In situ X-ray observation of phase transformation in Fe2O3 at high pressures and high temperatures

A laser-heated sample in a diamond anvil cell and synchrotron X-ray radiation was used to carry out structural characterization of the phase transformation of Fe₂O₃ at high pressures (30-96 GPa) and high temperature. The Rh₂O₃(II) (or orthorhombic perovskite) structure transforms to a new phase, which exhibits X-ray diffraction data that are indicative of a CaIrO₃-type structure. The CaIrO₃-type structure exhibited an orthorhombic symmetry (space group: Cmcm) that was stable at temperatures of 1200-2800 K and pressure of 96 GPa (the highest pressure used). Unambiguous assignment of such a structure requires experimental evidence for the presence of two Fe species. Based on the equation of state of gold, the phase boundary of the CaIrO₃-type phase transformation was P (GPa)=59+0.0022×(T−1200) (K).     

Structural phase transitions at high-temperature in double perovskite Sr2GdRuO6

The crystal structure evolution of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K≤T≤1273 K. Powder X-ray diffraction measurements at room temperature and Rietveld analysis show that this compounds crystallizes in a monoclinic perovskite-type structure with P2₁/n (#14) space group and the 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites, with lattice parameters a=5.81032(8) Å, b=5.82341(4) Å, c=8.21939(7) Å, V=278.11(6) Å³ and angle β=90.311(2)^o. The high-temperature analysis shows that this material suffers two-phase transitions. At 373 K it adopts a monoclinic perovskite structure with I2/m space group, and lattice parameters a=5.81383(2) Å, b=5.82526(4) Å, c=8.22486(1) Å, V=278.56(2) Å³ and angle β=90.28(2)^o. Above of 773 K, it suffers a phase transition from monoclinic I2/m to tetragonal I4/m, with lattice parameters a=5.84779(1) Å, c=8.27261(1) Å, V=282.89(5) Å³ and angle β=90.02(9)^o. The high-temperature phase transition from monoclinic I2/m to tetragonal I4/m is characterized by strongly anisotropic displacements of the anions.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.     

X-ray study of phase transitions in the system of solid solutions K2Pb4Nb10O30-Na2Pb4Nb10O30-K6W4Nb6O30

Boundaries of morphotropic phase transitions region in the system of solid solutions K₂Pb₄Nb₁₀O₃₀-Na₂Pb₄Nb₁₀O₃₀-K₆W₄Nb₆O₃₀ with the structure of the tetragonal tungsten bronze have been specified. Presence of the second morphotropic phase transition, perpendicular to the first one has been revealed. The temperature dependences of the structural parameters of some compounds have been investigated. The compounds with high values of Curie temperatures and working temperatures have been obtained.     

In situ pressure-induced solid-state amorphization in Sm2(MoO4)3, Eu2(MoO4)3 and Gd2(MoO4)3 crystals: chemical decomposition scenario

The effect of pressure on the phase transformations in Sm₂(MoO₄)₃, Gd₂(MoO₄)₃ and Eu₂(MoO₄)₃ crystals has been studied in situ using synchrotron radiation. All three isostructural compounds undergo a structural phase transition at 2.2-2.8 GPa to a new phase, which is interpreted as a possible precursor of amorphization. Amorphization in these crystals occurs irreversibly over a wide pressure range, and its mechanism, interpreted as a chemical decomposition, is found to be weakly affected by the degree of hydrostaticity.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

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.     

Critical phenomena of amorphous Nd4(Fe0.75Cr0.25)77.5B18.5 alloys

The magnetic behavior of amorphous Nd₄(Fe_0.75Cr_0.25)_77.5B_18.5 alloys was investigated in the critical region. The Curie temperature T_C and critical exponents β, γ and δ are found to be 141 K, 0.45±0.02, 1.64±0.08 and 4.66±0.10, respectively. The data are fitted to a magnetic equation of state characteristic of a second-order phase transition over a rather wide range of temperatures both above and below T_C. It is noted that the values of the exponents are in disagreement with those derived for a three-dimensional Heisenberg ferromagnet and show an enhancement. This anomalous critical behavior may originate from magnetic inhomogeneity.     

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.     

Electrical conductivity and amorphization of Sc2(WO4)3 at high pressures and temperatures

Impedance spectroscopy measurements and synchrotron X-ray diffraction studies of Sc₂(WO₄)₃ at 400°C have been carried out as a function of pressure up to 4.4 GPa. Ionic conductivity shows normal decrease with increase in pressure up to 2.9 GPa, but then increases at higher pressures. The XRD results show that Sc₂(WO₄)₃ undergoes pressure-induced amorphization at pressures coincident with the reversal in conductivity behavior. The loss of crystal structure at high pressure is consistent with growing evidence of pressure-induced amorphization in negative thermal expansion materials, such as Sc₂(WO₄)₃. The increase in conductivity in the amorphized state is interpreted as the result of an increase in structural entropy and a concomitant reduction of energy barriers for ionic transport.     

