High-resolution Neutron Powder Diffraction Study Research Articles

Potential of neutron powder diffraction for the study of solid triacylglycerols

Abstract We present a high-resolution neutron powder diffraction study of the triclinic β form of tripalmitin as well as in situ crystallisation experiments, monitored with neutron diffraction, conducted over three different cooling rates. We use the results from the high-resolution study to anticipate if neutron diffraction could be beneficial in differentiating the polymorphism in triacylglycerol systems. We extend on this to present analysis of a diffraction pattern of cocoa butter, to establish the potential for neutron diffraction to study the (hydrogenous) forms of triacylglycerols used in food production.

A high-resolution neutron powder diffraction study of the low-temperature structural phase transitions in RbCaF3 perovskite

The temperature dependence of the crystal structure and thermoelastic properties of RbCaF3 perovskite are reported from a high-resolution neutron powder diffraction investigation in the temperature interval 2–300 K. Two phase transitions have been identified from these data, the first from space group Pm3¯m to I4/mcm, at 200 K, has been well characterized in earlier studies, the second, from space group to I4/mcm to Pbnm has previously been less well understood. New results from the former transition show the order parameter evolves with a critical exponent of 0.284(5), and the phase is characterized by an Einstein temperature of 192(8) K, and a Debye temperature of 270(10) K. The temperature variation of the tetragonal spontaneous strain in this phase is consistent with an additional fourth order invariant in the Landau potential. The second phase transition is found to be strongly first order and a temperature dependent, and heating rate/cooling rate dependent phase coexistence is found at all temperatures below ~ 80 K on heating, and ~ 65 K on cooling. The evolution of the orthorhombic unit cell is atypical in the region of the phase transition, with a and b exhibiting strong negative linear thermal expansion, whilst the c axis behaves in the expected manner. The crystal structure of the Pbnm phase at 2 K is reported from an unconstrained Rietveld refinement for the first time.

Spontaneous structural distortion of the metallic Shastry-Sutherland system DyB4 by quadrupole-spin-lattice coupling

DyB4 has a two-dimensional Shastry-Sutherland (Sh-S) lattice with strong Ising character of the Dy ions. Despite the intrinsic frustrations, surprisingly, it undergoes two successive transitions: a magnetic ordering at TN = 20K, and a quadrupole ordering at TQ=12.5 K. From high-resolution neutron and synchrotron X-ray powder diffraction studies, we have obtained full structural information on this material in all phases, and demonstrate that structural modifications occurring at quadrupolar transition lead to the lifting of frustrations inherent in the Sh-S model. Our study thus provides a complete experimental picture of how the intrinsic frustration of the Sh-S lattice can be lifted by the coupling to quadrupole moments. We show that two other factors, i.e. strong spin-orbit coupling and long-range RKKY interaction in metallic DyB4, play an important role in this behavior.

Low-temperature performance of Li-ion batteries: The behavior of lithiated graphite

Safety issues along with the substantially reduced energy and power capabilities of Li-ion cells, operated at low temperatures, pose a technical barrier limiting their use in electric vehicles and aerospace applications. A combined in situ high-resolution neutron powder diffraction and electrochemical study on Li-ion cells of the 18650-type over a temperature range from 230 K to 320 K is reported with a focus on the graphite anode and the low temperature performance of the cell. Instead of a quasi-continuous behavior as observed at ambient temperatures, an anomalous behavior occurs upon discharge at low temperature, primarily reflected in the abrupt character of the LiC12 – to – graphite phase transformation and the unusual temperature dependence of the amount of LiC6. An instability of lithiated graphite phases at temperatures below 250 K is observed, which affects the performance of Li-ion batteries at low temperatures.

Effect of composition and pressure on phase transitions in FexO at low temperature

We report the results of high-resolution neutron powder diffraction studies of FexO (x = 0.925, 0.94) in the vicinity of the low-temperature antiferromagnetic transition and at pressures up to 8 GPa. Our analysis shows that the P-T phase diagram of FexO is strongly affected by the composition and defect structure of the material. We observe the divergence of critical temperatures of magnetic and structural transitions. In contrast to the stoichiometric antiferromagnet MnO, we find no correlation between the magnitude of the magnetic moment of iron and the degree of the rhombohedral distortion in FexO. We suggest that the defect structure of antiferromagnetic FexO significantly influences the temperature of the structural transition.

