High-temperature Neutron Diffraction Study Research Articles

Using neutron diffraction to explore lithium displacement within cubic phase stabilised Ga-doped Li6.75La3Zr1.75Ta0.25O12 lithium garnet oxides

Typical Li6.75La3Zr1.75Ta0.25O12 exists as a mixture of tetragonal and cubic arrangements, but adding small amounts of Ga3+ (Li6.75–3xGaxLa3Zr1.75Ta0.25O12 x ≥ 0.1) resulted in a single cubic (I a-3d) phase lithium garnet oxide. Following the stabilisation of the cubic phase, the effects on lithium distributions were explored with neutron powder diffraction concerning Ga3+ content and temperature. Increasing the amount of Ga3+ reduced the amount of lithium within the structure, directly decreasing the Li 96h site occupancy and showing a minimal effect on the Li 24d site occupancy. High-temperature neutron diffraction studies revealed the migration of lithium from the Li 24d site to the Li 96h with increasing temperature. The inclusion of Ga3+ improved the total ionic conductivity over the gallium-free system. However, with increasing gallium content (x > 0.1), a negative correlation between the garnet's gallium content and total lithium ionic conductivity is observed, showing how the total amount of free lithium ions impact the system's total ionic conductivity. Though the electrolytes explored here show some limitations, the lithium-ion displacement trends with doping and temperature give us further insight into how these lithium garnet systems respond to chemical and physical change.

Neutron diffraction investigation of copper tellurite glasses with high real-space resolution

High real-space resolution neutron diffraction measurements up to 34 Å−1 were performed on a series of xCuO–(100 − x)TeO2 (x = 30, 40 and 50 mol%) glasses that were synthesized by the melt-quenching technique. The Fourier transformation of neutron diffraction structure factors was used to generate the pair distribution functions, with the first peak at 1.90 Å due to the overlapping Te–O and Cu–O atomic pairs. Reverse Monte Carlo (RMC) simulations were performed on the structure factors and the six partial atomic pair distributions of Cu–Cu, Cu–Te, Cu–O, Te–Te, Te–O and O–O were calculated. The Te–O and Cu–O distributions are very similar and asymmetrical, which revealed that there is a significant short-range disorder in the tellurite network due to the existence of a wide range of Te—O and Cu—O bond lengths. A high-Q (magnitude of momentum transfer function) neutron diffraction study revealed that the average Te–O coordination number decreases steadily from 3.45 to 3.18 with an increase in CuO concentration from 30 to 50 mol% in the glass network. Similar coordination number modifications were earlier found by the RMC analysis of neutron diffraction data sets of copper tellurite glasses that were performed up to lower Q maximum values of 9.5 Å−1. The comparison of high-Q and low-Q neutron diffraction studies reveals that RMC is a powerful and possibly the only technique that is available to elucidate the glass short-range and medium-range structural properties when diffraction data are available up to low Q values of, say, 9.5 Å−1, and when cation–oxygen bond lengths are strongly overlapping and cannot be resolved by Fourier transformation. In situ high-temperature (473 K) neutron diffraction studies of 50CuO–50TeO2 glass revealed that significant distortion of the tellurite network occurs with heating.

High-Temperature Neutron and X-ray Diffraction Study of Fast Sodium Transport in Alluaudite-type Sodium Iron Sulfate

Sodium-ion battery is a potential alternative to replace lithium-ion battery, the present main actor in electrical energy storage technologies. A recently discovered cathode material Na2.5Fe1.75(SO4)3 (NFS) derives not only high energy density with very high voltage generation over 3.8 V, but also high-rate capability of reversible Na insertion as a result of large tunnels in the alluaudite structure. Here we applied high-temperature X-ray/neutron diffraction to unveil characteristic structural features related to major Na transport pathways. Thermal activation and nuclear density distribution of Na demonstrate one-dimensional Na diffusion channels parallel to [001] direction in full consistence with computational predictions. This feature would be common for the related (sulfo-)alluaudite system, forming emerging functional materials group for electrochemical applications.

High temperature neutron diffraction studies of PrInO3 and the measures of perovskite structure distortion.

