Analysis of magnetic and structural phase transition behaviors of La1−xSrxCrO3 for preparation of phase diagram

The phase transition behavior of perovskite-type compounds, La_1−xSr_xCrO₃, was investigated by differential scanning calorimetry (DSC), dilatometry, dc magnetic susceptibility measurement and X-ray diffraction analysis. Both second-order magnetic phase transition from antiferromagnetic to paramagnetic and first-order structural phase transition from orthorhombic to rhombohedral were observed in the DSC or dilatometric curve of every specimen. The temperatures of both these magnetic and structural phase transitions decreased linearly with an increase in Sr content. The structural phase transition temperature of La_1−xSr_xCrO₃ with x less than 0.11 is higher than the magnetic phase transition temperature; however, a larger decrease in structural phase transition temperature than in magnetic phase transition temperature was observed with an increase in Sr content, resulting in a structural phase transition temperature lower than the magnetic phase transition temperature for La_1−xSr_xCrO₃ with x of more than 0.12. It was also observed that the heat of absorption of the structural phase transition decreased with an increase in x. In the dependence of dc magnetic susceptibility on temperature, variations by not only magnetic but also structural phase transitions were observed. It was also revealed that thermal expansion coefficient is affected not only by structural phase transition but also magnetic phase transition. Magnetic and structural phase diagram of La_1−xSr_xCrO₃, suggesting the existence of two Sr contents and temperatures at which triple phases coexist, was proposed.     

Composition-induced structural phase transitions in the (Ba1−xLax)2In2O5+x (0?x?0.6) system

Composition-induced structural phase changes across the high temperature, fast oxide ion conducting (Ba_1−xLa_x)₂In₂O_5+x, 0?x?0.6, system have been carefully analysed using hard mode infrared (IR) powder absorption spectroscopy, X-ray powder diffraction and electron diffraction. An orthorhombic brownmillerite to three-dimensionally disordered cubic perovskite phase transition in this system is signalled by a drastic change in slope of both wavenumber and average line widths of IR spectra as a function of composition. Some evidence is found for the existence of an intermediate tetragonal phase (previously reported to exist from electron diffraction data) around x∼0.2. The new spectroscopic data have been used to compare microscopic and macroscopic strain parameters arising from variation in composition. The strain and spectroscopic data are consistent with first-order character for the tetragonal→orthorhombic transition, while the cubic→tetragonal transition could be continuous. Differences between the variation with composition of spectral parameters and of macroscopic strain parameters are consistent with a substantial order/disorder component for the transitions. There is also evidence for precursor effects within the cubic structure before symmetry is broken.     

Temperature behavior of the protonic conductor K4LiH3(SO4)4

The results of the X-ray structural study for the K₄LiH₃(SO₄)₄ single crystal are presented at a wide temperature range. The thermal expansion of the crystal using the X-ray dilatometry and the capacitance dilatometry from 8 to 500 K was carried out. The crystal structures data collection, solution and refinement at 125, 295, 443 and 480 K were performed. The K₄LiH₃(SO₄)₄ crystal has tetragonal symmetry with the P4₁ space group (Z=4) at room temperature as well as at the considered temperature range. The existence of a low-temperature, para-ferroelastic phase transition at about 120 K is excluded. The layered structure of the crystal reflects a cleavage plane parallel to (001) and an anisotropy of the protonic conductivity. The superionic high-temperature phase transition at T_S=425 K is isostructural. Nevertheless, taking into account an increase of the SO₄ tetrahedra libration above T_S, a mechanism of the Grotthus type could be applied for the proton transport explanation.     

Structural and magnetic anomalies among the spin-chain compounds, Ca3Co1+xIr1?xO6

The results of X-ray diffraction, andac anddc magnetisation as a function of temperature are reported for a new class of spin-chain oxides, Ca₃Co_1+x Ir_1−x O₆. While thex = 0.0, 0.3, 0.5 and 1.0 are found to form in the K₄CdCl₆-derived rhombhohedral (space group ) structure, thex = 0.7 composition is found to undergo a monoclinic distortion in contrast to a literature report. Apparently, the change in the crystal symmetry withx manifests itself as a change in the sign of paramagnetic Curie temperature for this composition as though magnetic coupling sensitively depends on such crystallographic distortions. All the compositions exhibit spin-glass anomalies with an unusuallylarge frequency dependence of the peak temperature inac susceptibility in a temperature range below 50 K, interestingly obeying Vogel-Fulcher relationship even for the stoichiometric compounds. Dedicated to Professor C N R Rao on his 70th birthday     

Photopyroelectric method

High resolution measurements have been performed with a photopyroelectric (PPE) technique to study the static and dynamic thermal parameters behaviour in the critical region of different liquid and solid samples. In particular the specific heat (c), the thermal diffusivity (D) and the thermal conductivity (k) have been simultaneously determined in the critical region around the antiferromagnetic/paramagnetic (AP) phase transition occurring in FeF₂ and around the Smectic-A/Nematic (AN) phase transition occurring in the 8S5 liquid crystal.The high resolution of the measuring technique has allowed the determination of thec andD critical exponents and amplitude ratio of their critical terms.     

