards CO2 adsorption, the results suggest two-step adsorption stages in the early stage of the gas adsorption process and the energy barrier of framework deformation required to incorporate gas molecules. Further, we determined the CO2 adsorption structure of CPL-1.

An In Situ Decarbonation Kinetic Study of Calcite Using Synchrotron Radiation XRD

ABSTRACT In situ decarbonation kinetics of calcite were investigated at high temperatures, up to 900 °C, using synchrotron radiation powder X-ray diffraction. The sequence of X-ray diffraction spectra reveals that calcite begins to thermally decompose into lime (CaO) and CO2 at 800 °C and ambient pressure. The decarbonation degree gradually increases with temperature, and calcite completely transforms into lime at 900 °C. The kinetic analysis of the isothermal data using an Avrami model involving nucleation and growth yields the values for the decarbonation rate and reaction order. Our results indicate that the decarbonation rate increases from 2.89 × 10–4s–1 to 3.48 × 10–3s–1 with elevated temperature from 840 to 880 °C, showing a positive temperature dependence on the reaction rate. The calculated Avrami exponent (n) values between 1.35 and 2.38 suggest that the thermal decomposition of calcite should be mainly dominated by homogeneous nucleation and CO2 diffusion-controlled growth. In natural carbonate fault rocks, the decarbonation of CaCO3 caused by frictional heating may be strengthened by the action of high shear velocity. In addition, the resulting ultrafine powder and CO2 pressurization can remarkably reduce the friction coefficient between two fault planes, which further leads to carbonate fault weakening. The yielding result will be conductive to better understanding the role of decarbonation of calcite in some active fault zones.

Structural Phase Transitions in closo-Dicarbadodecaboranes C2B10H12.

The crystal structures of three thermal polymorphs (I, II, and III) for each isomer of closo-dicarbadodecaboranes C2B10H12 (ortho, meta, and para) have been determined by combining synchrotron radiation X-ray powder diffraction and density functional theory calculations. The structures are in agreement with previous calorimetric and spectroscopic studies. The difference between rotatory phases (plastic crystals) I and II lies in isotropic rotations in the former and anisotropic rotations of the icosahedral clusters in the latter. Phase I is the cubic close packing (ccp) of rotating closo-molecules C2B10H12 in the space group Fm3̅. Phase II is the ccp of rotating closo-molecules C2B10H12 in the cubic space group Pa3̅. The preferred rotational axis in II varies with the isomer. The ordered phases III are orthorhombic (meta) or monoclinic (ortho and para) deformations of the cubic unit cell of the disordered phases I and II. The ordering in the phase III of the ortho-isomer carrying the biggest electrical dipole moment creates a twofold superstructure w.r.t. the cubic unit cell. The thermal polymorphism for C2B10H12 and related metal salts can be explained by division of the cohesive intercluster interactions into two categories (i) dispersive cohesive interaction with additional Coulombic components in the metal salts and (ii) anisotropic local interaction resulting from nonuniform charge distribution around icosahedral clusters. The local interactions are averaged out by thermally activated cluster dynamics (rotations and rotational jumps) which effectively increase the symmetry of the cluster. The C2B10H12 molecules resist at least as well as the CB11H12– anion to the oxidation, and both clusters form easily a mixed compound. This allows designing solid electrolytes such as Nax(CB11H12)x(C2B10H12)1–x, where the cation content may be varied and the temperature of transition into the disordered conducting phase is decreased.

Manganese crystalline phases developed in high lead glazes during firing

A High Temperature Synchrotron Radiation X-Ray Powder Diffraction experiment was performed to determine the manganese compounds formed during the heating and cooling of 70 wt% PbO – 30 wt% SiO2 mixture or the equivalent glass plus 10 wt% of MnO. The effect of adding calcite, dolomite and kaolinite were also studied. All mixtures were fired between 690 °C and 1020 °C in oxidizing conditions and analysed by Scanning Electron Microscopy.A sequence of manganese phases are formed during firing: bixbyite (Mn2O3), barysilite ((Pb,Mn)Si2O7), kentrolite (Pb2Mn2Si2O9) and braunite (Mn7SiO12). Kentrolite and braunite crystallise with different crystal habits during the heating and the cooling. If dolomite is present diopside ((Ca,Mg,Mn)2Si2O6) is formed. If calcite is present, ganomalite (Pb3(CaMn)2Si3O11), margarosanite (Pb(Ca,Mn)2Si3O9) and wollastonite ((Ca,Mn)SiO3) are also formed. Wollastonite can incorporate enough manganese to transform into bustamite ((Mn,Ca)3Si3O9) at high temperatures. This leaves less manganese available for the crystallisation of kentrolite and braunite.