The crystal structure of PrInO(3) was investigated in the temperature range 303-1123 K by high-resolution neutron-powder diffraction. The PrInO(3) adopts a highly distorted variant of the perovskite structure with the orthorhombic Pnma space group in the whole temperature range investigated. The bond length and bond-angle analysis revealed a very slow tendency to decrease structural distortion with increasing temperature. Comparison of different parameters quantifying perovskite structure distortion calculated for PrInO(3) and the similar PrAlO(3) and PrGaO(3) shows the advantage of using the tolerance factor t12 calculated for the 12-fold coordinated Pr by geometrical averaging of the individual interatomic distances. An additional advantage of the tolerance factor method results from the possibility of extending it to predict the average structural distortion and the geometrical stability of the perovskites at various temperatures once the accurate dependence of t(x,T,d) on the composition, temperature and oxygen content is found. By comparing PrInO(3) with several AMO(3) perovskites containing ions in the fixed oxidation state on the A and M crystal sites it was found that structural distortion and the tolerance factor t12 for PrInO(3) are consistent with the empirical thermal expansion coefficient based on the bond strength calculation [R. M. Hazen, and C. T. Prewitt, Am. Mineral., 1977, 62(3-4), 309]. In contrast to perovskites AMO(3-d) containing mixed-valent M ions, which allow for a wide range of changes of the tolerance factor t(12)(T,d) as a function of oxygen content, perovskites AMO(3) with M ions in the fixed oxidation state show much less flexibility. This flexibility is further reduced for the A(3+)M(3+)O(3) perovskites like PrInO(3) for which even a large change of the synthesis temperature has a minor effect on controlling the resulting t(12)(T) and the structural phase in comparison with A(2+)M(4+)O3 perovskites. The only parameter left for A(3+)M(3+)O(3) materials allowing formation of various perovskites and hexagonal phases is the total pressure, which may significantly change t(12)(T,P).

Nuclear and charge density distributions in ferroelectric PbTiO3: maximum entropy method analysis of neutron and X-ray diffraction data

We conducted in-situ high-temperature neutron and X-ray diffraction studies on tetragonal PbTiO3. Using a combination of Rietveld analysis and Maximum Entropy Method, the nuclear and charge density distributions were determined as a function of temperature up to 460 °C. The ionic states obtained from charge density distributions reveal that the covalency of Pb–O2 bonds gradually weakens with increasing temperature. The spontaneous polarizations calculated from the contributions of ionic state, ionic displacement, and nuclear polarization, are in good agreement with the experimental measurements. This method provides an effective approach to determine spontaneous polarizations in multiferroics with high-current leakage and low resistance.

High-temperature neutron diffraction study of deuterated brucite

To study the structural behavior of brucite at high temperature, we conducted in situ neutron diffraction experiments of a deuterated brucite powder sample, Mg(OD)2, in the temperature range 313–583 K. The sample was stable up to 553 K, above which it started to decompose into periclase (MgO) and D2O vapor. Rietveld analyses of the obtained data were performed using both single-site and three-site split-atom hydrogen models. Our results show that with increasing temperature, unit-cell parameter c increases at a rate ~7.7 times more rapidly than a. This large anisotropy of thermal expansion is primarily due to rapid increase in the interlayer thickness along the c-axis on heating. The amplitudes of thermal vibration for Mg, O, and D increase linearly with increasing temperature; however, the rate of the increase for the lighter D is much larger. In addition, D vibrates anisotropically with a higher magnitude within the (001) plane, as confirmed by our first-principles phonon calculations. On heating, the interatomic distances between a given D and its associated O and D from the adjacent [MgO6] layer increase, whereas the O–D bond length decreases. This behavior suggests weakened D···O and D···D interlayer interactions but strengthened O–D bonding with increasing temperature.

