High temperature structural studies of SrRhO3

High resolution X-ray powder diffraction studies have shown SrRhO₃ to transform from an orthorhombic Pnma structure at room temperature through an intermediate Imma phase to a tetragonal I4/mcm structure near 800 °C. The orthorhombic Imma phase exists over a very limited temperature range, of less than 20°. The diffraction data suggests the Pnma to Imma transition is continuous and demonstrates that the Imma-I4/mcm transition is first order.     

In situ energy dispersive X-ray diffraction study of iron disilicide thermoelectric materials

The dynamic phase transformation and structure of rapidly solidified Fe_1−xCo_xSi₂ (0.02?x?0.06) thermoelectric materials were in situ investigated under high temperatures and high pressures by energy dispersive X-ray diffraction using synchrotron radiation. The FeSi₂ alloys which solidified as α-Fe₂Si₅ and ε-FeSi eutectic structures, were transformed to the semiconducting β-FeSi₂ phase upon heating by the main reaction α+ε→β and the subsidiary reaction α→β+Si. The low heating rates and Co contents were found to be beneficial for the β phase formation. The decomposition temperature of β→α+ε was weakly dependent on heating rate, but significantly suppressed by the high pressures.     

Intermediate-temperature polymorphic phase transition in KH2PO4: A synchrotron X-ray diffraction study

We have used synchrotron X-ray diffraction to investigate the structural and chemical changes undergone by polycrystalline KH₂PO₄ (KDP) upon heating within the 30-250 °C temperature interval. Our data show evidence of a polymorphic transition at T∼190 °C from the room-temperature tetragonal KDP phase to a new intermediate-temperature monoclinic KDP modification (spacegroup P2₁/m and lattice parameters a=7.590, b=6.209, c=4.530 Å, and β=107.36°). The monoclinic RDP polymorph remains stable upon further heating to 235 °C, and is isomorphic to its RbH₂PO₄ and CsH₂PO₄ counterparts.     

Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na₂Al₂B₂O₇ samples aged for over 3 months, we found that Na₂Al₂B₂O₇ at room temperature is a mixture of two phases with space group and P6₃/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al₂B₂O₇]²⁻_∝ double layers stacking alternatively with Na⁺ ions along the c-direction, but differ at in-plane bond orientations of the BO₃/AlO₄ groups within the double layers: in P6₃/m phase B-O₁/Al-O₁ bonds of the two layers are perfectly aligned, whereas in phase they are twisted by 46.4/41.6° around c-axis against each other. It is also found that a freshly prepared sample contains only the phase, but part of the phase will transfer to P6₃/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.     

In situ high-pressure X-ray diffraction study of densification of a molecular chalcogenide glass

Structural mechanisms of densification of a molecular chalcogenide glass of composition Ge_2.5As_51.25S_46.25 have been studied in situ at pressures ranging from 1 atm to 11 GPa at ambient temperature as well as ex situ on a sample quenched from 12 GPa and ambient temperature using high-energy X-ray diffraction. The X-ray structure factors display a reduction in height of the first sharp diffraction peak and a growth of the principal diffraction peak with a concomitant shift to higher Q-values with increasing pressure. At low pressures of at least up to 5 GPa the densification of the structure primarily involves an increase in the packing of the As₄S₃ molecules. At higher pressures the As₄S₃ molecules break up and reconnect to form a high-density network with increased extended-range ordering at the highest pressure of 11 GPa indicating a structural transition. This high-density network structure relaxes only slightly on decompression indicating that the pressure-induced structural changes are quenchable.     

Pressure-induced polymorphism in enstatite (MgSiO3) at room temperature: clinoenstatite and orthoenstatite

The phase transition of a synthetic clinoenstatite in a diamond-anvil cell has been studied by using Raman spectroscopy at various pressures and room temperature. The phenomena observed in clinoenstatite have been compared with that observed in orthoenstatite. It is found that the pressure-induced phase transitions in the two enstatites are reversible, but with different transition pressures and transition behavior. An analysis of Raman spectra has revealed that the two enstatites have different high-pressure polymorphs. This result suggests that the space group of the high-pressure polymorph of orthoenstatite is not of C2/c, and that orthoenstatite and orthoferrosilite have different transition routes at room temperature and high pressure. The compressional behavior of the high-PC2/c enstatite is also discussed according to the pressure dependences of Raman frequencies.     

