High-temperature Neutron Powder Diffraction Research Articles

Comprehensive study of O2 and H2O interaction with La0.9Sr0.1ScO3–δ oxide

In this paper we study the processes of O2 and H2O interaction with La0.9Sr0.1ScO3−δ. Using high temperature neutron powder diffraction (HT-NPD), the evolution of crystal structure and water uptake was investigated in the temperature range 25–700 °C at 21.3 kPa in O2 and O2 + D2O atmospheres. The results of HT-NPD show the influence of the water uptake by La0.9Sr0.1ScO3–δ bulk on the thermal expansion of the material. The kinetics of oxygen surface exchange was studied using pulsed isotope exchange technique. The measurements were carried out in the temperature range 300−900 °C at pO2 = 5.1, 12.2 and 21.3 kPa in dry and humid (pH2O = 2.6 kPa) conditions. For the first time the physical model describing the kinetics of oxygen surface exchange between the mixture of oxygen and water in the gas phase and a solid oxide has been suggested. The model includes following steps: oxygen dissociative adsorption, oxygen incorporation, hydrogen surface diffusion, water adsorption and dissociation. The rate-determining step of the exchange is considered. The influence of O2 humidification on the oxygen surface exchange is discussed.

Intermetallic compounds of the Cr–Mn system investigated using in situ powder neutron diffraction: The reported order-disorder transformation of the σ phase elucidated

Cr–Mn system is very important for many applications. In the present work, the reported intermetallic compounds have been studied using in situ high temperature neutron powder diffraction. For the first time, site occupancies in a σ phase are obtained as a function of temperature by Rietveld analysis. They are compared with calculated data obtained from DFT. The previously reported order-disorder transition in the σ phase around 1000 °C is shown to be in fact a congruent phase transformation to the bcc Cr–Mn solid solution. This solid solution is probably continuous between pure Cr and δ-Mn. The second reported intermetallic α’ phase (Cr2Mn3) phase is shown to be in fact a ternary nitride of the Cr–Mn–N system that should not appear in the binary phase diagram. The present work clarifies and explains all the features of the previous literature on this system and brings major revisions to the phase diagram. General conclusions are drawn concerning mistakes that can be done in the evaluations of phase diagrams.

High-temperature neutron powder diffraction studies on atomic structure of (1−x)PbTiO3-xBi(Zn0.5Ti0.5)O3

The atomic structure of (1−x)PbTiO3-xBi(Zn0.5Ti0.5)O3 (x=0.1, 0.2, 0.25) is investigated across the tetragonal to cubic phase transition by neutron powder diffraction measurements up to temperature T=750°C. Both the long-range average structure and the local atomic distributions are studied from Rietveld refinements and atomic pair distribution function (PDF) analysis combined with reverse Monte-Carlo (RMC) modeling, respectively. In the tetragonal phase, the average structure is featured with a large shift of oxygen, which is responsible for an off-centering of cations. With increasing concentration x, the shift of the planar oxygen along the c-axis increases further. The increment of the oxygen displacement is larger than that of the Ti/Zn displacement. This makes the off-centering of cations larger at higher x, which enhances the stability of the tetragonal phase with higher transition temperature. On thermal displacements, the heaviest ion Pb/Bi in the structure exhibits the largest value and is also increased by the concentration x in both the tetragonal and cubic phases. The local structure from PDF and RMC modeling shows that the abnormal thermal displacements are due to a large distribution of Pb off-centering and a bond-length mismatch between Ti–O and Zn–O bonds. Based on these complementary local and average structures, a splitting between Ti and Zn displacements is discussed.