High-temperature neutron diffraction and first-principles study of temperature-dependent crystal structures and atomic vibrations in Ti3AlC2, Ti2AlC, and Ti5Al2C3

Herein we report on the thermal expansions and temperature-dependent crystal structures of select ternary carbide Mn+1AXn (MAX) phases in the Ti–Al–C phase diagram in the 100−1000 °C temperature range. A bulk sample containing 38(±1) wt. % Ti5Al2C3 (“523”), 32(±1) wt. % Ti2AlC (“211”), 18(±1) wt. % Ti3AlC2 (“312”), and 12(±1) wt. % (Ti0.5Al0.5)Al is studied by Rietveld analysis of high-temperature neutron diffraction data. We also report on the same for a single-phase sample of Ti3AlC2 for comparison. The thermal expansions of all the MAX phases studied are higher in the c direction than in the a direction. The bulk expansion coefficients—9.3(±0.1)×10−6 K−1 for Ti5Al2C3, 9.2(±0.1) ×10−6 K−1 for Ti2AlC, and 9.0(±0.1)×10−6 K−1 for Ti3AlC2—are comparable within one standard deviation of each other. In Ti5Al2C3, the dimensions of the Ti–C octahedra for the 211-like and 312-like regions are comparable to the Ti–C octahedra in Ti2AlC and Ti3AlC2, respectively. The isotropic mean-squared atomic displacement parameters are highest for the Al atoms in all three phases, and the values predicted from first-principles phonon calculations agree well with those measured.

Crystal Structure and Oxide-Ion Diffusion of Nanocrystalline, Compositionally Homogeneous Ceria–Zirconia Ce0.5Zr0.5O2 up to 1176 K

The crystal structure, phase stability, and oxide-ion diffusion of nanocrystalline ceria–zirconia materials are unresolved important issues, in particular at high temperatures, where the ceria–zirconia catalysts work efficiently. Here, we report a high-temperature neutron diffraction study of nanocrystalline [10.1(7) nm], compositionally homogeneous, tetragonal Ce0.5Zr0.5O2. Contrary to the previous work, we have observed no tetragonal-to-cubic phase transition in the nanocrystalline Ce0.5Zr0.5O2 up to 1176 K. The axial ratio c/aF and oxygen displacement along the c-axis from the fluorite regular 8c 1/4,1/4,1/4 position of the nanocrystalline Ce0.5Zr0.5O2 are almost independent of temperature. It was found that the c/aF ratio and oxygen displacement in nanocrystalline Ce0.5Zr0.5O2 are smaller than those of the bulk sample. It was shown that the refined atomic displacement parameter and spatial (maximum entropy method nuclear density) distribution of oxygen atom in nanocrystalline Ce0.5Zr0.5O2 are larger than those in nanocrystalline CeO2, which are factors of the high bulk oxygen diffusivity and catalytic activity in nanocrystalline Ce0.5Zr0.5O2. Possible diffusion pathways of oxide ions along the fluorite ⟨100⟩ and ⟨110⟩ directions were visualized in the spatial distribution of bond valence sums calculated using the present refined crystal structure of nanocrystalline Ce0.5Zr0.5O2 at 1023 K. The diffusion path is not straight but curved and forms a three-dimensional network.

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.

A high temperature neutron diffraction study of the double perovskite Ba2154SmMoO6

Ba2LnMoO6 double perovskites have been recently shown to display a wide range of interesting magnetic and structural properties; Ba2154SmMoO6 exhibits simultaneous antiferromagnetic order and a Jahn–Teller distortion. Here we report a high temperature neutron diffraction study of Ba2154SmMoO6 from 353 to 877K. The results evidence a tetragonal to cubic phase transition at 423K. Above this temperature the thermal displacement parameters of the oxygen atoms are modelled anisotropically as a result of a transverse vibration of the bridging oxygen. A smooth increase in the cell parameter a is observed with temperature for Ba2154SmMoO6.

Thermal expansion and decomposition of jarosite: a high-temperature neutron diffraction study

The structure of deuterated jarosite, KFe3(SO4)2(OD)6, was investigated using time-of-flight neutron diffraction up to its dehydroxylation temperature. Rietveld analysis reveals that with increasing temperature, its c dimension expands at a rate ~10 times greater than that for a. This anisotropy of thermal expansion is due to rapid increase in the thickness of the (001) sheet of [Fe(O,OH)6] octahedra and [SO4] tetrahedra with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α = α0 + α1 T, where α0 = 1.01 × 10−4 K−1 and α1 = −1.15 × 10−7 K−2. On heating, the hydrogen bonds, O1···D–O3, through which the (001) octahedral–tetrahedral sheets are held together, become weakened, as reflected by an increase in the D···O1 distance and a concomitant decrease in the O3–D distance with increasing temperature. On further heating to 575 K, jarosite starts to decompose into nanocrystalline yavapaiite and hematite (as well as water vapor), a direct result of the breaking of the hydrogen bonds that hold the jarosite structure together.

