Variable-temperature Synchrotron X-ray Diffraction Research Articles

Structural Properties of Some Vacancy-Ordered Platinum Halide Perovskites.

The structural changes that accompany the dehydration of Na2PtX6·6H2O (X = Cl, Br) were studied using in situ variable temperature synchrotron X-ray diffraction. The two hexahydrates are isostructural, containing isolated PtX6 octahedra separated by Na cations. Removal of the water results in the formation of the anhydrous vacancy ordered double perovskites Na2PtX6. The Na cation is too small for the cuboctahedron site of the parent cubic structure, resulting in cooperative tilting of the PtX6 octahedra and lowering of the symmetry. Replacing Na with a larger alkali metal (K, Rb, or Cs) invariably enabled the isolation of the anhydrous hexahalide, and we found no evidence that these readily hydrated. For all cations, other than Na, it was possible to observe the archetypical cubic structure, although for the two potassium salts K2PtBr6 and K2PtI6, this was only observed above a critical temperature of 175 and 460 K, respectively. As these two samples were cooled, symmetry lowering was observed, yielding a tetragonal structure initially and ultimately a monoclinic structure: Fm3̅m → P4/mnc → P21/n. These phase transitions are associated with the onset of long-range cooperative tilting of the PtX6 octahedra described using the Glazer tilt notation as a0a0a0 → a0a0c+ → a-a-c+.

Enhanced PEBA composite membrane performance by solution-free melt coating for phenol pervaporation from aqueous solution

Solution-based membrane fabrication methods, which are widely used in the membrane industry, have a negative impact on the environment and fabrication effectiveness. In this study, a green solution-free melt coating method was proposed to rapidly fabricate a poly(ether-block-amide) copolymer (PEBA) composite membrane. Based on its melting point and viscosity, PEBA 1074 was chosen as a suitable membrane material for fabrication by solution coating and melt coating methods, which were compared by characterizations such as variable-temperature synchrotron X-ray diffraction and nano-scratching test to reveal the differences in such as crystallinity etc. The separation performance of these membranes for the pervaporation of phenol from aqueous solutions under different operating conditions was evaluated. The results showed that the PEBA composite membrane fabricated by the melt coating method possessed the same separation factor as the corresponding freestanding membrane because the sol-gel transition was avoided, and a separation factor of 74 for the separation of 1.5 wt% phenol/water solution at 70 °C can be obtained due to high density and crystallinity even with a thickness of 11 μm, which is superior to previously reported results. This study provides an ecologically friendly and effective approach for the fabrication of thermoplastic polymer membranes, thereby promoting their industrial applications.

Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor.

In the pursuit of urgently needed, energy dense solid-state batteries for electric vehicle and portable electronics applications, halide solid electrolytes offer a promising path forward with exceptional compatibility against high-voltage oxide electrodes, tunable ionic conductivities, and facile processing. For this family of compounds, synthesis protocols strongly affect cation site disorder and modulate Li+ mobility. In this work, we reveal the presence of a high concentration of stacking faults in the superionic conductor Li3YCl6 and demonstrate a method of controlling its Li+ conductivity by tuning the defect concentration with synthesis and heat treatments at select temperatures. Leveraging complementary insights from variable temperature synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, density functional theory, and electrochemical impedance spectroscopy, we identify the nature of planar defects and the role of nonstoichiometry in lowering Li+ migration barriers and increasing Li site connectivity in mechanochemically synthesized Li3YCl6. We harness paramagnetic relaxation enhancement to enable 89Y solid-state NMR and directly contrast the Y cation site disorder resulting from different preparation methods, demonstrating a potent tool for other researchers studying Y-containing compositions. With heat treatments at temperatures as low as 333 K (60 °C), we decrease the concentration of planar defects, demonstrating a simple method for tuning the Li+ conductivity. Findings from this work are expected to be generalizable to other halide solid electrolyte candidates and provide an improved understanding of defect-enabled Li+ conduction in this class of Li-ion conductors.

Thermal properties of TiNiSn and VFeSb half-Heusler thermoelectrics from synchrotron x-ray powder diffraction

Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with α av = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced α av = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are linked to low thermal conductivities, but also reduce the bandgap and lower the onset of thermal bipolar transport.