Neutron powder diffraction study of nuclear and magnetic structures of multiferroic (Bi0.8Ba0.2)(Fe0.8Ti0.2)O3: Evidence for isostructural phase transition and magnetoelastic and magnetoelectric couplings

We report here the results of a high-resolution neutron powder diffraction study on the multiferroic solid solution system (Bi${}_{0.8}$Ba${}_{0.2}$)(Fe${}_{0.8}$Ti${}_{0.2}$)O${}_{3}$ in the temperature range 4 to 700 K. Using irreducible representation theory to analyze the magnetic structure by Rietveld refinement, we show that the magnetic structure is collinear $G$-type antiferromagnetic. Further, we confirm the occurrence of an isostructural phase transition (IPT) accompanying the magnetic ordering around \ensuremath{\sim}625 K in (Bi${}_{0.8}$Ba${}_{0.2}$)(Fe${}_{0.8}$Ti${}_{0.2}$)O${}_{3}$. It is shown that as a result of the IPT, the positions of all the atoms change significantly in the magnetically ordered phase, leading to an excess polarization which scales linearly with the sublattice magnetization obtained by Rietveld refinement of the magnetic structure. Structural evidence for magnetoelastic coupling for the magnetic transitions below room temperature is also presented.

Structural study of K x Na1 − x NbO3 (KNN) for compositions in the range x = 0.24–0.36

The structure of the A-site substituted perovskite K(x)Na(1 - x)NbO(3), x = 0.24-0.36, where a phase boundary was previously reported, has been determined by high-resolution X-ray powder and neutron powder diffraction studies. The structure of the x = 0.3 compound was refined in the monoclinic space group Pm at 293 K and in P4mm at 523 K. The Glazer tilt system of the room-temperature monoclinic phase is a(0)b(+)c(0), which has implications for the nature of the next symmetry change with composition towards pure potassium niobate. A phase-coexistence region at the transition between monoclinic and tetragonal phases was also identified, consistent with a first-order phase boundary. There is also evidence for an intermediate oxygen-octahedra tilted tetragonal phase.

A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La 9.67Si 6O 26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La 9.67Si 6O 26.5 are reported for temperatures ranging between 25 and 900 °C. The best fit to the data was obtained for space group P6 3 and there was no evidence for any change in symmetry over the temperature range studied. Interstitial oxide ions are identified lying in sites similar to those predicted by previous computer modelling studies, and in agreement with structural studies on related materials. Furthermore, occupancy of these sites is enhanced by Frenkel-type disorder from neighbouring positions. The results thus add further weight to the interpretation that, in these apatite-type systems, the silicate substructure is important for the accommodation of interstitial oxide ions and their migration.

Processing control of phase separation, cation ordering, and the dielectric properties of Ba3(Co0.6Zn0.4)Nb2O9

The dielectric and crystallographic properties of the ceramic Ba3(Co0.6Zn0.4)Nb2O9 have been investigated in relation to the processing schedule. Regimes incorporating slow cooling or a lower-temperature annealing stage are found to enhance the dielectric properties with the unloaded quality factor value increasing up to Qu=24300 at 3.21GHz, as compared with Qu=14425 at 3.21GHz using the standard schedule, while high relative permittivity εr=34 is retained. High-resolution neutron powder diffraction studies reveal the materials to be biphasic assemblages of trigonal perovskites which exhibit varying degrees of cation order. The higher Q samples are found to contain the highest proportion of the fully cation-ordered phase.

High-resolution neutron powder diffraction study on the phase transitions in BaPbO 3

Phase transitions that occurred in perovskite BaPbO 3 have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO 6 octahedra around the two-fold axis (tilt system a 0 b − b −). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO 3 undergoes a discontinuous phase transition to a tetragonal structure (space group I4 /mcm) with the tilting of the PbO 6 octahedra being about the four-fold axis of the cubic aristotype (tilt system a 0 a 0 c −). With further increasing the temperature, BaPbO 3 experiences a continuous phase transition to a simple cubic structure (space group Pm3¯ m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→ I2/ m phase transition above room temperature has not been observed.