Synthesis and structural, magnetic, electric, and thermoelectric characterization of layered Rh1−xIrxTe2 (0≤x≤1)

Crystallographic analysis and thermoelectric studies of solid solutions ${\mathrm{Rh}}_{1\text{\ensuremath{-}}x}{\mathrm{Ir}}_{x}{\mathrm{Te}}_{2}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$ are reported. All compositions show layered structures belonging to the $P\overline{3}m1$ space group at room temperature. ${\mathrm{IrTe}}_{2}$ presents a first-order phase transition from the hexagonal to the triclinic lattice ($P\overline{1}$ space group), which is monitored by synchrotron radiation x-ray powder diffraction. In the cooling-down process the transition appears at 240 K while in the warming-up process it begins at 280 K, showing a remarkable hysteresis. All compositions show a strong metallic behavior with enhanced Pauli paramagnetism and two regimes in the electrical resistivity. These regimes are associated with electron-electron scattering (at low temperature $\ensuremath{\rho}\ensuremath{\sim}{T}^{2}$) and electron-phonon coupling (higher temperatures $\ensuremath{\rho}\ensuremath{\sim}T$). The Seebeck coefficient shows hole-type carriers for all the compounds.

Hydrogen storage properties and reaction mechanisms of K2Mn(NH2)4–8LiH system

Hydrogen storage properties of K2Mn(NH2)4–8LiH were investigated by considering its de/re-hydrogenation properties and reaction mechanisms. Experimental results show that the dehydrogenated K2Mn(NH2)4–8LiH can be almost re-hydrogenated completely at 230 °C and 50 bar of H2 with a hydrogenation rate more than 1.0 wt%/min. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and FTIR investigations reveal that during ball milling K2Mn(NH2)4 reacts with LiH to form LiNH2 and K–Mn-species1 which is probably a K–Mn-containing hydride. The ball milled sample releases hydrogen in a multi-step reaction with the formation of K3MnH5 and K–Mn-species2 as intermediates and Li2NH, Mn3N2 and MnN as final products. The full hydrogenated products are LiH, LiNH2, and K–Mn-species2. The K–Mn-species2 may play a critical role for the fast hydrogeneration. This work indicates that transition metal contained amide-hydride composite holds potentials for hydrogen storage.

Critical Length for Lattice Expansion of SnO2 Nanorods and Nanosheets: Implications for Lithium-Ion Batteries

Lattice expansion and surface stress for tetragonal SnO2 nanorods and nanosheets were systematically investigated using lattice constants determined from in situ synchrotron radiation X-ray powder diffraction of hydrothermal synthesis. A particle size dependence of lattice constants a and c for the nanorods was identical to that for the nanosheets in the same size within experimental uncertainties. The fact suggests the lattice constants of SnO2 nanostructured materials only depend on the particle size parallel to the crystallographic axis. The average atomic volume of the nanorods and nanosheets linearly depends on the ratio of the particle surface area A to volume V in A/V < 0.5 nm–1. The surface stress estimated in A/V < 0.5 nm–1 was −1.7(2) N/m with a reasonable average atomic volume for bulk. Significant deviations from the linear line were observed in A/V > 0.5 nm–1 for the nanorods and nanosheets. A boundary of expansion behavior was found to be the particle size of ∼9 nm. The critical length of 9 nm has an effect on the study of their application for Li-ion batteries since SnO2 nanostructured materials with size from a few nanometers to tens of nanometers have been fabricated and investigated for anodes in lithium-ion batteries.