Reactions of chlorine and related molecules with graphite intercalation compounds

K-graphite intercalation compounds (GICs), metal chloride-GICs and reduced products of CuCl₂-GICs were allowed to react with chlorine. Decomposition of K-GICs took place through the reaction with chlorine. The extent of the decomposition was found to be dependent not on the gas pressure of the chlorine but on the stage number of the starting GIC. Metal chloride-GICs were unreactive with chlorine, whereas chlorine could react with copper particles generated in the interlayer of graphite through the reduction of CuCl₂-GICs. The reaction products were CuCl and CuCl₂; the former exists as crystals in the interlayer, and the latter can form CuCl₂-GICs. Also reported on are reactions of GICs with chlorobenzene, benzoyl bromide and iodine monochloride.     

High-pressure powder diffraction study of TaO2F

TaO₂F, with a ReO₃-type structure, has been studied at up to 12.8 GPa using monochromatic synchrotron powder diffraction and diamond anvil cells. Two-phase transitions at ∼0.7 and 4 GPa were observed on compression. Below ∼0.7 GPa the cubic material was found to have a bulk modulus (K₀) of 36(3) GPa (K_p fixed at 4.0), similar to that reported for NbO₂F but much smaller than that of ReO₃. Immediately above 0.7 GPa on compression, the diffraction data were not fully consistent with a VF₃-type structure as previously proposed for NbO₂F. On decompression, the data between 8 and 4 GPa could be satisfactorily attributed to a single R-3c phase with a VF₃-type structure and an average bulk modulus of 60(2) GPa.     

On the criteria of formation and lattice distortion of perovskite-type complex halides

The famous Goldschmidt's tolerance factor gives us a necessary but not sufficient condition for the formation of perovskite-type compounds (ABX₃). In this work, computerized data analysis has been used to find some complementary criteria for the formation and lattice distortion of perovskite-type complex halides. It has been found that the radius ratio (R_A/R_X) and (R_B/R_X), affecting the stability of BX₆ octahedra and AX₁₂ cubo-octahedra (they are basic units of perovskite structure), are also dominating factors for the formation and lattice distortion of perovskite-type compounds. Besides, it has been found that the transition between the perovskite structure (with corner-sharing BX₆ octahedra) to BaNiO₃ structure (with face-sharing BX₆ octahedra) can be predicted by a criterion based on the relative magnitude of ionic radii and electronegativity. Based on multivariate data analysis, several complementary criteria for the formation and lattice distortion of perovskite-type complex halides have been obtained, and some empirical equations expressing the relationships between the ionic radii (R_A,R_B,R_X) and the lattice constants of perovskite-type complex halides have been found. The physical meaning of these empirical relationships has been discussed based on Pauling's rules of the crystal lattice stability of complex ionic compounds.     

X-ray diffraction and infrared spectroscopy of monazite-structured CaSO4 at high pressures: Implications for shocked anhydrite

X-ray diffraction and infrared spectroscopy of CaSO₄ are conducted to pressures of 28 and 25 GPa, respectively. A reversible phase transition to the monoclinic monazite-structure occurs gradually between 2 and ∼5 GPa with a highly pressure-dependent volume change of ∼6-8%. A second-order fit of the X-ray data to the Birch-Murnaghan equation of state yields a bulk modulus (K) of 151.2 (±21.4) GPa for the high-pressure monoclinic phase. In the high-pressure infrared spectrum, the infrared-active asymmetric stretching and bending vibrations of the sulfate tetrahedra split at the phase transition, in accord with the results of factor group analysis. Additionally, the tetrahedral symmetric stretching vibration, which is weak in the anhydrite phase, becomes strongly resolved at the transition to the monazite structure. The infrared results indicate that the sulfate tetrahedra are more distorted in the monazite-structured phase than in anhydrite. Kinetic calculations indicate that the anhydrite to monazite transformation may generate the phase transition observed near 30 GPa under shock loading in CaSO₄. Our results indicate that the anhydrite- and monazite-structured phases may be the only phases that occur under shock loading of CaSO₄ to pressures in excess of 100 GPa.     