High temperature neutron diffraction studies of 0.9BiFeO3–0.1PbTiO3

Neutron diffraction data were collected from sintered polycrystalline 0.9BiFeO3–0.1PbTiO3 at temperatures between 293 and 793 K and the crystal and magnetic structures refined by the Rietveld method. An antiferromagnetic Néel temperature of 592 K was determined from a Brillouin fit to the refined magnetic moments with a ground state of 4.34μB, showing no loss of moment per Fe site compared to BiFeO3. Some magnetic order was observed to persist above the Néel temperature. A maximum in the spontaneous rhombohedral strain was observed close to the Néel temperature, in a significant departure from typical Landau behavior for a ferroelectric and suggesting a coupling between the magnetic and nuclear structures.

Rapid intracrystalline exchange of octahedrally-coordinated divalent cations in amphiboles: an in situ high-temperature neutron diffraction study of synthetic potassic richterite AKB(NaCa)C(Mg2.5Ni2.5)Si8O22(OH)2

AbstractThe distribution of Mg and Ni over the octahedrally-coordinated sites M(l,2,3) in Ni-substituted potassic richterite solid solution, AKB(NaCa)c(Mg2.5Ni2.5)Si8O22(OH)2, synthesized at 0.1 GPa/750°C (25 days), has been studied in situ to 700°C by neutron powder diffraction with Rietveld structure refinement. Using a 2.7 g sample it was possible to make short data collections (3 h) at each temperature at intervals of 50° from 50 to700°C and locate the onset of cation exchange. Above 700°C the amphibole decomposes rapidly and so only an on-heating dataset was collected. Unit-cell parameters increase smoothly upon heating, with no discontinuities evident. Site occupancies of M(l), M(2) and M(3) sites were refined from site-scattering values. The initial XNi values of M(l), M(2) and M(3) sites, corresponding to the synthesis temperature of 750°C, are 0.57(1), 0.34(1) and 0.68(2), respectively. Above 400°C, there is initial ordering whereby thermal annealing allows the disordered Mg-Ni distribution of the quenched synthesis product to adjust to be closer to those appropriate for lower temperatures of the heating sequence. This ordering involves the exchange of Mg and Ni between M(l) and M(2), with no significant change in M(3) occupancy. At 700°C, XNi values of the M(l), M(2) and M(3) sites are 0.64(1), 0.27(1) and 0.69(2), respectively. No disordering was observed, due to the short duration of the high-temperature data collections and decomposition at 750°C. The results of this study indicate that divalent cations exchange between octahedral sites in amphiboles in a matter of hours.

Diffraction, structure and phase stability studies on aluminium titanate

The crystal structure studies of aluminium titanate (Al 2TiO 5) in the temperature range of 25–1200 °C have been investigated by X-ray and neutron powder diffraction. The use of simultaneous refinement of the X-ray and neutron data has enhanced the refinement precision of the structure model of Al 2TiO 5 with the concomitant improvement in values such as atomic coordinates, site occupancies and unit-cell parameters. Results from Rietveld refinements showed that the disordering of Ti and Al atoms in the metal 1 and metal 2 sites. High-temperature neutron diffraction studies of the Al 2TiO 5 crystal structure revealed an increasing substitution of Al for the Ti atoms in the crystal structure. It is proposed that this increased substitution is responsible for the decomposition of Al 2TiO 5 that is observed between 900 and 1280 °C.