X-ray diffraction studies on crystallite size evolution of CoFe2O4 nanoparticles prepared using mechanical alloying and sintering

Nanosized cobalt ferrite spinel particles have been prepared by using mechanically alloyed nanoparticles. The effects of various preparation parameters on the crystallite size of cobalt ferrite which includes milling time; ball-to powder weight ratio (BPR) and sintering temperature, were studied using X-ray diffractometer (XRD). Scherrer's equation was used to study the crystallite size evolution of the as-prepared materials. The results of the as-milled sample revealed that both milling time and BPR plays a role in determining the crystallite size of the milled powder. However, where sintering is involved, the sintering temperature results in grain growth, and thus plays a dominant role in determining the final crystallite size of the samples sintered at higher temperature (above 900 °C). From the vibrating-sample magnetometer (VSM) measurement it was observed that the coercivity of the as-milled samples without sintering is almost negligible, which is a type characteristic of superparamagnetic material. However, for the sintered samples, the saturation increases while coercivity decreases with increases sintering temperature.     

Synthesis of Al–Cr decagonal quasicrystal nanoparticles and their temperature of phase transformation to stable crystal phase

The synthesis of Al–Cr single quasicrystal (QC) nanoparticles of the decagonal phase was achieved by introducing an advanced gas flow evaporation method. By obtaining successive electron diffraction patterns for single-QC nanoparticles, the phase transformation temperature of a single-QC nanoparticle was determined to be 700 °C. It was also determined that part of the QC nanoparticle decomposed into hex-Al₈Cr₅ and Al during the phase transformation. Since the grain growth did not occur during the phase transformation in the present experiment, the inherent phase transformation temperature could be measured.     

Photocatalytic abilities of gel-derived P-doped TiO2

P-doped TiO₂ nanoparticles were synthesized through hydrolysis and condensation of Ti(OC₂H₅)₄ with H₃PO₄ additions. Effects of [H₃PO₄]/[Ti(OC₂H₅)₄] molar ratios on the anatase-to-rutile phase transformation, crystallite sizes, surface areas, and photocatalytic abilities of the gel-derived P-doped TiO₂ were investigated. The P-doped TiO₂ nanoparticles prepared by [H₃PO₄]/[Ti(OC₂H₅)₄]=0.03 were composed of anatase monophase even at 900 ^oC and possessed very strong photocatalytic ability. Kinetic studies on the P-doped TiO₂ to photocatalytically decompose methylene blue under irradiation of 365 nm UV light found that the P-doped TiO₂ prepared by [H₃PO₄]/[Ti(OC₂H₅)₄]=0.03 and calcined at 800 ^oC had the specific reaction rates, at 25 °C, k_A,m=0.76 m³/(kg min) (based on the mass of P-doped TiO₂) and k_A,BET=46.2×10⁻⁶ m/min (based on the BET surface area of P-doped TiO₂), which is superior to the performance of a commercial product, P25 (k_A,m=0.22 m³/(kg min) and k_A,BET=4.8×10⁻⁶ m/min).     

Determination of kinetic parameters of thermally stimulated luminescence of Cu-doped BaSO4

Studies of thermally stimulated luminescence (TSL) of doped BaSO₄ with Cu activator have been carried out. The polycrystalline sample of Cu-doped BaSO₄ is prepared by recrystallisation method. The characterization and elemental analysis of BaSO₄:Cu compound are carried out by X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). The compound has orthorhombic structure at room temperature. The TSL studies of Cu-doped BaSO₄ sample show two glow peaks at 160 and 227 °C. Comparison of TL intensity of the most intensive glow peak of Cu- and Mn-doped BaSO₄ compounds with that of undoped BaSO₄ shows that addition of Cu and Mn impurity in BaSO₄ compound enhances the TL intensity by about 9 and 3 times, respectively. Among the studied samples, namely undoped, Cu- and Mn-doped BaSO₄, Cu-doped BaSO₄ is found to be the most sensitive. The trap parameters namely order of kinetics (b), activation energy (E), frequency factor (s) and Balarin parameter (γ) associated with the most intensive glow peak, i.e. the 227 °C glow peak of BaSO₄:Cu phosphor were determined by using isothermal decay method and glow curve shape (Chen's) method and these parameters are in good agreement.     