Low temperature structural phase transition in hafnium and zirconium tetrafluoride trihydrates

From time-differential perturbed angular correlation (TDPAC) measurements, the monoclinic and triclinic crystal structures in hafnium and zirconium tetrafluoride trihydrates are found to be present simultaneously in both the compounds. From previous TDPAC and XRD investigations, a monoclinic crystal structure for HfF₄·3H₂O but, for its analogues zirconium compound, a triclinic structure was reported. Contrary to earlier reports, the triclinic fraction in HfF₄·3H₂O is found to be maximum (80%) at room temperature. In fact, the triclinic crystal structure of HfF₄·3H₂O is reported here which was not known prior to this report. In ZrF₄·3H₂O, a strong signal (80–90%) for the triclinic structure is found at room temperature while the monoclinic fraction appears as a weak signal (10–15%). Structural phase transitions in these trihydrate compounds have been observed in the temperature range 298–333 K.     

Importance of hydrogen bonding for second harmonic generation effect: X-ray diffraction and DFT study on S-benzyl isothiouronium chloride

X-ray diffraction study of the nonlinear optical (NLO) material S-benzyl isothiouronium chloride (C₈H₁₁N₂SCl) (SBTC) is reported for the first time. The single crystal of SBTC is orthorhombic with space group Pbca. SBTC exhibits second-order NLO susceptibility, and this study shows that hydrogen bonding is, in part, responsible for this. The present work shows that C-H?Cl and N-H?Cl hydrogen bonds direct the nature of the three-dimensional lattice. Such intermolecular interactions help to extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Density functional theory (DFT) calculation and atom-in-molecule (AIM) analysis has been carried out to study the nature of hydrogen involved in the SBTC complex.     

Preparation and thermal decomposition studies of l-tyrosine intercalated MgAl, NiAl and ZnAl layered double hydroxides

l-Tyrosine (represented as l-Tyr) intercalated MgAl, NiAl and ZnAl layered double hydroxides (LDHs) have been obtained by the method of coprecipitation. In situ FT-IR, in situ HT-XRD and TG-DTA measurements allow a detailed understanding of the thermal decomposition process for the three intercalated composites. In situ HT-XRD reveals that the layered structure of l-Tyr/MgAl-LDH collapses completely at 450 °C with the first appearance of reflections of a cubic MgO phase, while the corresponding temperature for l-Tyr/NiAl-LDH is some 50 °C lower. In contrast, there is a major structural change in l-Tyr/ZnAl-LDH at 250 °C as shown by the disappearance of its (0 0 6) and (0 0 9) reflections at this temperature accompanied by the appearance of reflections of ZnO. In situ FT-IR experiments give information about the decomposition of the interlayer -Tyr ions. The decomposition temperature of l-Tyr in the ZnAl host is about 50 °C lower than the corresponding values for the MgAl and NiAl hosts. TG-DTA curves show a significant weight loss step (170-260 °C) in l-Tyr/ZnAl-LDH which is due to the dehydroxylation of the host layers, with a corresponding weak endothermic peak at 252 °C. This temperature range is much lower than that observed for MgAl and NiAl hosts, indicating that the ZnAl-LDH layers are relatively unstable. The data indicate that the order of thermal stability of the three intercalates is: l-Tyr/MgAl-LDH > l-Tyr/NiAl-LDH > l-Tyr/ZnAl-LDH.     

Thermal trapped iron(II) high spin state investigated by X-ray diffraction

The possibilities to trap by flash cooling the high spin (HS) state of iron(II) in the [Fe(PM-BiA)₂(NCS)₂] complex have been investigated by X-ray diffraction. This study reveals that trapping the HS state is possible under some conditions depending on the final temperature. If the latter is lower than the T(LIESST) temperature, the HS→LS (low spin) relaxation is slow enough to determine the trapped HS crystal structure by X-ray diffraction. The crystal structure of this complex in the 30 K trapped HS state shows differences from either the room temperature (HS) or the 30 K (LS) crystal structures, as for example differences in the strength of the S?H-C hydrogen bond like intermolecular interaction or the π-π interactions, known to play a crucial role in this compound for the propagation of the change in spin at the spin crossover (SCO), i.e. the cooperativity. The differences in intermolecular interactions are directly linked to the differences between the crystallographic unit cell modifications induced by pure thermal effects and those induced by the SCO.     