Phase transitions to superionic Li2Te and Li2Se – A high-temperature neutron powder diffraction study, atom displacements, probability density functions and atom potentials

Despite much research on Li ion batteries at ambient temperature, Li-ion conductors at high temperature have received less attention. In the series of ionic Li2X compounds (X = O, S, Se, Te), superionic phase transitions have been reported for Li2O and Li2S. The lower transition temperature of Li2S, supposedly caused by the larger anion size of S, inspired the investigation of the superionic behavior of Li2Se and Li2Te. Their thermal expansion and thermally excited spatial distribution of Li cations and Se/Te anions was monitored by neutron diffraction up to 800 °C for Li2Te and up to 1000 °C for Li2Se. Both compounds exhibit positive thermal expansion with changing coefficient of expansivity at the superionic transition temperatures. No deviation from cubic metrics or abrupt changes of the cell volume were observed. Rietveld refinements of the fluorite-type structures (space group Fm3¯m) employing Gaussian spatial distribution functions reveal discontinuities in the slope of the increasing mean square displacement parameters as a function of temperature. At ~600 °C (Li2Te) and ~780 °C (Li2Se), the mean square displacements of Li atoms increase abruptly, indicating a superionic transition. For the Te/Se anions, similar discontinuities occur at slightly lower temperatures. Difference Fourier syntheses reveal strongly anisotropic Li spatial distribution density around the 8c (¼ ¼ ¼) positions of cubic site symmetry m3¯m. Moving the maximum of the density distribution to adjacent 32f (xxx) (site symmetry .3 m) positions introduces four virtual split atoms. In addition to the small displacement x along 〈111〉, denoting the mean positions of the split atoms, anisotropic spatial distributions may then be refined. Whereas x deviates from ¼ as an almost linear function of temperature, the abrupt changes in the increase of mean square amplitudes of thermal displacements with temperature are retained. Significantly improved profile refinement residuals support this flexible structure model. Temperature-dependent joint probability density function maps demonstrate the increasing delocalization of Li atoms and yield potential energy distributions, i.e. the energy barriers of ionic transport. The potential barrier between adjacent Li positions at the octahedral site at (½ ½ ½) shows an abrupt drop at the superionic phase transition temperature. The derivative of the potentials, i.e. the restoring forces of Li atom motions show clear deviations from linearity. The anharmonicity of thermally excited spatial atomic distribution refined using the Gram-Charlier series expansion with third- and fourth-order tensor components confirms the enhanced thermal motion in the high-temperature regime. The overall cubic crystal symmetry is maintained.

In-depth study of the Ruddlesden-Popper LaxSr2−xMnO4±δ family as possible electrode materials for symmetrical SOFC

The Ruddlesden Popper (RP) manganites LaxSr2−xMnO4±δ with compositions 0.25 ≤ x ≤ 0.6 have been successfully synthesized as single phases by solid-state reaction in air. All those materials are not only stable in reducing atmosphere but they also maintain the K2NiF4-type structure with I4/mmm symmetry under redox cycling conditions with limited volume changes. The x = 0.5 phase was analyzed by in situ high temperature neutron powder diffraction (HTNPD), under flowing hydrogen, showing the formation of oxide-ion vacancies on the equatorial sites of the perovskite planes, during reduction process. The total electrical conductivity was optimized and found maximum for x = 0.5 with values of 35.6 S cm−1 and 1.9 S cm−1 at 800 °C in air and 3% H2/Ar, respectively, what is judged to be sufficient for an active layer of symmetrical SOFC electrode. First Electrochemical Impedance Spectroscopy (EIS) measurements in both oxidizing and reducing conditions, using an YSZ electrolyte and a GDC buffer layer, are presented giving rise to promising values.

Structural and Magnetic Properties of the Trirutile-type 1D-Heisenberg Anti-Ferromagnet CuTa2O6.

We prepared trirutile-type polycrystalline samples of CuTa2O6 by low-temperature decomposition of a Cu-Ta-oxalate precursor. Diffraction studies at room temperature identified a slight monoclinic distortion of the hitherto surmised tetragonal trirutile crystal structure. Detailed high-temperature X-ray and neutron powder diffraction investigations as well as Raman scattering spectroscopy revealed a structural phase transition at 503(3) K from the monoclinic structure to the tetragonal trirutile structure. GGA+U density functional calculations of the spin-exchange parameters as well as magnetic susceptibility and isothermal magnetization measurements reveal that CuTa2O6 is a new 1D Heisenberg magnet with predominant anti-ferromagnetic nearest-neighbor intrachain spin-exchange interaction of ∼50 K. Interchain exchange is a factor of ∼5 smaller. Heat capacity and low-temperature high-intensity neutron powder diffraction studies could not detect long-range order down to 0.45 K.