Phase transition in (1−x)Ba(Y1/2Nb1/2) O3-xPbTiO3 solid solution with two morphotropic phase boundaries studied by Raman spectroscopy

New (1−x)Ba(Y_1/2Nb_1/2)O₃-xPbTiO₃ ceramics have been synthesized by conventional mixed-oxide process. Two morphotropic phase boundaries (MPBs) corresponding to the rhombohedral-orthorhombic and orthorhombic-tetragonal phases are observed with increasing PT. The appearance of E(2TO₁), E(2TO₂), silent, R_l and E(4LO) modes indicates the presence of an orthorhombic symmetry. Remarkable variation with two MPBs appears in the whole frequency region, where the rhombohedral modes change into the tetragonal ones in the system.     

Magnetic properties of the ferrimagnetic cobaltite CaBaCo4O7

The magnetic properties of the ferrimagnetic cobaltite CaBaCo₄O₇ are systematically investigated. We find that the susceptibility exhibits a downward deviation below ∼360 K, suggesting the occurrence of short-range magnetic correlations at a temperature well above T_C. The effective moment is determined to be ., which is consistent with that expected for the Co²⁺/Co³⁺ high spin species. Using a criterion given by Banerjee [Phys. Lett. 12 (1964) 16], we demonstrate that the paramagnetic to ferrimagnetic transition in CaBaCo₄O₇ has a first order character.     

Tuning hydrogen storage properties and reactivity: Investigation of the LiBH4-NaAlH4 system

Tuning the hydrogen storage properties of complex metal hydrides is of vast interest. Here, we investigate the hydrogen release and uptake pathways for a reactive hydride composite, LiBH₄−NaAlH₄ utilizing in situ synchrotron radiation powder X-ray diffraction experiments. Sodium alanate transforms to sodium borohydride via a metathesis reaction during ball milling or by heating at T∼95 °C. NaBH₄ decomposes at ∼340 °C in dynamic vacuum, apparently directly to solid amorphous boron and hydrogen and sodium gas and the latter two elements are lost from the sample. Under other conditions, T=400 °C and p(H₂)=∼1 bar, NaBH₄ only partly decomposes to B and NaH. On the other hand, formation of LiAl is facilitated by dynamic vacuum conditions, which gives access to the full hydrogen contents in the LiBH₄−NaAlH₄ system. Formation of AlB₂ is observed (T∼450 °C) and other phases, possibly AlB_x or Al_1−xLi_xB₂, were observed for the more Li-rich samples. This may open new routes to the stabilization of boron in the solid state in the dehydrogenated state, which is a challenging and important issue for hydrogen storage systems based on borohydrides.     

Pressure-induced phase transition in defect Chalcopyrites HgAl2Se4 and CdAl2S4

Results of angle dispersive X-ray diffraction (ADXRD) measurements on the defect chalcopyrites (DCP), HgAl₂Se₄ and CdAl₂S₄ up to 22.2 and 34 GPa, respectively, are reported. The ambient tetragonal phase is retained in HgAl₂Se₄ and CdAl₂S₄ up to 13 and 9 GPa respectively. The values of the bulk modulus estimated from the Equation of State is 66(1.5) and 44.6(1) GPa for HgAl₂Se₄ and CdAl₂S₄ in the chalcopyrite phase. At higher pressure a disordered rock-salt structure and on pressure release a disordered zinc blende structure with broad X-ray diffraction lines are observed as is the case for several defect chalcopyrites.     

X-ray diffraction, Cs and H NMR and thermal studies of CdZrCs1.5(H3O)0.5(C2O4)4·xH2O displaying Cs and water dynamic behavior

A novel mixed cadmium zirconium cesium oxalate with an open architecture has been synthesized from precipitation methods at room pressure. It crystallizes with an hexagonal symmetry, space group P3₁12 (no. 151), a=9.105(5) Å, c=23.656(5) Å, V=1698(1) Å³ and Z=3. The structure displays a [CdZr(C₂O₄)₄]²⁻ helicoidal framework built from CdO₈ and ZrO₈ square-based antiprisms connected through bichelating oxalates, which generates channels along different directions. Cesium cations, hydronium ions and water molecules are located inside the voids of the anionic framework. They exhibit a dynamic disorder which has been further investigated by ¹H and ¹³³Cs solid-state NMR. Moreover a phase transition depending both upon ambient temperature and water vapor pressure was evidenced for the title compound. The thermal decomposition has been studied in situ by temperature-dependent X-ray diffraction and thermogravimetry. The final product is a mixture of cadmium oxide, zirconium oxide and cesium carbonate.