Variable-temperature Synchrotron X-ray 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.

Anomalous and colossal thermal expansion, photoluminescence, and dielectric properties in lead halide-layered perovskites with cyclohexylammonium and cyclopentylammonium cations

SummaryA detailed study of lead halide-layered perovskites with general formula A2PbX4 (where A is cyclohexylammonium (CHA) or cyclopentylammonium (CPA) cation and X is Cl− or Br− anion) is presented. Using variable temperature synchrotron X-ray powder diffraction, we observe that these compounds exhibit diverse crystal structures above room temperature. Very interestingly, we report some unconventional thermomechanical responses such as uniaxial negative thermal expansion and colossal positive thermal expansion in a perpendicular direction. For the polymorphs of (CHA)2PbBr4, the volumetric thermal expansion coefficient is among the highest reported for any extended inorganic crystalline solid, reaching 480 MK−1. The phase transitions are confirmed by calorimetry and dielectric measurements, where the dielectric versus temperature curves show anomalies related with the order-disorder phase transitions. In addition, these compounds exhibit a broad photoluminescence (PL) emission with a large Stokes shift, which is related with an exciton PL emission.

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.

Structural Features in Some Layered Hybrid Copper Chloride Perovskites: ACuCl4 or A2CuCl4.

We present three new hybrid copper(II) chloride layered perovskites of generic composition ACuCl4 or A2CuCl4, which exhibit three distinct structure types. (m-PdH2)CuCl4 (m-PdH22+ = protonated m-phenylenediamine) adopts a Dion-Jacobson (DJ)-like layered perovskite structure type and exhibits a very large axial thermal contraction effect upon heating, as revealed via variable-temperature synchrotron X-ray powder diffraction (SXRD). This can be attributed to the contraction of an interlayer block, via a slight repositioning of the m-PdH22+ moiety. (3-AbaH)2CuCl4 (3-AbaH+ = protonated 3-aminobenzoic acid) and (4-AbaH)2CuCl4 (4-AbaH+ = protonated 4-aminobenzoic acid) possess the same generic formula as Ruddlesden-Popper (RP) layered perovskites, A2BX4, but adopt different structures. (4-AbaH)2CuCl4 adopts a near-staggered structure type, whereas (3-AbaH)2CuCl4 adopts a near-eclipsed structure type, which resembles the DJ rather than the RP family. (3-AbaH)2CuCl4 also displays static disorder of the [CuCl4]∞ layers. The crystal structures of each are discussed in terms of the differing nature of the templating molecular species, and these are compared to related layered perovskites. Preliminary magnetic measurements are reported, suggesting dominant ferromagnetic interactions.

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.

Characterisation of oxygen defects and nitrogen impurities in TiO2 photocatalysts using variable-temperature X-ray powder diffraction

TiO2-based powder materials have been widely studied as efficient photocatalysts for water splitting due to their low cost, photo-responsivity, earthly abundance, chemical and thermal stability, etc. In particular, the recent breakthrough of nitrogen-doped TiO2, which enhances the presence of structural defects and dopant impurities at elevated temperatures, exhibits an impressive visible-light absorption for photocatalytic activity. Although their electronic and optical properties have been extensively studied, the structure-activity relationship and photocatalytic mechanism remain ambiguous. Herein, we report an in-depth structural study of rutile, anatase and mixed phases (commercial P25) with and without nitrogen-doping by variable-temperature synchrotron X-ray powder diffraction. We report that an unusual anisotropic thermal expansion of the anatase phase can reveal the intimate relationship between sub-surface oxygen vacancies, nitrogen-doping level and photocatalytic activity. For highly doped anatase, a new cubic titanium oxynitride phase is also identified which provides important information on the fundamental shift in absorption wavelength, leading to excellent photocatalysis using visible light.

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.