High temperature neutron powder diffraction study of the Cu12Sb4S13 and Cu4Sn7S16 phases

Ternary copper-containing sulfides Cu12Sb4S13 and Cu4Sn7S16 have attracted considerable interest since few years due to their high-efficiency conversion as absorbers for solar energy and promising thermoelectric materials. We report therein on the decomposition study of Cu12Sb4S13 and Cu4Sn7S16 phases using high temperature in situ neutron powder diffraction. Our results obtained at a heating rate of 2.5K/min indicate that: (i) Cu12Sb4S13 decomposes above ≈792K into Cu3SbS3, and (ii) Cu4Sn7S16 decomposes above ≈891K into Sn2S3 and a copper-rich sulfide phase of sphalerite ZnS-type structure with an assumed Cu3SnS4 stoichiometry. Both phase decompositions are associated to a sulfur volatilization. While the results on Cu12Sb4S13 are in fair agreement with recent published data, the decomposition behavior of Cu4Sn7S16 differs from other studies in terms of decomposition temperature, thermal stability and products of reaction. Finally, the crystal structure refinements from neutron powder diffraction data are reported and discussed for the Cu4Sn7S16 and tetrahedrite Cu12Sb4S13 phases at 300K, and for the high temperature form of skinnerite Cu3SbS3 at 843K.

Unravelling Ultraslow Lithium-Ion Diffusion in γ-LiAlO2: Experiments with Tracers, Neutrons, and Charge Carriers

Lithium aluminum oxide (γ-LiAlO2) has been discussed and used for various applications, e.g., as electrode coating, membrane, or tritium breeder material. Although lithium-ion diffusion in this solid is essential for these purposes, it is still not sufficiently understood on the microscopic scale. Herein, we not only summarize and assess the available studies on diffusion in different crystalline forms of γ-LiAlO2, but also complement them with tracer-diffusion experiments on (001)- and conductivity spectroscopy on (100)-oriented single crystals, yielding activation energies of 1.20(5) and 1.12(1) eV, respectively. Scrutinous crystal-chemical considerations, Voronoi–Dirichlet partitioning, and Hirshfeld surface analysis are employed to identify possible diffusion pathways. The one-particle potential, as derived from high-temperature powder neutron diffraction data presented as well, reveals the major path to be strongly curved and to run between adjacent lithium positions with a migration barrier of 0.72(...

Tuning the high-temperature properties of Pr2NiO4+δby simultaneous Pr- and Ni-cation replacement

Novel Pr2−xSrxNi1−xCoxO4±δ (x = 0.25; 0.5; 0.75) oxides with the tetragonal K2NiF4-type structure have been prepared. Room-temperature neutron powder diffraction (NPD) study of x = 0.25 and 0.75 phases together with iodometric titration results have shown the formation of hyperstoichiometric oxide for x = 0.25 (δ = 0.09(2)) and a stoichiometric one for x = 0.75. High-temperature X-ray powder diffraction (HT XRPD) showed substantial anisotropy of the thermal expansion coefficient (TEC) along the a- and c-axis of the crystal structure, which increases with increasing the Co content from TEC(c)/TEC(a) = 2.4 (x = 0.25) to 4.3 (x = 0.75). High-temperature NPD (HT NPD) study of the x = 0.75 sample reveals that a very high expansion of the axial (Ni/Co)–O bonds (75.7 ppm K−1 in comparison with 9.1 ppm K−1 for equatorial ones) is responsible for such behaviour, and is caused by a temperature-induced transition between low- and high-spin states of Co3+. This scenario has been confirmed by high-temperature magnetization measurements on a series of Pr2−xSrxNi1−xCoxO4±δ samples. For compositions with high Ni content (x = 0.25 and 0.5) we synthesised K2NiF4-type oxides Pr2−x−ySrx+y(Ni1−xCox)O4±δ, y = 0.0–0.75 (x = 0.25); y = 0.0–0.5 (x = 0.5). The studies of the TEC, high-temperature electrical conductivity in air, chemical stability of the prepared compounds in oxygen and toward interaction with Ce2−xGdxO2−x/2 (GDC) at high temperatures reveal optimal behaviour of Pr1.35Sr0.65Ni0.75Co0.25O4+δ. This compound shows stability in oxygen at 900 °C and does not react with GDC at least up to 1200 °C. It features low TEC of 13 ppm K−1 and high-temperature electrical conductivity in air of 280 S cm−1 at 900 °C, thus representing a promising composition for use as a cathode material in intermediate temperature solid oxide fuel cells (IT-SOFC).