Structural stability of orthorhombic and rhombohedralLa0.75Nd0.25CrO3: a high-temperature neutron diffraction study

A high-temperature neutron diffraction study ofLa0.75Nd0.25CrO3 compound has been carried out in the temperature range 300–1400 K. The sample has beenstudied for its structural stability of the two observed crystallographic phases at elevatedtemperatures. It was found to contain orthorhombic and rhombohedral phases with weightpercentages 85% and 15%, respectively, at ambient temperature. It was observed that thetwo phases coexist even up to 1400 K; however, the fraction of the rhombohedral phaseincreases with increasing temperature at the expense of the orthorhombic phase. At 1400 K,the weight fractions of the orthorhombic and rhombohedral phases are about 11% and89%, respectively. This study reveals a negative thermal expansion along theb-axis for the orthorhombic phase and along thec-axis for the rhombohedral phase from 950 K up to 1200 K, where a rapid transformation ofone phase to the other occurs. Below 950 K as well as above 1200 K, both thephases show a normal positive thermal expansion. The La–La and Cr–Cr bonddistances show an abrupt drop from 950 K up to 1200 K and a sharp rise afterthat, which corroborates the negative thermal expansion of the cell volume andb-axis of the orthorhombic phase. The Cr–Cr distance in the rhombohedral phase shows a reduction inmagnitude in the temperature range 950–1200 K, explaining the negative thermal expansion of thec-axis of this phase. The coefficients of volume thermal expansion and linear thermalexpansion have been determined for both the phases. The significance of the present studyhas been discussed for practical applications of the studied compound in solid oxide fuelcells.

Anisotropic thermal expansion and hydrogen bonding behavior of portlandite: A high-temperature neutron diffraction study

The structure of deuterated portlandite, Ca(OD) 2, was investigated using time-of-flight neutron diffraction in the temperature range 308–643 K. Rietveld analysis reveals that with increasing temperature, the c dimension expands at a rate ∼4.5 times larger than that for a. This anisotropy of thermal expansion is due to rapid increase in the interlayer thickness along c with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α= α 0+ α 1 T, where α 0=5.966×10 −5 K −1 and α 1=3.333×10 −8 K −2. On heating, hydrogen-mediated interatomic interactions within the interlayer become weakened, as reflected by increases in the interlayer D⋯O and D⋯D distances with increasing temperature. Correspondingly, the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at high temperatures.

Structural study of La0.75Sr0.25CrO3 at high temperatures

A high-temperature neutron diffraction study has been carried out onLa0.75Sr0.25CrO3 compound in the temperature range 300–1400 K. On doping the parent compoundLaCrO3 with Sr at the La site,the orthorhombic (Pbnm) to rhombohedral () structural transition shifts to lower temperatures. From quantitative Rietveld analysis it is foundunequivocally that there is a two-phase coexistence (orthorhombic and rhombohedral phases with∼89 and 11 weight%, respectively) in the temperature range 300–470 K anda three-phase coexistence (with a new cubic phase with space groupPm3m) in the temperature range 480–1400 K. The weight percentages ofthe orthorhombic, rhombohedral and cubic phases were found to be∼49%, 37% and 14%, respectively, in the temperature range 480–1300 K, while over 1350–1400 K, theaverage weight percentages of orthorhombic, rhombohedral and cubic phases were found to be∼41%, 41% and 18%, respectively. The coefficients of volume thermal expansion and linearthermal expansion have been determined for all three phases. The importance of thepresent study has been discussed for practical applications of the studied compound insolid oxide fuel cells.

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn 2O 5+x and TbBaMn 2O 5.5−y

The miscibility of TbBaMn 2O 5+ x and TbBaMn 2O 5.5− y has been investigated at 100–600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric ( x , y = 0 ) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn 2O 5 is Mn 2+/Mn 3+ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.

High temperature neutron diffraction study of the Al 2O 3–Y 2O 3 system

The phase diagram of the Al 2O 3–Y 2O 3 system has been investigated with five different compositions by XRD and in situ high temperature neutron diffractometry. High purity YAG, YAP and YAM compounds have been produced successfully through a melt extraction technique. High temperature neutron diffraction has made it possible to follow, in real time, the reactions involved in this system, especially to determine the temperature range of each reaction, which would have been impossible to determine by other means. A good agreement between the experimental results and the phase diagram of the Al 2O 3–Y 2O 3 system has been observed.

High-temperature neutron diffraction study of the crystal structure of ceria-zirconia solid solutions.

High-temperature neutron diffraction study of the crystal structure of ceria-zirconia solid solutions.