Effect of electric field on phase separation of polymer dispersed liquid crystal

The kinetics of the polymerization induced phase separation of liquid crystal (LC)/monomer mixture has been investigated by means of depolarized light intensity technique and polarized light microscope (PLM). To examine the effect of the electric field, a DC electric field was applied across the mixtures during the phase separation process. The kinetic study indicates that the phase separation process is accelerated when the electric field is applied. The morphologies of the formed polymer dispersed liquid crystal (PDLC) films were observed by PLM. The electric field applied during the phase separation process yields the PDLC with small LC domains and fine morphologies. The clearing temperature (T_NI) of the formed PDLC films was measured by the PLM and it is found that the T_NI increases with the applied electric field intensity.     

DSC study of the phase transitions in [Ni(D2O)6](ClO4)2 and [Ni(H2O)6](ClO4)2

DSC measurements were carried out for [Ni(H₂O)₆](ClO₄)₂ (sampleH) and [Ni(D₂O)₆](ClO₄)₂ (sampleD) in the temperature range 300–380 K. For both compounds two anomalies on the DSC curves were detected. The results for sampleH are compared to those previously obtained using adiabatic calorimetry method. For both compounds studied in this work the high-temperature transition appears at the same temperature while the low-temperature one is shifted towards higher temperatures in sampleD. Disorder connected with H₂O or D₂O groups is suggested in the intermediate phase between the low- and high-temperature transitions.     

Determination of charge density of anionic polyacrylamide flocculants by NMR and DSC

New methods are suggested for the determination of the charge density of acrylamide/acrylate copolymers.¹³C nuclear magnetic resonance spectroscopy was used to determine the comonomer ratio by comparing the peak intensities of the methine carbon in acrylamide and acrylate monomers. Results were compared with those obtained by conductometric and potentiometric titration and were found to be in good agreement. Differential scanning calorimetry was employed to determine the glass transition temperatures (T _g) of the copolymers. A master curve was established by plottingT _g versus charge density of the copolymers previously determined by NMR and conductometric titration. Compositions of poly(acrylamide-co-acrylate) samples can thus be determined by measuring theT _g and reading the percent composition directly from the master curve.     

Neutron powder diffraction study of A2BeF4 (A=K, Rb, Cs): Structure refinement and analysis of background

The crystal structure of potassium, rubidium and caesium fluoroberyllates have been re-examined by neutron powder diffraction at room temperature and at 1.5 K. Previously, their structures, obtained from X-ray data, were described in the Pn2₁a space group. However, the results obtained from Rietveld refinements, using powder neutron diffraction, at both temperatures, indicated that all structures are orthorhombic with space group Pnma. The known phase transition at high temperature is probably related to the appearance of a hexagonal pseudo-symmetry instead of the elimination of the mirror plane between the above mentioned orthorhombic space groups. A possible phase transition, at very low temperature, was discarded considering the stereochemical criteria concerning the structural stability of A₂BX₄ compounds. This was confirmed by thermal analysis. On the other hand, a modulated background has been detected in all samples during the refinements. This is compatible with the presence of an amorphous phase, coexisting with the crystalline phase, or with a disordered component within the main crystalline phase. Instead of using a polynomial function, the background was modelled by Fourier filtering improving the fit for all patterns. The radial distribution function (RDF) was obtained from the analysis of the calculated background and compared with the RDF from the average crystal structure. The advantages of neutron with respect to X-ray diffraction were evidenced for this type of compound with β-K₂SO₄-type structure.     

A structure, conductivity and dielectric properties investigation of A3CoNb2O9 (A=Ca, Sr, Ba) triple perovskites

The room temperature structures as well as the temperature-dependent conductivity and dielectric properties of the A₃CoNb₂O₉ (A=Ca²⁺, Sr²⁺ and Ba²⁺) triple perovskites have been carefully investigated. A constrained modulation wave approach to Rietveld structure refinement is used to determine their room temperature crystal structures. Correlations between these crystal structures and their physical properties are found. All three compounds undergo insulator to semiconductor phase transitions as a function of increasing temperature. The hexagonal Ba₃CoNb₂O₉ compound acts as an insulator at room temperature, while the monoclinic Ca₃CoNb₂O₉ compound is already a semiconductor at room temperature. The measured dielectric frequency characteristics of the A=Ba compound are excellent.     

