Flux growth and magnetic properties of FeVO4 single crystals

FeVO₄ (I) single crystals are grown by the flux method using V₂O₅ as the self-flux. The grown crystals exhibit a characteristic morphology with natural facets. The quality of the crystals is confirmed by X-ray diffraction and EPMA techniques. Magnetic properties are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. Two magnetic phase transitions are observed at ∼13 and ∼20 K. Such unusual magnetic behaviors are suggested to originate from two different Fe ligand environments of octahedral FeO₆ and trigonal bipyramidal FeO₅ in a six-column doubly bent chain.     

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.     

Magnetic and Calorimetric Studies on One-Dimensional Ln3RuO7(Ln=Pr, Gd)

Magnetic and calorimetric properties of Ln₃RuO₇ (Ln=Pr, Gd) have been investigated. Magnetic susceptibility and specific heat measurements indicate that both Pr₃RuO₇ and Gd₃RuO₇ compounds show magnetic transitions at 55 K and 15 K, respectively. In addition, a clear structural phase transition has been found at 382 K for Gd₃RuO₇ from the specific heat measurements. From the temperature dependence of the magnetic specific heat, the magnetic entropy change is estimated and the magnetic ground states of each ion are determined.     

Phase transitions in molecular crystals of carboxylic acids

A comparative analysis of phase transitions in molecular crystals of carboxylic acids (n-alkano acids) with various chain lengths, (CH₃(CH₂) _n COOH) is performed with the use of DSC. A number of new effects related to first-order phase transitions are discovered. The temperature dependence of heat capacity is quantitatively analyzed in terms of the theory of blurred (Λ-shaped) phase transitions of the first order.     

Magnetic properties of Co2V2O7 single crystals grown by flux method

Based on the phase diagram of CoO–V₂O₅ system, single crystals of Co₂V₂O₇ are grown using V₂O₅ as self-flux at a slow cooling rate. The quality of grown crystals is analyzed by X-ray powder diffraction and electron probe microanalysis techniques. Magnetic properties are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results suggest that Co₂V₂O₇ is a three-dimensional antiferromagnet, in which two magnetic transitions may occur at low temperature and a spin-flop-like transition may occur at the applied field along the b-axis. By contrast to Ni₂V₂O₇, it is suggested that similar and different magnetic properties may arise from their similar crystal structures and different magnetic ions, respectively.     

Effect of mechanical activation on the hydrogen reduction kinetics of magnetite concentrate

The combined thermal analysis techniques thermal expansion: and differential scanning calorimetry were used to characterize various phase transitions that exist in the solid solutions of PbZr_1?xSn_xO₃. Using thermodynamic quantities, i.e., thermal expansion and specific heat to distinguish first-order transitions from second-order ones, we demonstrate that some perovskite antiferroelectrics can exhibit continuous transition at their Curie temperature T _C. We observed such a transition in antiferroelectric crystals of solid solutions based on PbZrO₃. Although pure PbZrO₃ is a classical example of antiferroelectric crystal with a first-order transition at T _C, the solid solutions of PbZr_1?xSn_xO₃ in the range of composition of x > 0.25 seem to exhibit a second-order phase transition.     

Analysis of structural and magnetic phase transition behaviors of La1−xSrxCrO3 by measurement of heat capacity with thermal relaxation technique

The heat capacity, C_p, of the La_1−xSr_xCrO₃ system and its temperature dependence have been measured by a thermal relaxation technique. Both structural and magnetic phase transitions were detected at temperatures that can be surmised from the phase diagram proposed in previous studies. The observed variation in enthalpy after the first-order structural phase transition, ΔH, showed agreement with those measured by differential scanning calorimetry (DSC). A decrease in the variation in C_p in the second-order magnetic phase transition, ΔC_p, with an increase in Sr content was detected, which can be attributed to a decrease in electronic spin configuration entropy with an increase in Sr content. In the dependence of ΔC_p on Sr content, a bending point was also observed at x  0.12, at which the crystal system varies from an orthorhombic-distorted perovskite structure to a rhombohedral-distorted perovskite structure.     

The Specific Heat of Tetramethylammonium Salts

New measurements of the (N(CH₃ )₄ )₂ MnBr₄ specific heat by adiabatic calorimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are compared with previous calorimetric studies on similar tetramethylammonium bromide compounds. The thermodynamic behaviour of the tribromides and tetrabromides derivatives together with the influence on the phase transition parameters of the cation and halogen molecular substitutions are examined. The thermal relaxation experiments permit to study the behaviour of the crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions is used to describe the thermodynamic behaviour of some of these crystals. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low Temperature Specific Heat Capacity of 3d Transition Metal Chlorine Boracites (T3B7O13Cl; T=Cr,Mn, Fe,Co, Ni,Cu, Zn or Mg)

The heat capacities of eight chlorine boracites T₃B₇O₁₃Cl (T=Cr, Mn, Fe, Co, Ni, Cu, Zn or Mg) have been measured in the temperature range 2 to 100 K. Magnetic phase transitions occur below 20 K in the compounds studied except in the two non-magnetic substances Zn₃B₇O₁₃Cl and Mg₃B₇O₁₃Cl. The magnetic specific heat capacities give information on magnetic ground state of the transition metals and the entropy related to the phase transitions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Metal-insulator transition in VnO2n−1

Research on phase relationships and structure studies by electron diffraction confirm V_nO_2n?1 (n = 3–9) phases between V₂O₃ and VO₂. Metal-insulator phase transitions have been found in all phases but V₃O₅ and V₇O₁₃. Electrical, magnetic and thermodynamic properties associated with the transitions are reported for sintered samples or for single crystals prepared by a vapor-transport method. The results are collated and reviewed in summarized form.     

