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).     

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.     

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.     

Phase transitions in BaCeO3: neutron diffraction and Raman studies

Polycrystalline samples and small single crystals of the perovskite BaCeO₃ were studied by neutron diffraction and Raman spectrometry between 300 and 1200 K. The controversy about the phase transitions originally deduced from our previous Raman study and those observed since by neutron diffraction by Knight has stimulated this work. Pretransitional effects which are detected by Raman much before long-range ordering takes place can partly explain the above disagreement. A continuous monitoring of the structural changes by neutron diffraction and by Raman spectroscopy including polarization analysis has allowed discussion of the transition mechanisms: The first transition Pnma–Imma takes place at 573 K and is of second order. Although some modes soften when the temperature is raised as in many of these perovskite compounds the transition is likely partly displacive partly order–disorder. The Raman modes which disappear transform in modes at the X point of the Brillouin zone of the Imma phase. The second transition Imma–R c takes place at 673 K and is first order. The last transition R c–Pm3m occurs above 1200 K and the transition temperature which can be deduced by extrapolation to zero Raman intensity is in good agreement with neutron results. This second order transition is progressive and begins at about 400 K, the intermediate R c structure appearing as an attempt for slowing down the structural evolution toward the cubic perovskite form.     

High pressure X-ray diffraction study of ReS2

The high-pressure behavior of rhenium disulfide (ReS₂) has been investigated to 51.0 GPa by in situ synchrotron X-ray diffraction in a diamond anvil cell at room temperature. The results demonstrate that the ReS₂ triclinic phase is stable up to 11.3 GPa, at which pressure the ReS₂ transforms to a new high-pressure phase, which is tentatively identified with a hexagonal lattice in space group P6?m2. The high-pressure phase is stable up to the highest pressure in this study (51.0 GPa) and not quenchable upon decompression to ambient pressure. The compressibility of the triclinic phase exhibits anisotropy, meaning that it is more compressive along interlayer directions than intralayer directions, which demonstrates the properties of the weak interlayer van der Waals interactions and the strong intralayer covalent bonds. The largest change in the unit cell angles with increasing pressures is the increase of β, which indicates a rotation of the sulfur atoms around the rhenium atoms during the compression. Fitting the experimental data of the triclinic phase to the third-order Birch-Murnaghan EOS yields a bulk modulus of K_OT=23±4 GPa with its pressure derivative K_OT′= 29±8, and the second-order yields K_OT=49±3 GPa.     

High pressure Raman spectroscopy of spinel-type ferrite ZnFe2O4

An in-situ Raman spectroscopic study was conducted to explore the pressure induced phase transformation of spinel-type ferrite ZnFe₂O₄. Results indicate that ferrite ZnFe₂O₄ initially transforms to an orthorhombic structure phase (CaFe₂O₄-polymorph) at a pressure of 24.6 GPa. Such a phase transformation is complete at 34.2 GPa, and continuously remains stable to the peak pressure of 61.9 GPa. The coexistence of the two phases over a wide range of pressure implies a sluggish mechanism upon the spinel-to-orthorhombic phase transition. Upon release of pressure, the high pressure ZnFe₂O₄ polymorph is quenchable at ambient conditions.     

Electrical and thermal studies in the commensurate incommensurate phase region of (NH4)2ZnCl4 single crystal

DC electrical conductivity for a virgin and poled annealed (NH₄)₂ZnCl₄b-axis single crystal shows a defect controlled property. A Schottky mechanism is a probable mechanism of conduction in regions of strong structural transitions. The rise of conductivity in the incommensurate and paraelectric phases is linked to an increase in discommensurations density. The activation energies (ΔE) in the three phases region were calculated. DTA measurements shows that the crystal is stable up to 200 °C and the phase transition temperatures were observed at 42, 94.8 and 137 °C. The effective activation energy (E_e) was obtained using Kissinger and Mahadevan equations. It was found to be equal to 0.49 eV. This correlates with the value obtained through DC conductivity.     

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.     