Cr2(WO4)3和Cr2(MoO4)3的负热膨胀特性研究

用液相反应-前驱物烧结法制备了Cr₂(WO₄)₃和Cr₂(MoO₄)₃粉体。298～1 073 K的原位粉末X射线衍射数据表明Cr₂(WO₄)₃和Cr₂(MoO₄)₃的晶胞体积随温度的升高而增大, 本征线热膨胀系数分别为(1.274±0.003)×10^-6 K^-1和(1.612±0.003)×10^-6 K^-1。用热膨胀仪研究了Cr₂(WO₄)₃和Cr₂(MoO₄)₃在静态空气中298～1 073 K范围内热膨胀行为，即开始表现为正热膨胀，随后在相转变点达到最大值，最后表现为负热膨胀，其负热膨胀系数分别为(-7.033±0.014)×10^-6 K^-1和(-9.282±0.019)×10^-6 K^-1。     

Why pressure induces an abrupt structural rearrangement in PdTe2 but not in PtTe2

High-pressure X-ray diffraction measurements were carried out for polymeric CdI₂-type compounds MTe₂ (M=Pt, Pd) to investigate if they undergo a structural phase transition under pressure as does IrTe₂. Up to 27 GPa at room temperature PtTe₂ does not undergo any structural phase transition. In contrast, however, an abrupt change in the inter-atomic distances occurs in PdTe₂ above 15.7 GPa at room temperature, and above 5 GPa at 300 °C, but the volume vs. pressure curve exhibits no discontinuity. To account for the differences between the isostructural compounds PtTe₂, PdTe₂ and IrTe₂, their electronic structures and bonding were analyzed on the basis of first principles electronic band structure calculations.     

Crystal structure and magnetic properties of the UFe7Al5 uranium-iron aluminide

The ternary compound UFe₇Al₅ was synthesized by arc melting, followed by annealing at 850°C. The crystal structure was determined by single-crystal X-ray diffraction and refined to a residual value of R=0.039 (S=1.030), with lattice parameters a=8.581(2) Å and c=4.946(1) Å. This compound is a new extreme composition in the family of intermetallics with general formula UFe_xAl_12−x crystallizing in the tetragonal ThMn₁₂-type structure, space group I4/mmm. In contrast to UFe_xAl_12−x within the composition range 4?x?6, in UFe₇Al₅ the additional iron atom is found in the 8i equipositions. Magnetization measurements versus temperature show two magnetic transitions at 363 and 275 K, respectively, with a ferromagnetic behavior below the highest temperature transition. ⁵⁷Fe Mössbauer data indicate that the high-temperature transition is related to the ordering of the iron atoms. The dependence of the isomer shifts and magnetic hyperfine fields on the crystallographic site and on the number of the iron nearest neighbors is similar to that observed in the other UFe_xAl_12−x and rare-earth analogues. The magnetic hyperfine field values of iron atoms on 8i sites is larger than in the other sites, in agreement with previous data obtained for other ThMn₁₂-type compounds.     

Magnetic properties of Mn2V2O7 single crystals

Magnetic properties of Mn₂V₂O₇ single crystals are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. A structural phase transition of the α-β forms is clearly observed at the temperature range of 200-250 K and an antiferromagnetic ordering with magnetic anisotropy is observed below 20 K. A spin-flop transition is observed with magnetic field applied along the [110] axis of β-Mn₂V₂O₇, of which corresponds to the [001] axis of α-Mn₂V₂O₇, suggesting that the spins of Mn²⁺ ions locate within honeycomb layers which point likely in the [110] direction of β-Mn₂V₂O₇ or the [001] axis of α-Mn₂V₂O₇. However, a rather small jump of magnetization at spin-flop transition suggests a possible partition of crystal to some domains through β-to-α transition on cooling or much complex spin structure in honeycomb lattice with some frustration.     

Influence of cubic α-Fe2O3 particles on the thermal stability of poly(methyl methacrylate) synthesized by in situ bulk polymerization

Poly(methyl methacrylate)/α-Fe₂O₃ composites were prepared by in situ bulk radical polymerization of methyl methacrylate in the presence of the cubic α-Fe₂O₃ particles using 2,2′-azobisisobutyronitrile as initiator. The cubic α-Fe₂O₃ particles were synthesized by forced hydrolysis of FeCl₃ and characterized by X-ray diffraction analysis and transmission electron microscopy. The molar masses and molar mass distribution of synthesized PMMA samples were determined by gel permeation chromatography. The influence of α-Fe₂O₃ filler particles on the thermal properties of the PMMA matrix was investigated using thermogravimetry and differential scanning calorimetry. The molar mass and polydispersity of PMMA extracted from composite samples were not influenced by cubic α-Fe₂O₃ particles. The obtained composites have better thermal and thermooxidative stability than pure PMMA. On the other hand, the values of the glass transition temperature of composite samples were identical to the glass transition temperature of pure PMMA.     