Crystal structure, magneto-structural correlation, thermal and electrical studies of an imidazolium halometallate molten salt: (trimim)[FeCl4

A novel imidazolium halometallate molten salt with formula (trimim)[FeCl4] (trimim: 1,2,3-trimethylimidazolium) was synthetized and studied with structural and physico-chemical characterization. Variable-temperature synchrotron X-ray powder diffraction (SXPD) from 100 to 400 K revealed two structural transitions at 200 and 300 K. Three different crystal structures were determined combining single crystal X-ray diffraction (SCXD), neutron powder diffraction (NPD), and SXPD. From 100 to 200 K, the compound exhibits a monoclinic crystal structure with space group P21/c. At 200 K, the former crystal system and space group are retained, but a disorder in the organic cations is introduced. Above 300 K, the structure transits to the orthorhombic space group Pbcn, retaining the crystallinity up to 400 K. The study of the thermal expansion process in this temperature range showed anisotropically evolving cell parameters with an axial negative thermal expansion. Such an induction occurs immediately after the crystal phase transition due to the translational and reorientational dynamic displacements of the imidazolium cation within the crystal building. Electrochemical impedance spectroscopy (EIS) demonstrated that this motion implies a high and stable solid-state ionic conduction (range from 4 × 10−6 S cm−1 at room temperature to 5.5 × 10−5 S cm−1 at 400 K). In addition, magnetization and heat capacity measurements proved the presence of a three-dimensional antiferromagnetic ordering below 3 K. The magnetic structure, determined by neutron powder diffraction, corresponds to ferromagnetic chains along the a-axis, which are antiferromagnetically coupled to the nearest neighboring chains through an intricate network of superexchange pathways, in agreement with the magnetometry measurements.

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.

A Variable Temperature Synchrotron X-ray Diffraction Study of Colossal Magnetoresistant NdMnAsO0.95F0.05.

The recent discovery of high temperature superconductivity in Fe arsenides has invigorated research into transition metal pnictides. Colossal magnetoresistance (CMR) has recently been reported for NdMnAsO1-xFx for x = 0.05–0.08, with a maximum magnetoresistance achieved at low temperature (MR9T(3 K)) = −95%). This appears to be a novel mechanism of CMR, which is as a result of a second order phase transition in field from an insulating antiferromagnet to a semiconducting paramagnet. Here we report a variable temperature synchrotron X-ray powder diffraction study of the CMR oxypnictide NdMnAsO0.95F0.05 between 4 K–290 K. An excellent fit to the tetragonal unit cell with space group P4/nmm is obtained over the entire temperature range, with no change in crystal structure detected down to 4 K. A coupling of the lattice and magnetic order is observed, where subtle discontinuities in the temperature variation of a and the c/a ratio are apparent as the Nd spins order antiferromagnetically and the Mn moments reorient into the basal plane at TSR. The results suggest that very small changes in lattice parameters effect the coupling between lattice, electronic and magnetic degrees of freedom.

Coordination Site Disorder in Spinel-Type LiMnTiO4

LiMnTiO4 was prepared through solid-state syntheses employing different heating and cooling regimes. Synchrotron X-ray and neutron powder diffraction data found quenched LiMnTiO4 to form as single phase disordered spinel (space group Fd3̅m), whereas slowly cooled LiMnTiO4 underwent partial phase transition from Fd3̅m to P4332. The phase behavior of quenched and slowly cooled LiMnTiO4 was confirmed through variable-temperature synchrotron X-ray and neutron powder diffraction measurements. The distribution of Li between tetrahedral and octahedral sites was determined from diffraction data. Analysis of the Mn/Ti distribution in addition required Mn and Ti K-edge X-ray absorption near-edge structure spectra. These revealed the presence of Mn(3+) in primarily octahedral and Ti(4+) in octahedral and tetrahedral environments, with very slight variations depending on the synthesis conditions. Magnetic measurements indicated the dominance of antiferromagnetic interactions in both the slowly cooled and quenched samples below 4.5 K.

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.