Structural origin of the anisotropic and isotropic thermal expansion of K2NiF4-Type LaSrAlO4 and Sr2TiO4.

K2NiF4-type LaSrAlO4 and Sr2TiO4 exhibit anisotropic and isotropic thermal expansion, respectively; however, their structural origin is unknown. To address this unresolved issue, the crystal structure and thermal expansion of LaSrAlO4 and Sr2TiO4 have been investigated through high-temperature neutron and synchrotron X-ray powder diffraction experiments and ab initio electronic calculations. The thermal expansion coefficient (TEC) along the c-axis (αc) being higher than that along the a-axis (αa) of LaSrAlO4 [αc = 1.882(4)αa] is mainly ascribed to the TEC of the interatomic distance between Al and apical oxygen O2 α(Al-O2) being higher than that between Al and equatorial oxygen O1 α(Al-O1) [α(Al-O2) = 2.41(18)α(Al-O1)]. The higher α(Al-O2) is attributed to the Al-O2 bond being longer and weaker than the Al-O1 bond. Thus, the minimum electron density and bond valence of the Al-O2 bond are lower than those of the Al-O1 bond. For Sr2TiO4, the Ti-O2 interatomic distance, d(Ti-O2), is equal to that of Ti-O1, d(Ti-O1) [d(Ti-O2) = 1.0194(15)d(Ti-O1)], relative to LaSrAlO4 [d(Al-O2) = 1.0932(9)d(Al-O1)]. Therefore, the bond valence and minimum electron density of the Ti-O2 bond are nearly equal to those of the Ti-O1 bond, leading to isotropic thermal expansion of Sr2TiO4 than LaSrAlO4. These results indicate that the anisotropic thermal expansion of K2NiF4-type oxides, A2BO4, is strongly influenced by the anisotropy of B-O chemical bonds. The present study suggests that due to the higher ratio of interatomic distance d(B-O2)/d(B-O1) of A2(2.5+)B(3+)O4 compared with A2(2+)B(4+)O4, A2(2.5+)B(3+)O4 compounds have higher α(B-O2), and A2(2+)B(4+)O4 materials exhibit smaller α(B-O2), leading to the anisotropic thermal expansion of A2(2.5+)B(3+)O4 and isotropic thermal expansion of A2(2+)B(4+)O4. The "true" thermal expansion without the chemical expansion of A2BO4 is higher than that of ABO3 with a similar composition.

Experimental visualization of the diffusion pathway of sodium ions in the Na3[Ti2P2O10F] anode for sodium-ion battery.

Sodium-ion batteries have attracted considerable interest as an alternative to lithium-ion batteries for electric storage applications because of the low cost and natural abundance of sodium resources. The materials with an open framework are highly desired for Na-ion insertion/extraction. Here we report on the first visualization of the sodium-ion diffusion path in Na3[Ti2P2O10F] through high-temperature neutron powder diffraction experiments. The evolution of the Na-ion displacements of Na3[Ti2P2O10F] was investigated with high-temperature neutron diffraction (HTND) from room temperature to 600°C; difference Fourier maps were utilized to estimate the Na nuclear-density distribution. Temperature-driven Na displacements indicates that sodium-ion diffusion paths are established within the ab plane. As an anode for sodium-ion batteries, Na3[Ti2P2O10F] exhibits a reversible capacity of ~100 mAh g−1 with lower intercalation voltage. It also shows good cycling stability and rate capability, making it promising applications in sodium-ion batteries.