Determination of the Crystal Structures in the A-Site-Ordered YBaMn2O6 Perovskite

We present a complete structural study of the successive phase transitions observed in the YBaMn2O6 compound with the layered ordering of cations on the perovskite A-site. We have combined synchrotron radiation X-ray powder diffraction and symmetry-adapted mode analysis to describe the distorted structures as pseudosymmetric with respect to the parent tetragonal structure. The YBaMn2O6 compound shows three consecutive phase transitions on cooling from 603 K down to 100 K. It undergoes a first-order structural transition at T1 ≈ 512 K from a C2/m cell with a single Mn site to a P21/c cell with two nonequivalent Mn sites. No checkerboard ordering of the two types of MnO6 octahedra is revealed, and there is no significant charge segregation. A second transition is observed below T2 ≈ 460 K giving rise to a duplication of the c-axis and the occurrence of four nonequivalent Mn sites. These sites are grouped in two pairs, producing, in this case, a checkerboard arrangement in the ab-plane with an average charge segregation of Δq ≈ 0.4 e–. The observed distortions in this phase disagree with the formation of an orbital-ordered phase. Finally, another structural transition is observed coupled to the magnetic transition at TN ≈ 200 K and the c-axis is no longer duplicated. The low-temperature phase is polar with SG P21. It also contains four nonequivalent Mn sites grouped in two pairs. The charge difference between these pairs is increased, achieving a value of Δq ≈ 0.7 e–. In this phase, an asymmetric stretching mode favors a Jahn–Teller-like distortion in the expanded MnO6 octahedra that could be associated with an ordering of eg (3dx2–z2/3dy2–z2) orbitals. Our refinements disclose that this phase is ferroelectric with significant polar displacements of the Mn and Obasal atoms along the b-axis. The simultaneous occurrence of ferroelectricity and magnetic ordering indicates that YBaMn2O6 can be considered as a type II multiferroic compound and can present magnetoelectric coupling.

Structure stabilization of zeolite Y induced by yttrium and its role in promoting n-docosane conversion

Rare earth (RE)-exchanged zeolite Y is the major active component of fluid catalytic cracking (FCC) catalysts. Herein, a series of yttrium (Y3+)-exchanged zeolites Y was prepared by the ion-exchange method, and the location of Y3+ ions in the Y3+-exchange zeolite NaY was determined by fitting synchrotron radiation X-ray powder diffraction (XRPD) pattern combined with the infrared spectra in the hydroxyl stretching region. In analogy to lanthanum (La3+)-exchanged zeolite Y, Y3+ ions were preferentially located inside the sodalite units near the double six-membered rings (D6R's), octahedrally coordinating to three framework oxygens and to three H2O molecules (or OH groups) in close proximity. The substitution of Y3+ ions for counter-ions Na + caused a noticeable shrinkage of the unit cell due mainly to the relatively small ionic radius and high charge density of Y3+ ions. As a consequence, the Y3+-exchanged HY-based FCC catalyst exhibited improved steam stability, and higher selectivity for high-value products (liquefied petroleum gas and C5+gasoline) in n-docosane cracking compared with the La3+-exchanged HY-based catalyst.

Structural and dynamic studies of Pr(11BH4)3

Rare earth borohydrides RE (BH4)3 are studied in the context of energy storage, luminescence and magnetic applications. We have investigated the structural behavior of praseodymium borohydride Pr (11BH4)3 containing 11B isotope because of the previously reported negative thermal expansion. Differential scanning calorimetry (DSC), in-situ variable temperature synchrotron radiation powder X-ray diffraction (SR-PXD) and infrared studies reveal that Pr (11BH4)3 undergoes to a volume contraction during the phase transition from alpha α-Pr (11BH4)3 to rhombohedral r-Pr (11BH4)3 phase upon heating to 493 K. Surprisingly, the phase transition persists upon cooling at room temperature. Vibrational analysis also shows that the stretching frequency of BH4− anion does not change upon heating which indicates that the B–H bond length remains constant during the structural phase transition from α-Pr (11BH4)3 to r-Pr (11BH4)3 phase. Additionally, the energy barrier of reorientation motion of the BH4− anion in the α-phase was estimated to be ca 23 kJ/mol by quasi-elastic neutron scattering (QENS) and Raman spectroscopy.