Precise structural analysis of polymer materials using synchrotron X-ray scattering and spectroscopic methods

Polymers can exist in many different conformations and exhibit a variety of unique and complicated hierarchical structures, which are dominated mainly by entropy and sometimes by kinetics rather than thermodynamics due to the intrinsic high viscosity, polydispersity, and so on. The physical properties of polymeric materials are determined not only by their primary structures but also by their higher order (hierarchical) structures. Hence, precise determination of the structures at all spatial scales is essential to control the physical properties of polymeric materials. This focus review details the use of various characterization techniques, including variable-temperature (VT) synchrotron (SR) X-ray scattering, VT SR X-ray diffraction, and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS), to determine the structure of fully liquid-crystalline ABA-type triblock copolymers, highly crystalline aromatic polyimides, and thin-film polyimides, respectively. The advantages of these methods are briefly reviewed, and the relationship between the macroscopic physical properties and the inherent microstructure is discussed. Physical properties of polymer materials are deeply related to not only the primary structure but also the higher ordered structure such as periodic structures and molecular orientation. If the target material consists of ordered and non-ordered region, the former structure can be selectively detected by X-ray scattering method, whereas structural information of both the regions is obtained by infrared absorption spectroscopy. Particularly for thin film materials, infrared pMAIRS (p-polarized multiple-angle incidence resolution spectroscopy) and GI-XRD (grazing incidence X-ray diffraction methods are very useful for precise structural analyses.

Magneto-structural coupling in SrTcxRu1-xO3 (x = 0.25,0.5) perovskites

The structure and magnetic properties of the mixed technetium-ruthenium perovskites SrTcxRu1-xO3 (x ​= ​0.25 and 0.5) have been investigated using variable temperature synchrotron X-ray diffraction, magnetic susceptibility measurements, and DFT calculations. Both oxides are isostructural with SrRuO3 and SrTcO3, and there is no evidence of long-range cation order. Magnetostriction is observed along the c-axis for both oxides. The inverse susceptibility and DFT calculations suggest a cation-disordered ferrimagnetic ground state for SrTc0.5Ru0.5O3.

Thermal expansion of ammonium pertechnetate and ammonium perrhenate

The thermal expansion of NH4TcO4 and NH4ReO4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the BO4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH4TcO4 and NH4ReO4 is a consequence of differences in the HBO4 interactions due to changes in the BO bond lengths within the BO4 groups. Both ammonium salts decompose when heated above ∼500 K.

Crystal structures and phase transition behaviour in the 5d transition metal oxides AReO4 (A = Ag, Na, K, Rb, Cs and Tl).

The structures of the six perrhenates (AReO4 A = Ag, Na, K, Rb, Cs and Tl) at room temperature have been established using powder neutron diffraction methods. These demonstrate the rigid nature of the ReO4 tetrahedra, with the Re-O distances decreasing very slightly and the O-Re-O bond angles approaching the regular tetrahedron value of 109.5° as the size of the A-type cation increases. Variable temperature synchrotron X-ray diffraction measurements show that RbReO4 undergoes a I41/a to I41/amd transition near 650 K that is associated with a change in the orientation of the ReO4- tetrahedra about the scheelite b-axis associated with a Γ3+ mode. CsReO4 has an orthorhombic pseudo scheelite structure at room temperature with rotation of the ReO4 tetrahedra about the c-axis described by mode M4+ and this undergoes a first order orthorhombic to tetragonal (Pnma to I41/a) transition near 450 K with a transition to the I41/amd structure occurring above this. TlReO4 is a rare example of a crystalline material displaying a re-entrant phase transition; 141/a to P21/c to 141/a. The monoclinic structure can be described as a scheelite superstructure that contains an ordering of tetrahedral rotations around the c-axis and along the b-axis with the irrep Γ3+ and M4+ both present. This behaviour is different to that described recently for the analogous Tc oxide TlTcO4, which highlights the differences in the chemistry of these two systems.