Chlorine ion mobility in Cl-mayenite (Ca12Al14O32Cl2): An investigation combining high-temperature neutron powder diffraction, impedance spectroscopy and quantum-chemical calculations

Abstract The crystal structure of Cl-mayenite (Ca 12 Al 14 O 32 Cl 2 ) is very similar to that of the well-known oxygen ion conductor O-mayenite (Ca 12 Al 14 O 33 ), showing zeolite-type cages with partial occupancy by oxygen anions. In Cl-mayenite chlorine ions occupy the cage centers instead of oxygen ions, and it is of interest whether these chlorine ions are mobile and whether Cl-mayenite is a chlorine ion conductor. The answer for these questions is the focus of the paper. High temperature neutron powder experiments and impedance spectroscopy measurements were performed. For information on possible chloride migration pathways and activation energies, quantum-chemical calculations based on density-functional theory were carried out. The behavior of Cl − is in clear contrast to O 2 − in O-mayenite: even at high temperatures it only shows normal harmonic thermal displacement without indications for long range diffusion between the cages. The total ionic conductivity was found to be very low with a value of σ ≈ 10 − 6 S cm − 1 at 1073 K. Quantum-chemical calculations result in a very high activation barrier for Cl − migration of 3.07 eV.

Polymorphism, phase transitions, and thermal expansion of K3Lu(PO4)2

Alkali rare-earth double phosphates have been studied for use as long-wavelength scintillators for γ-ray detection using Si photodiodes. Single-crystal and powder X-ray diffraction (XRD) and powder neutron diffraction have been used to study the structure as a function of temperature. K3Lu(PO4)2 crystallizes with a hexagonal unit cell at room temperature, space group P3¯. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Two lower-temperature phases were characterized using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230K with a transformation to a monoclinic P21/m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130K, with a large change in the cell volume, keeping the same P21/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. High-temperature powder neutron diffraction and high-temperature powder XRD have been used to study the thermal expansion of K3Lu(PO4)2 and indicate a large thermal expansion anisotropy. The crystallographic axes with largest changes account for the structural collapse, which rotates the phosphate group to increase the Lu coordination. The lowest temperature form of K3Lu(PO4)2 is the same as the room temperature form for all the lighter RE compounds of the same type, which is not surprising, given the lighter (larger) RE ions would prefer a higher coordination number.

Order/disorder phenomena in Zn1−xMnxGa2Se4 ordered vacancy compounds: high temperature neutron powder diffraction experiments

We present a study of order–disorder phenomena in the series of tetrahedral ordered vacancy compounds Zn1−xMnxGa2Se4 by means of time-of-flight neutron diffraction at high temperature together with dc magnetic susceptibility, Raman spectroscopy, differential thermal analysis and optical absorption experiments. Samples of nominal composition x = 0, 0.24, 0.5, 0.77 and 1 have been studied. An order–disorder phase transition has been detected, with Tc ranging from 472 to 610 ° C, which involves a structural change from a defect chalcopyrite phase, with space group (s.g.) and three different cation sites, to a partially disordered defect stannite, in which Zn, Mn and half of the Ga ions share the 4d site in s.g. Neither the vacancies nor the Ga ions occupying site 2a are involved in the phase transition. An additional ordering process is observed on approaching the phase transition from below, which is attributed to several factors: the activation of cation diffusion at ∼300 ° C, the partially disordered cation distribution exhibited by the as-grown single crystals and the preference of Mn atoms for the 2d crystallographic site in the structure. The reversibility of the phase transition is analysed with the aid of magnetic, optical and Raman experiments.