Symmetry mode analysis of distorted polar/nonpolar structures in A -site ordered SmBaMn2O6 perovskite

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the $A$-site ordered $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In $\mathrm{SmBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at ${T}_{\mathrm{CO}1}\ensuremath{\approx}380\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {T}_{\mathrm{CO}2}\ensuremath{\approx}190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and ${T}_{\mathrm{N}}\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The main difference between the two charge-ordered phases concerns the stacking sequence along the $c$ axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about $0.35{e}^{\ensuremath{-}}$ in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above $\ensuremath{\approx}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the $a$ axis.

In situ thermal, optical, and structural investigations on reversible thermochromism in bismuth molybdate

Reversible thermochromic inorganic materials show stable and perceptible color change with changing environmental temperatures. This property makes them excellent materials for fabricating temperature indicators, military camouflage materials, and thermal warning devices. However, the mechanism of thermochromism has not been fully understood. Here, we prepared novel bismuth molybdate materials by calcination of Bi2O3 and MoO3 mixtures. The materials exhibit reversible thermochromism, gradually changing color from milky white at 25 °C to bright yellow at 500 °C. The phase composition of the product is tunable by adjusting the ratio of Bi2O3 and MoO3 in the starting materials, producing either single phase of α-Bi2Mo3O12 or mixed phases of α-Bi2Mo3O12,β-Bi2Mo2O9 and γ-Bi2MoO6. The mechanism of reversible thermochromism was studied by in situ UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), in situ synchrotron radiation powder X-ray diffraction (SRPXRD), and differential scanning calorimetry (DSC). In situ SRPXRD revealed no phase transition during heating and cooling, which agrees with DSC analysis with no thermal event detected until the melting temperature of α-Bi2Mo3O12 at 661.6 °C. With increasing temperature, in situ SRPXRD also revealed anisotropic expansion of the α-Bi2Mo3O12 lattice parameters, while in situ UV/Vis DRS showed a continuous red shift in the absorption edge and gradual decrease of band gap, suggesting that lattice expansion and shrinking upon heating and cooling is the main reason for the thermochromic behavior in bismuth molybdate materials.

Size effect of the guest cation on the AlO4 framework in aluminate sodalite-type oxides M8[Al12O24](SO4)2 (M= Sr2+ and Ca2+) in the I43m phase.

Sr8[Al12O24](SO4)2 (SAS) and Ca8[Al12O24](SO4)2 (CAS) are members of the aluminate sodalite-type oxides with the general chemical formula M8[Al12O24](XO4)2 (M2+ is the guest cation and XO42- is the guest anion). To discuss the role of the guest cations (M2+ = Sr2+ and Ca2+) on the rotation of AlO4 in the oxygen tetrahedral framework in the I43m phase, the crystal structure parameters and the probability density function of the guest ions in SAS and CAS have been investigated via synchrotron radiation X-ray powder diffraction by considering Gram-Charlier expansions. The interatomic distances between the M2+ and O2- ions evaluated from the maximum positions in the probability density distribution are almost equal to the sum of the ideal ionic radii of the M2+ and O2- ions. This result suggests that the geometry of the AlO4 tetrahedral framework and the fluctuation of the guest ions are mainly caused by steric effects between the M2+ and O2- ions.

Transformation of copper oxide and copper oxide nanoparticles in the soil and their accumulation by Hordeum sativum.