Structure and hydrogen bonding inCaSiD1+x: Issues about covalent bonding

We report here our high-resolution neutron powder diffraction and neutron vibrational spectroscopy study of ${\mathrm{CaSiD}}_{1+x}$ along with first-principles calculations, which reveal the deuterium structural arrangements and bonding in this novel alloy hydride. Both the structural and spectroscopic results show that, for $xg0$, D atoms start occupying a ${\mathrm{Ca}}_{3}\mathrm{Si}$ interstitial site. The corresponding $\mathrm{Si}\text{\ensuremath{-}}\mathrm{D}$ bond length is determined to be $1.82\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, fully $0.24\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ larger than predicted by theory. Thus, our neutron measurements are at odds with the strongly covalent $\mathrm{Si}\text{\ensuremath{-}}\mathrm{H}$ bonding in ${\mathrm{CaSiH}}_{1+x}$ that such calculations suggest, a result which may have implications for a number of ongoing studies of metal-hydrogen systems destabilized by Si alloying.

Lanthanide distribution in some doped alkaline earth aluminates and gallates

High-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB 2O 4 ( A=Ca 2+, Sr 2+ and Ba 2+ and B=Al 3+ and Ga 3+). These oxides all adopt a stuffed tridymite type structure, the precise nature of which depends on both the A- and B-type cations. Bond valence calculations reveal a range of values for the various A-type cations, in all cases at least one site is significantly underbonded. Conversely the tetrahedral B-type sites invariably exhibit unexceptional bond valencies. Attempts to dope the oxides with various lanthanides to the 1% level invariably resulted in some segregation into alternate phases located at the grain boundaries. The identity of the impurity phases is presented and the importance of bond valencies in understanding this segregation is highlighted.

High-resolution neutron powder diffraction study on the structure of BaPbO 3

Using high-resolution time-of-flight neutron powder diffraction, the crystal structure of BaPbO 3 has been reinvestigated at room temperature and 4.2 K. By comparing different structural models, i.e. the orthorhombic Imma and the monoclinic I2/ m, it is concluded that the former one describes correctly the structure of BaPbO 3, and no Imma→ I2/ m phase transition exists in the temperature range investigated. The apparent monoclinic distortion is likely due to the existence of twins that introduce the micro strain resulting in anisotropic line broadening of the observed profiles.

A high-resolution neutron powder diffraction study of ammonia dihydrate (ND3⋅2D2O) phase I

We have measured the thermal expansivity of ammonia dihydrate (ND3⋅2D2O) phase I from 4.2 to 174 K at ambient pressure, and the incompressibility at 174 K from 0 to 0.45 GPa, using time-of-flight neutron powder diffraction. The unit cell volume as a function of temperature, V(T), was fitted with a Grüneisen approximation to the zero-pressure equation of state (with the lattice vibrational energy calculated from a double-Debye model fitted to heat capacity data) having the following parameters at zero pressure and temperature: V0,0=356.464±0.005 Å3, (K0,0/γ)=7.163±0.024 GPa, and K0,0′=5.41±0.33 (where VP,T is the unit cell volume at pressure P and temperature T, KP,T is the isothermal bulk modulus, KP,T′ is its first pressure derivative, and γ is the Grüneisen ratio). The two Debye temperatures are θDA=165±3 K and θDB=729±4 K. The unit cell volume at 174 K as a function of pressure, V(P), was fitted with a third-order Birch–Murnaghan equation of state having the following parameters: V0,174=365.69±0.16 Å3, K0,174=7.02±0.25 GPa, and K0,174′=9.56±1.28. The volume thermal expansion coefficient, αV, at 174 K and atmospheric pressure is 281.3×10−6 K−1. The proton disorder manifested at high homologous temperatures is seen to be frozen in, on the time scale of these experiments, down to 4.2 K. A high-pressure polymorph of ammonia dihydrate was observed following melting of the sample at 179 K and 0.46 GPa.

High-resolution neutron powder diffraction study on the structure of Sr 2SnO 4

From the high-resolution time-of-flight neutron powder diffraction data, the crystal structure of Sr 2SnO 4 at the temperature range between 4 and 300 K has been investigated. The Rietveld refinement has shown that Sr 2SnO 4 belongs to the space group Pccn, which can be derived from the tetragonal K 2NiF 4 structure by tilting the SnO 6 octahedra along the [100] T- and [010] T-axis, respectively, with non-equal tilts. The earlier reported first-order phase transition in Sr 2SnO 4, from Bmab to P4 2 /ncm, has not been observed.