Experimental Visualization of the Diffusional Pathway of Oxide Ions in a Layered Perovskite-type Cobaltite PrBaCo2O5+δ

Layered perovskite-type cobaltite PrBaCo2O5+δ is an oxide-ion and electronic mixed conductor with high oxide-ion conductivity and is promising for potential use in SOFC (solid oxide fuel cell) cathodes and oxygen membranes. The crystal structure and oxide-ion diffusional pathway of PrBaCo2O5+δ at 596 and 1000 °C have been investigated by in situ high-temperature neutron powder diffraction and a maximum-entropy method (MEM)-based nuclear-density analysis. The oxide ions of PrBaCo2O5+δ two-dimensionally and anisotropically diffuse through equatorial and deficient apical oxygen sites along the ⟨101⟩ directions in the Pr–Co–O ion-conducting slab.

Specific features of the crystal structure of polymorphous modifications of KFeO2 and their correlation with ionic conductivity

The structure of potassium ferrite KFeO2 has been studied by high-temperature powder neutron diffraction over a wide temperature range. Based on the structural data, the sizes of potassium cation migration channel have been found in the low- and high-temperature KFeO2 modifications using the TOPOS program package. In the low-temperature form, the channels are nonequivalent, and, as a result, strong anisotropy of the potassium-cation conductivity is observed. During the phase transition, the migration channel sizes increase and are equalized, which is the main cause of the jump in the conductivity.

Phase stability of multiferroic GaFeO3 up to 1368 K from in situ neutron diffraction

We report a detailed high-temperature powder neutron diffraction investigation of the structural behavior of the multiferroic GaFeO3 between 296 and 1368 K. Temperature dependent neutron diffraction patterns do not show any appreciable change either in intensity or appearance/disappearance of the observed peaks up to 1368 K, ruling out any structural transition in the entire temperature range. Evolution of the distortion of the oxygen polyhedra around Ga1, Ga2, Fe1, and Fe2 cations sites suggest that the Ga1-O tetrahedron is least distorted and Fe1-O is most distorted. Structural features regarding the distortion of polyhedral units would be crucial to understand the temperature dependence of the microscopic origin of polarizations. The electric polarization has been estimated using a simple ionic model and its value is found to decrease with increasing temperature.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2(x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28−xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD2O and pO2 at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10−6 K−1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

Neutron diffraction measurements and first-principles study of thermal motion of atoms in selectMn+1AXnand binaryMXtransition-metal carbide phases

Herein, we compare the thermal vibrations of atoms in select ternary carbides with the formula ${M}_{n+1}A{X}_{n}$ (``MAX phases,'' $M$ $=$ Ti, Cr; $A$ $=$ Al, Si, Ge; $X$ $=$ C, N) as determined from first-principles phonon calculations to those obtained from high-temperature neutron powder diffraction studies. The transition metal carbides TiC, TaC, and WC are also studied to test our methodology on simpler carbides. Good qualitative and quantitative agreement is found between predicted and experimental values for the binary carbides. For all the MAX phases studied---Ti${}_{3}$SiC${}_{2,}$ Ti${}_{3}$GeC${}_{2}$, Ti${}_{2}$AlN, Cr${}_{2}$GeC and Ti${}_{4}$AlN${}_{3}$---density functional theory calculations predict that the $A$ element vibrates with the highest amplitude and does so anisotropically with a higher amplitude within the basal plane, which is in line with earlier results from high-temperature neutron diffraction studies. In some cases, there are quantitative differences in the absolute values between the theoretical and experimental atomic displacement parameters (ADPs), such as reversal of anisotropy or a systematic offset of temperature-dependent ADPs. The mode-dependent Gr\"uneisen parameters are also computed to explore the anharmonicity in the system.

High-temperature properties of lithium tetraborate Li2B4O7

High-temperature coherent neutron powder diffraction experiments were carried out on Li2B4O7 with boron isotope ratio 11B : 10B as high as 99.6 : 0.4%. Neither traces of phase transformations nor discontinuous changes of physical properties were observed. Anomalies in the thermal expansion of lithium tetraborate were considered in terms of first-order Grüneisen approximation. Extended bond length analysis revealed significant modifications of the boron–oxygen framework which is supplemented by dynamic lithium disorder. Impedance spectroscopy studies revealed a complicated conduction mechanism in single crystalline lithium tetraborate. The lithium diffusion pathway for bulk conductivity along the polar axis was established using both maximum entropy and anharmonic refinement techniques.