Structure and phase transitions in chloroantimonate(V) crystals: [(C2H5)3NH]SbCl6 and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl

New triethylammonium salts: [(C₂H₅)₃NH]SbCl₆ (TCA) and [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P2₁/n and P2₁/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C₂H₅)₃NH]SbCl₆ consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C₂H₅)₃NH]SbCl₆·1/2[(C₂H₅)₃NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.     

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.     

Decoupling of the order-disorder and displacive contributions to the phase transition in NH4H(ClH2CCOO)2

The effects of hydrostatic pressure and substitution of Rb⁺for the ammonium cations on the ferroelectric phase transition temperature in NH₄H(ClH₂CCOO)₂ have been studied by electric permittivity measurements. The transition temperature (T_c) decreases with increasing pressure up to 800 MPa and the pressure coefficient dT_c/dp=−1.4×10⁻² [K/MPa] has been experimentally determined. The substitution of Rb⁺ for the ammonium cations has been shown to considerably lower the ferroelectric phase transition temperature T_c. In mixed crystals, additional electric permittivity anomaly has been clearly evidenced. The results are discussed assuming a model, which combines polarizability effects, related to the heavy ion units, with the pseudo-spin tunnelling.     

Brillouin scattering study of phase transitions in LiK0.80(NH4)0.20SO4 mixed crystals

Brillouin spectroscopy was used to study the phase transitions of LiK_0.80(NH₄)_0.20SO₄ mixed crystals in the temperature range 10-300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ₁₆ and the quasi-transverse γ₁₇ mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO₄ and LiK_0.96(NH₄)_0.04SO₄ crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.     

Structure of the low-temperature phase of Rb3D(SeO4)2 by single-crystal neutron diffraction

Crystal structure of Rb₃D(SeO₄)₂ has been investigated at 25 K (below the transition temperature T_c=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO₄ groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO₄ groups was identified to be in the state closer to a HSeO₄⁻ ion or to a SeO₄²⁻ ion and the dipole arrangement of SeO₄-D-SeO₄ dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.     

Induction of B1 phase in binary mixtures of rod- and bent-shaped mesomorphic compounds

The miscibility of a rod-shaped (R) and a bent-shaped (BC) compound forming an induced B₁ phase is reported. B₁ phase is not present in both R and BC in the entire phase sequence. R exhibits partially bilayered smectic A_d phase with cholesteric and TGB while BC shows a B₂ phase. It is interesting to show that the B₂ phase is totally diminished in low composition (weight%) of BC, B₁ phase is induced at lower temperature range (~90-135 °C). A dielectric study is also incorporated to confirm the induction of B₁ phase in the binary mixture.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

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.     

One-step synthesis and electrochemical behavior of LiMnO2 and its composite from MnO2 in the presence of glucose

LiMnO₂ and 0.23Li₂MnO₃·0.77LiMnO₂ were prepared by a convenient one-step solid-state reaction from MnO₂ using glucose as organic carbon resource. The crystal structure and morphology of the as-prepared materials was examined by X-ray powder diffraction and field emission scanning electron microscopy, respectively. The ration of Li to Mn was determined by means of atomic absorption spectrometry and the Li/Mn molar ratio in the products was 1.23. The electrochemical properties were investigated by charge-discharge test and electrochemical impedance measurements. The prepared composite material presented an initial discharge capacity of 45 mAh g^-1 and a good cycling performance with reversible capacity of 218 mAh g^-1 after 30 cycles. On the basis of the experimental results, the discharge efficiency of this composite material more than 100% was also discussed.     

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.     

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.     

Heat capacity and magnetic phase transitions of rare-earth orthoferrite HoFeO3

Heat capacity of two rare-earth orthoferrites HoFeO₃ and LuFeO₃ were measured between 1.8 and 200 K. A distinctly large and two small heat capacity anomalies were detected for HoFeO₃ under zero magnetic field around 3.3, 53 and 58 K, respectively. The low-temperature anomaly with a peak at 3.3 K is due to the ordering of Ho³⁺ ions and the estimated magnetic entropy for this transition was favorably compared with the expected (R ln 2). Application of magnetic field significantly affects the positions and the magnitudes of the anomaly at 3.3 K. Energies of low-lying levels of the lowest J-term of Ho³⁺ ion were roughly estimated through analysis of the Schottky heat capacity.