Depressed Phase Transitions and Thermally Persistent Local Distortions in CsPbBr3 Quantum Dots

The optoelectronic properties of CsPbX3 quantum dots (where X = Cl, Br, or I) are influenced by both their local and average structures. Variable-temperature synchrotron X-ray diffraction measurements of CsPbBr3 quantum dots show that the temperatures for both the orthorhombic-to-tetragonal (50 °C < Tγ-β < 59 °C) and tetragonal-to-cubic (108 °C < Tβ-α < 117 °C) phase transitions of the average structure are depressed relative to their temperatures in bulk CsPbBr3. Simultaneously, pair distribution function analysis of synchrotron X-ray total scattering measurements indicates that the local crystal structure of the quantum dots is best described as orthorhombically distorted over the temperature range of 22 °C < T < 160 °C, with only small changes in the magnitude of the distortion occurring during the observed changes in the average structure. Taken together, these results suggest that phase transitions in CsPbBr3 quantum dots are order–disorder, involving the gradual ordering of individually coherent dom...

Dicyanamide Salts that Adopt Smectic, Columnar, or Bicontinuous Cubic Liquid-Crystalline Mesophases.

Although dicyanamide (i.e., [N(CN)2 ]- ) has been commonly used to obtain low-viscosity, halogen-free, room-temperature ionic liquids, liquid-crystalline salts containing such anions have remained virtually unexplored. Here we report a series of amphiphilic dicyanamide salts that, depending on their structures and compositions, adopt smectic, columnar, or bicontinuous cubic thermotropic liquid-crystalline mesophases, even at room temperature in some cases. Their thermal properties were explored by polarized light optical microscopy, differential scanning calorimetry, thermogravimetric analysis (including evolved gas analysis), and variable-temperature synchrotron X-ray diffraction. Comparison of the thermal phase characteristics of these new liquid-crystalline salts featuring "V-shaped" [N(CN)2 ]- anions with those of structural analogues containing [SCN]- , [BF4 ]- , [PF6 ]- , or [CF3 SO3 ]- anions indicated that not only the size of the counterion but also its shape should be considered in the development of mesomorphic salts. Collectively, these discoveries may be expected to facilitate the design of thermotropic ionic liquid crystals that form inverted-type bicontinuous cubic and other sophisticated liquid-crystalline phases.

Structural and Thermal Properties of BaTe2O6: Combined Variable-Temperature Synchrotron X-ray Diffraction, Raman Spectroscopy, and ab Initio Calculations.

Variable-temperature Raman spectroscopic and synchrotron X-ray diffraction studies were performed on BaTe2O6 (orthorhombic, space group: Cmcm), a mixed-valence tellurium compound with a layered structure, to understand structural stability and anharmonicity of phonons. The structural and vibrational studies indicate no phase transition in it over a wider range of temperature (20 to 853 K). The structure shows anisotropic expansion with coefficients of thermal expansion in the order αb ≫ αa > αc, which was attributed to the anisotropy in bonding and structure of BaTe2O6. Temperature evolution of Raman modes of BaTe2O6 indicated a smooth decreasing trend in mode frequencies with increasing temperature, while the full width at half-maximum (fwhm) of all modes systematically increases due to a rise in phonon scattering processes. With the use of our earlier reported isothermal mode Grüneisen parameters, thermal properties such as thermal expansion coefficient and molar specific heat are calculated. The pure anharmonic (explicit) and quasiharmonic (implicit) contribution to the total anharmonicity is delineated and compared. The temperature dependence of phonon mode frequencies and their fwhm values are analyzed by anharmonicity models, and the dominating anharmonic phonon scattering mechanism is concluded in BaTe2O6. In addition to the lattice modes, several external modes of TeOn (n = 5, 6) are found to be strongly anharmonic. The ab initio electronic structure calculations indicated BaTe2O6 is a direct band gap semiconductor with gap energy of ∼2.1 eV. Oxygen orbitals, namely, O-2p states in the valence band maximum and the sp-hybridized states in the conduction band minimum, are mainly involved in the electronic transitions. In addition a number of electronic transitions are predicted by the electronic structure calculations. Experimental photoluminescence results are adequately explained by the ab initio calculations. Further details of the structural and vibrational properties are explained in the manuscript.