Synthesis and structural study of a new NASICON-type solid solution: Li1−xLax/3Zr2(PO4)3

A new complete solid solution of NASICON-type compounds between LiZr₂(PO₄)₃ and La_1/3Zr₂(PO₄)₃ was evidenced with the general formula Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1). These phases were synthesized by a complex polymerizable method and structurally characterized from Rietveld treatment of their X-ray and neutron powder diffraction data. This solid solution results from the substitution mechanism Li⁺→1/3La³⁺+2/3□ leading to an increase of the vacancies number correlated to an increase of the La content. According to this substitution mechanism, the general formula can then be written Li_1−xLa_x/3□_2x/3Zr₂(PO₄)₃ (0?x?1) in order to underline the correlation between the La content and the vacancies rate. For all the compounds, the structure is clearly related to that of the NASICON family with three crystallographic domains evidenced. For 0?x?0.5, all the members adopt at high temperature the typical NASICON-type structure (s.g. R3¯c), while at lower temperature, their structure distorts to a triclinic form (s.g. C 1¯), as observed for LiZr₂(PO₄)₃ prepared above 1100 °C. Moreover, in this domain, the reversible transition is clearly soft and the transition temperature strongly depends of the x value. For 0.6?x?0.9, the compounds crystallize in a rhombohedral cell (s.g. R3¯), while for x=1, the phase La_1/3Zr₂(PO₄)₃ is obtained (s.g. P3¯, Z=6, a=8.7378(2) Å, c=23.2156(7) Å).This paper is devoted to the structure analysis of the series Li_1−xLa_x/3Zr₂(PO₄)₃ (0?x?1), from X-ray and neutron powder thermo diffraction and transmission electron microscopy (TEM) studies.     

Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO3 Perovskites at High Temperatures by Neutron Diffraction

The influence of small amounts of Fe³⁺ on the phase transitions of CaTiO₃ perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO₃ is shown by both CaTi_0.9Fe_0.1O_2.95 and CaTi_0.8Fe_0.2O_2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3m at high temperature, with an intermediate I4/mcm tetragonal phase which exists over a temperature range of about 100°C. The two phase boundaries in the temperature vs composition phase diagram of the system CaFe_xTi_1−xO_3−x/2 (0≤x≤0.4) decrease in a quasi-linear manner with increasing Fe content up to x=0.2 and then they both drop abruptly to RT. The existence of a second orthorhombic phase (Cmcm), which has been postulated for CaTiO₃, is ruled out in the Fe-doped CaTiO₃ perovskites in view of the behavior of specific diffraction peaks. Strain analysis shows first-order thermodynamic character for the Pnma→I4/mcm transition, while the character of the Pm3m→I4/mcm transition could be second order or tricritical. Shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models. The volume strain has an anomalous coupling with the order parameter components, which appears to be temperature-dependent.     

Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3-PbTiO3 System

A complete solid solution between relaxor ferroelectric Pb(Fe_2/3W_1/3)O₃ (PFW) and ferroelectric PbTiO₃ (PT), (1−x)PFW-xPT, was synthesized by a B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. A phase diagram between PFW and PT has been established. The diffuse phase transition temperature (T_max≈180 K) of PFW was found to continuously increase with the increasing amount of Ti⁴⁺ ions on the B-site. At the same time, the relaxor ferroelectric behavior of PFW is gradually transformed toward a normal ferroelectric state, as evidenced by sharp and nondispersive peaks of dielectric permittivity around T_C for x≥0.25. At room temperature, a transition from a cubic to a tetragonal phase takes place with x increased up to 0.25. A morphotropic phase boundary is located within the composition interval 0.25≤x≤0.35, which separates a pseudocubic (rhombohedral) phase from a tetragonal phase.     

Thermal properties of high-T c superconductors

The termal stabilities of several families of high-T _c superconductors (HTSC), as well as the dependence of phase transitions on temperature and stoichiometry, have been studied by X-ray diffraction, DTA, TG and DSC. Experimental results are discussed in the context of decomposition models.     

High-pressure transitions in MgAl2O4 and a new high-pressure phase of Mg2Al2O5

Phase transitions in MgAl₂O₄ were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al₂O₃ corundum that are high-pressure dissociation products of MgAl₂O₄ spinel combines into calcium-ferrite type MgAl₂O₄ at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al₂O₃ corundum and a new phase with Mg₂Al₂O₅ composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg₂Al₂O₅ phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å³, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O₆ octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O₆ octahedra. The structure is related with that of ludwigite (Mg, Fe²⁺)₂(Fe³⁺, Al)(BO₃)O₂. The molar volume of the Mg₂Al₂O₅ phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al₂O₃ corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl₂O₄ among various A²⁺B³⁺₂O₄ compounds.