Defect Engineering for Control of Polarization Properties in SrBi2Ta2O9

Defect engineering to design for remanent polarization (Pr) and coercive field (Ec) in SrBi2Ta2O9 (SBT) is proposed. Cation vacancies coupled with substituents or oxide vacancies are shown to play an essential role in determining the polarization properties. High-resolution neutron powder diffraction studies revealed that trivalent-cation (Bi, La, and Nd) substitution induces Sr vacancies for the requirement of charge neutrality. Polarization measurements of dense ceramics indicated that Bi substitution led to an increase in 2Pr and the 2Pr value for Sr0.73Bi2.18Ta2O9 was twice as large as that of SBT. Structural refinements revealed that Bi substitution enhances spontaneous polarization (Ps) and the improvement of Ps is responsible for the larger 2Pr observed. La substitution with Sr vacancies increased Pr slightly, but the 2Ec value (41 kV/cm) of La0.33Sr0.5Bi2Ta2O9 was much smaller than that for SBT (57 kV/cm). Nd-substituted SBT showed almost the same Pr of La–SBT, while a high 2Ec (125 kV/cm) was attained for Nd0.33Sr0.5Bi2Ta2O9. It is suggested that the higher Ec found in Nd–SBT is attributed to oxide vacancies produced by the substitution of a very small amount of Nd at the Ta site. Defect engineering is expected to open the way for the design of the polarization properties in SBT.

Pressure Dependence Investigation of the Low-Temperature Structure of TlAu(CN)(2) by High-Resolution Neutron Powder Diffraction and Optical Studies.

In contrast to the anomalous increase of the lattice parameter b and associated reorientation of the [Au(3)(CN)(2)](-) ion with decreasing temperature, the unit cell volume and the lattice parameters of TlAu(CN)(2) are found to decrease (at T = 62 K) linearly for external hydrostatic pressures up to 4.5 kbar. The [Au(CN)(2)](-) ions do not essentially change orientation with pressure. The structural results suggest that the change in luminescence energy with pressure is due to changes in Tl.Au as well as in Au.Au interatomic separations, which again illustrates the importance of Tl.Au covalent interactions in this compound.

Mean-square displacements of atoms in hexaborides

High-resolution neutron powder and single-crystal X-ray diffraction studies has been used to investigate the temperature dependencies of atomic mean-square displacements in rare-earth hexaborides. The experimental data on elastic constants and phonon frequencies have been used to improve the valence force field parameters for SmB 6 and the temperature dependencies of atomic mean-square displacements in SmB 6 is compared with theoretical values. The correlation between ionic radii and mean-square displacements of rare-earth atoms are shown.

Structural behavior, crystal chemistry, and phase transitions in substituted leucite; high-resolution neutron powder diffraction studies

High-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi206, RbAlSi20, CsAlSi206, and KFeSi206. The temperature-dependent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-0 bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of comer-linked (AI,Si)04 tetrahedra about [001] leads to collapse of the (111) structural channels and concomitant volume reduction.

Non-stoichiometry, structural defects and properties of LaMnO3+δ with high δ values (0.11≤δ≤0.29)

Strongly oxygenated LaMnO3+δperovskites, with nominal MnIV contents up to 58%, have been prepared by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (200 bar). A high-resolution neutron powder diffraction study of four representative samples with 0.11≤δ≤0.29 reveals the presence of both La and Mn vacancies. Contrary to previous studies, it is found that there are a substantially higher proportion of Mn vacancies, depending rather sensitively on the oxidation conditions. The oxidation state for Mn calculated for the refined stoichiometry La1–xMn1–yO3 is in good agreement with the δ values previously determined by thermal analysis. Further to this, it is also found that as δ increases the Mn–O bond lengths shorten, the Mn–O–Mn angles progressively increase and the perovskite structure becomes more regular, which is consistent with the incorporation of MnIV cations. The presence of Mn vacancies (as much as 13% in samples prepared under high oxygen pressure) perturbs the conduction paths for the transport of holes across Mn–O–Mn, weakening the double-exchange interaction. This structural disorder explains the observed decrease of the ferromagnetic Curie temperature (TC) as δ increases.