Structural change and phase coexistence upon magnetic ordering in the magnetodielectric spinelMn3O4

Cooperative Jahn-Teller ordering is well-known to drive the cubic $Fd\overline{3}m$ to tetragonal $I$4$_1$/$amd$ structural distortion in Mn$_3$O$_4$ at 1170 $^{\circ}$C. Further structural distortion occurs in Mn$_3$O$_4$ upon magnetic ordering at 42 K. Employing high-resolution variable-temperature synchrotron x-ray diffraction we show that tetragonal $I$4$_1$/$amd$ and orthorhombic $Fddd$ phases coexist, with nearly equal fractions, below the N\'eel temperature of Mn$_3$O$_4$. Significant variation of the orthorhombic $a$ and $b$ lattice constants from the tetragonal $a$ lattice constant is observed. Structural phase coexistence in Mn$_3$O$_4$ is attributed to large strains due to the lattice mismatch between the tetragonal $I$4$_1$/$amd$ and the orthorhombic $Fddd$ phases. Strain tensors determined from Rietveld refinement show a highly strained matrix of the $I4_1/amd$ phase that accommodates the nucleated orthorhombic $Fddd$ phase in the phase coexistence regime. A comparison of the deformation observed in Mn$_3$O$_4$ to structural deformations of other magnetic spinels shows that phase coexistence may be a common theme when structural distortions occur below 50 K.

Diffraction Studies of Tungsten Bronze Type Relaxor Ferroelectrics

Ferroelectric materials are essential for modern electronic applications, from consumer electronics to sophisticated technical instruments. Relaxor ferroelectric materials provide the advantage of high dielectric constants over broad temperature ranges not seen in traditional ferroelectrics. Tungsten bronze type compounds have been shown to display a variety of industrially relevant optical and electronic properties amongst others. There is a fundamental relationship between the physical properties displayed by ferroelectrics and the crystal structures in which they form. Of particular interest are compositions and temperatures near phase transition. These are import because near phase transitions, particularly morphotropic phase transitions, electromechanical properties are often dramatically enhanced. [1,2] This work focuses on the structural investigation of the tungsten bronze type relaxor ferroelectric materials in the BaxSr3-xTi1-yZryNb4O15 (0 ≤ x ≤ 3; 0 ≤ y ≤ 1) system. A combination of X-ray, neutron (ToF and constant wavelength) and electron diffraction were employed to map the entire room temperature phase space. In addition, morphotropic phase boundary compositions were determined accurately. Variable temperature synchrotron X-ray diffraction studies were utilised to further explore the phase diagram for non-ambient conditions. Temperature dependent phase transitions were determined and the relationship between composition and transition temperature analysed. Structural models used in this work resulted from Rietveld refinements against powder diffraction data. [3] This work will shed light on new lead free relaxor ferroelectric materials.

Phase behavior and mixed ionic–electronic conductivity of Ba4Sb2O9

The 6H-type perovskite phase Ba4Sb2O9, which decomposes in air below 600K, is found to survive to room temperature in a CO2-free atmosphere. It shows substantial mixed protonic, oxide ionic and electronic conductivity. Compared to Ba4Nb2O9 and Ba4Ta2O9, Ba4Sb2O9 shows higher ionic conductivity due to the relatively easy reducibility of Sb5+, but lower electronic conductivity due to the predominantly n-type conductivity provided by the Sb5+/Sb3+ redox couple which leads to reduced hole concentration under oxidizing conditions. Variable temperature synchrotron X-ray diffraction studies carried out in situ under controlled atmospheres reveal a strong monoclinic distortion below 1150K. The hexagonal to monoclinic transition is slow, does not show second-order behavior, is strongly dependent on atmosphere, and coincides with the loss of ~0.4 molecules of H2O per formula unit of Ba4Sb2O9. All of this suggests an important structural role for protons or hydroxide ions in the monoclinic phase.