X-ray and AFM studies of polydisperse TiO2 (anatase) particles

Titanium dioxide (TiO₂) materials of a high chemical purity, as-prepared by the thermal hydrolysis, as well as subsequently modified by adsorption of different metal cations (Fe³⁺, Co²⁺, Cu²⁺), have been investigated by the X-ray diffraction, X-ray fluorescence and AFM microscopy methods. All TiO₂ powders have a fine-dispersated anatase structure and consist of grown together nanocrystallites of ∼8-17 nm. TiO₂ particles, usually ranging from 100 to 600 nm, show the ability to form large agglomerates, up to 2 μm in size. Contrary to the pure anatase, metal-modified TiO₂ particles possess a positive charge on their surface and can be lifted away by the AFM tip from the substrate surface during the scanning. This effect is mostly pronounced for the Fe-modified TiO₂ sample, where particles up to 250 nm are removed. The possible interaction mechanisms between different TiO₂ particles and the silicon tip are discussed. The electrostatic force has been found to play an essential role in the sample-tip interaction processes, and its value depends on the type of metal cation used.     

Powder-neutron diffraction studies of geometric isotope and hydrogen-bonding effects in β-CrOOH

Powder-neutron diffraction studies have been done on β-CrOOH and β-CrOOD to confirm whether or not the reported exceptionally large geometric isotope effect is real, and also to study hydrogen-bonding effects in β-CrOOH compared to the prototype CrO₂. The R(O?O) distance in the H-compound at room temperature is confirmed to be 2.472(3) Å, but in the D-compound it is significantly shorter, 2.518(3) Å, than in the earlier work. The resulting magnitude of the geometric isotope effect, 0.046(4) Å, is thus not as exceptionally large as earlier reported, but is still within the maximum range. Structural comparison with CrO₂ clarified the hydrogen-bonding effect in β-CrOOH: the formation of hydrogen bonds brings CrO₆ closer to a regular octahedron.     

Tunable luminescent properties by adjusting the Sr/Ba ratio in Sr1.97−xBaxMgSi2O7:Eu0.01, Dy0.02 phosphors

The long afterglow phosphors Sr_1.97−xBa_xMgSi₂O₇:Eu²⁺_0.01, Dy³⁺_0.02 (x=0, 0.4, 0.8, 1.2, 1.6 and 1.97) were synthesized via high temperature solid-state reaction. The phase identification reveals that the crystal plane spacing becomes greater with the decrease in the Sr/Ba ratio. Phase transition occurs when x=1.97. A nonlinear relationship between the emission peak and the crystal plane spacing is obtained with the decrease of the Sr/Ba ratio. This ascribes to the splitting of the 5d level of the Eu²⁺ and the change of the crystal field strength. The duration of the afterglow becomes shorter with the decrease of the Sr/Ba ratio. It may ascribe to deeper trap depth, lower trap concentration and the embarrassment of the transfer of carriers.     

Tetrahedral coordination for Zn in hexacyanometallates: Structures of Zn3A2[M(CN)6]2·xH2O with A=K, Rb, Cs and M=Ru, Os

Zinc hexacyanoruthenate (II) and hexacyanoosmate (II) were prepared and studied from X-ray diffraction (XRD), infrared (IR), and thermogravimetric (TG) data. These compounds were found to be isomorphous with the iron analogues, crystallizing with a rhombohedral unit cell (R−3c space group), where the zinc atom has tetrahedral coordination to N ends of CN groups. For Cs, compounds with formula unit ZnCs₂[M(CN)₆] and a cubic unit cell (Fm−3m) were also obtained. The crystal structures for the eight compositions were refined from the corresponding X-ray powder diffraction patterns using the Rietveld method. Related to the tetrahedral coordination for the Zn atom, the rhombohedral phase has a porous framework with ellipsoidal cavities of about 12.5×9×8 Å, communicated by elliptical windows of ∼5 Å. Within these cavities the exchangeable alkali metal ions are found. The filling of the cavity volume is completed with water molecules. IR spectrum senses certain charge delocalization from the inner metal, through the π-back donation mechanism. For Os compounds this effect is particularly pronounced, related to a more diffuse d orbitals for this metal.     

Formability of ABO3 cubic perovskites

In this study, 223 binary oxide systems (of which, 34 systems can form cubic perovskites) are collected to explore the regularity of cubic perovskites formability. It is found that the octahedral factor (r_B/r_O) take the same important role as the tolerance factor (t) to form cubic perovskites in complex oxide system. Regularities governing cubic perovskites formability are obtained by using empirical structure map constructed by these two parameters, on this structure map, sample points representing systems of forming (cubic structure) and non-forming are distributed in distinctively different regions. Prediction criteria for the formability of cubic perovskites are squeezed out, which may be applied to design new substrate or buffer materials with cubic perovskite structure in compound semiconductor epitaxy.