In recent years, the study of the influence of nanoparticles (NPs) on the environment has attracted much interest as nanotechnology is becoming the key technology of the future generation. The comparative studies on the effects of macro- and nanosized copper oxide (CuO) on plants rarely cover the state and behaviour of CuO in the soil-plant system. This work considers the transformation of CuO in Haplic Chernozem depending on the degree of dispersion and its toxic effects on spring barley (Hordeum sativum) growth. To investigate the transformation of the studied particles of metal oxide in the soil and plant, both chemical method of analysis and synchrotron radiation X-ray powder diffraction, X-ray absorption near-edge structure spectroscopy (XANES) and X-ray absorption fine-structure spectroscopy (EXAFS) were used. It was shown that CuO NPs underwent a stronger transformation due to the high reactivity of smaller particles. The Cu mobility was observed to increase within the soil profile as confirmed by the model pollution experiment. This is mainly due to the formation of complex forms of metal with organic matter. A dose of 300mg/kg of macro- and nanosized CuO did not significantly affect the development and productivity of spring barley. The effect of high doses of macro- and nanosized CuO (2000 and 10,000mg/kg) had a negative impact on the growth of spring barley. The application of nanosized CuO had a greater toxic effect than the macrosized CuO on the plants. The XANES and EXAFS data revealed that CuO NPs accumulated in the soil and plants. The linear combination fit shown that Cu atoms, incorporated into the plants, have environment typical of CuO. This indicates a high environmental risk when soil is contaminated with CuO NPs compared with its arrival as CuO.

Structure and phase transitions in A -site ordered RBaMn2O6 ( R=Pr,Nd ) perovskites with a polar ground state

We report here a structural study of $R\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ ($R=\mathrm{La}$, Pr, and Nd) compounds by means of synchrotron radiation x-ray powder diffraction and Raman spectroscopy. The three compounds are $A$-site ordered perovskites adopting the prototypical tetragonal structure at high temperature. A ferromagnetic transition is observed in the $\mathrm{LaBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ sample and the lattice parameters undergo anisotropic changes at ${T}_{C}$ related to the orientation of the magnetic moments. Both $\mathrm{PrBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ and $\mathrm{NdBa}{\mathrm{Mn}}_{2}{\mathrm{O}}_{6}$ have a structural transition coupled to an electronic localization and an antiferromagnetic transition. In both cases, the x-ray diffraction patterns reveal that the low-temperature phase is orthorhombic with lattice parameters $a+b,b\ensuremath{-}a$, and $c$ with respect to the tetragonal phase. Two possible solutions belonging to the space groups Pmam and $P{2}_{1}am$ can yield accurate refinements of the x-ray patterns. However, the active modes in the low-temperature phase disclosed by the Raman spectroscopy clearly point to the noncentrosymmetric space group, $P{2}_{1}am$. The symmetry analysis of this transition unveils that the primary modes belong to the irreducible representations M5\ensuremath{-} and GM5\ensuremath{-} and the main distortions correspond to rotations of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra and an asymmetric combination of stretching and scissoring modes of the basal oxygens in these octahedra. We conclude that the low-temperature phase is polar and the main contribution comes from the displacement of oxygen atoms from their centrosymmetric positions. However, negligible contribution from the asymmetric stretching associated with a Jahn-Teller distortion is found in this structural transition, suggesting the lack of ferroic orbital ordering of ${e}_{g}$ ($3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) orbitals in the orthorhombic $ab$ plane. There is only one inequivalent site for the Mn atom in the low-temperature polar phase so charge ordering cannot account for the electronic localization having a structural origin.

Interplay between the Reorientational Dynamics of the B3H8 - Anion and the Structure in KB3H8.

The structure and reorientational dynamics of KB3H8 were studied by using quasielastic and inelastic neutron scattering, Raman spectroscopy, first-principles calculations, differential scanning calorimetry, and in situ synchrotron radiation powder X-ray diffraction. The results reveal the existence of a previously unknown polymorph in between the α′- and β-polymorphs. Furthermore, it was found that the [B3H8]− anion undergoes different reorientational motions in the three polymorphs α, α′, and β. In α-KB3H8, the [B3H8]− anion performs 3-fold rotations in the plane created by the three boron atoms, which changes to a 2-fold rotation around the C2 symmetry axis of the [B3H8]− anion upon transitioning to α′-KB3H8. After transitioning to β-KB3H8, the [B3H8]− anion performs 4-fold rotations in the plane created by the three boron atoms, which indicates that the local structure of β-KB3H8 deviates from the global cubic NaCl-type structure. The results also indicate that the high reorientational mobility of the [B3H8]− anion facilitates the K+ cation conductivity, since the 2-orders-of-magnitude increase in the anion reorientational mobility observed between 297 and 311 K coincides with a large increase in K+ conductivity.