A variable temperature synchrotron X-ray diffraction study of the ferroelastic double perovskite Ba2GdMoO6

A study of the magnetic and structural properties of the double perovskite Ba2GdMoO6 has been performed. The crystal structure distorts from the ideal cubic (Fm3m) structure to the tetragonal space group I4/m at 220 K, before undergoing a second distortion to a triclinic system (I1) at 80 K. The phase transition to triclinic symmetry is also evident in magnetic susceptibility measurements. The variable temperature synchrotron powder X-ray diffraction results reveal that Ba2GdMoO6 is ferroelastic, with the onset of ferroelastic domain formation occurring at the cubic-tetragonal phase transition. A number of Rietveld refinement techniques for modelling diffuse scattering from ferroelastic domain boundaries have been explored.

A new phase in stoichiometric Cu6Sn5

The intermetallic compound Cu6Sn5 is a significant microstructural feature of many electronic devices where it is present at the solder–substrate interfaces. The time- and temperature-dependent thermomechanical properties of Cu6Sn5 are dependent on the nature and stability of its crystal structure, which has been shown to exist in at least four variants (η, η′, η6 and η8). This research details an additional newly identified monoclinic-based structure in directly alloyed stoichiometric Cu6Sn5 using variable-temperature synchrotron X-ray diffraction (XRD) and transmission electron microscopy. The phase is associated with a departure from the equilibrium temperature of the polymorphic monoclinic–hexagonal transformation temperature. The new monoclinic phase can be treated as a modulation of four η8-Cu5Sn4 unit cells plus one η′-Cu6Sn5 unit cell. It has been labeled as η4+1 and has cell parameters of a=92.241Å, b=7.311Å, c=9.880Å and β=118.95° determined from electron diffraction patterns. The XRD results could be fitted well to a Rietveld refinement using the new crystal parameters.

The structure, thermal properties and phase transformations of the cubic polymorph of magnesium tetrahydroborate

The structure of the cubic polymorph of magnesium tetrahydroborate (γ-Mg(BH(4))(2)) has been determined in space group Ia3d from a structural database of the isoelectronic compound SiO(2); this has been corroborated by DFT calculations. The structure is found to concur with that recently determined by Filinchuk et al. (Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov and H. Hagemann, Angew. Chem. Int. Ed., 2011, DOI: 10.1002/anie.201100675). The phase transformations and subsequent decomposition of γ-Mg(BH(4))(2) on heating have been ascertained from variable-temperature synchrotron X-ray diffraction data combined with thermogravimetric and mass spectrometry measurements. At ~160 °C, conversion to a disordered variant of the β-Mg(BH(4))(2) phase (denoted as β') is observed along with a further unidentified polymorph. There is evidence of amorphous phases during decomposition but there is no direct crystallographic indication of the existence of Mg(B(12)H(12)) or other intermediate Mg-B-H compounds. MgH(2) and finally Mg are observed in the X-ray diffraction data after decomposition.

Kinetics of the η–η′ transformation in Cu 6Sn 5

Cu 6 Sn 5 is an important intermetallic compound used in lead-free soldering and anode materials for Li-ion batteries. Cu 6 Sn 5 exists in at least two crystal structures, with a transformation from hexagonal η-Cu 6 Sn 5 at temperatures higher than ∼186 °C to monoclinic η′-Cu 6 Sn 5 at lower temperatures. Using variable temperature synchrotron X-ray diffraction analysis, the kinetics of the η–η′ transformation are investigated and a time–temperature transformation diagram is partially developed. The significance of the results in terms of phase and dimensional stability in solder joints are discussed.

Structural Studies of the Ferroelectric Phase Transition in Bi4Ti3O12.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural Studies of the Ferroelectric Phase Transition in Bi4Ti3O12

A variable-temperature synchrotron X-ray diffraction study of the phase transitions in the ferroelectric n = 3 Aurivillius oxide Bi4Ti3O12 is described. At room temperature the structure of Bi4Ti3O12 is orthorhombic in space group B2eb and this continuously transforms to the high-temperature tetragonal I4/mmm structure via an intermediate orthorhombic phase. The possible space groups of this intermediate orthorhombic phase have been identified by using group theory.