A study of the NMR relaxation mechanisms of ABCl3 (A=Rb, Cs, B=Cd, Mn) single crystals with the electric quadrupole and the electric magnetic types

The ⁸⁷Rb and ¹³³Cs spin-lattice relaxation rates of RbCdCl₃ and CsCdCl₃ single crystals grown using the slow evaporation method were measured over the temperature range 160-400 K. The changes in the ⁸⁷Rb spin-lattice relaxation rate near 340, 363, and 395 K correspond to phase transitions of the RbCdCl₃ crystal. The jump in T₁⁻¹ at 395 K is due to a shortening in the c-direction as a result of a phase transition from a cubic to a tetragonal structure. We suggest that the cubic Rb environment is favored above 395 K due to the fast motions and soft modes, which cause relaxation and average out the quadrupolar splittings. The temperature dependence of the relaxation rate below 340 K in RbCdCl₃ can be represented by and is thus in accordance with a Raman process. The ¹³³Cs nuclei in the CsCdCl₃ crystal produce only one resonance line, which indicates that the local structure around the Cs atoms is cubic. The temperature dependence of the relaxation rate of the Cs nuclei can also be described with the quadratic equation . In the case of the RbCdCl₃ and CsCdCl₃ crystals, which are of electric quadrupolar type, their relaxations proceed via Raman processes, whereas in RbMnCl₃ and CsMnCl₃ crystals, which are of magnetic relaxation type, the relaxations proceed via single phonon processes. Therefore, the relaxation mechanisms of these different types of ABCl₃ crystals (quadrupolar and magnetic) are completely different NMR behavior.     

An X-ray scattering study of the lamellar/nematic/isotropic sequence of phases in decylammonium chloride/H2O/NH4Cl

Abstract

An X-ray scattering study is presented of the lamellar/nematic/isotropic sequence in the lyotropic system DACI/H₂O/NH₄Cl. The whole reciprocal space of monocrystalline samples oriented in magnetic fields are reconstructed from their two dimensional sections on photographic films. Intense diffuse scatterings are observed in the lamellar phase, around and away from the Bragg spots. Their evolution close to the lamellar/nematic transition reveals the presence of intense structural fluctuations. They take place over temperature ranges which are significantly greater than those associated with the smectic/nematic transitions in thermotropic liquid crystals. A similar situation is observed in the isotropic phase in the vicinity of the nematic/isotropic transition.     

Microscopic mechanisms of magnetic transitions in chain polymeric copper(II) complexes with nitronyl nitroxide radicals

Parameters of exchange interactions in heterospin chain polymeric complexes of Cu(hfac)₂ (hfac is hexafluoroacetylacetonate anion) with pyrazolyl-substituted nitronyl nitroxides L^R (R = Me, Et, Pr, Bu) were estimated using quantum chemical computational methods. The magnetic properties of the considered chain polymeric complexes can be described within the framework of the model of isolated exchange clusters. Experimental data on the structural dynamics of chains polymeric with “head-to-tail” (R = Me) and “head-to-head” (R = Et, Pr, Bu) motifs are discussed in the context of the concept of gradual phase transitions. Based on the analysis performed, a hypothesis of microscopic mechanisms of magnetic transitions in crystals of this type of compounds was proposed.     

An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic–Inorganic Perovskite

The driving forces for the phase transitions of ABX₃ hybrid organic–inorganic perovskites have been limited to the octahedral tilting, order–disorder, and displacement. Now, a complex structural phase transition has been explored in a HOIP, [CH₃NH₃][Mn(N₃)₃], based on structural characterizations and ab initio lattice dynamics calculations. This unusual first‐order phase transition between two ordered phases at about 265 K is primarily driven by changes in the collective atomic vibrations of the whole lattice, along with concurrent molecular displacements and an unusual octahedral tilting. A significant entropy difference (4.35 J K^?1 mol^?1) is observed between the low‐ and high‐temperature structures induced by such atomic vibrations, which plays a main role in driving the transition. This finding offers an alternative pathway for designing new ferroic phase transitions and related physical properties in HOIPs and other hybrid crystals.     

Defect chemistry of uranium oxides

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO₂ and doped UO₂ [(U, M)O₂; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U₄O₉, U₃O₈) are shown against oxygen partial pressure. From the results of doped UO₂, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po₂ (x: deviation from the stoichiometric composition and Po₂: oxygen partial pressure) and log vs. log Po₂ (: electrical conductivity) relations. The defect structures of UO₂ and doped UO₂ are derived based on the Willis model for UO_2+x. The detect structure of U₄O₉ phase is similar to that of UO_2+x, but the defect structures of U₃O₈ phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO₂ doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U₄O₉ and U₃O₈ clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.     

High-Temperature and Large-Polarization Ferroelectric with Second Harmonic Generation Response in a Novel Crown Ether Clathrate

Molecular ferroelectrics of high-temperature reversible phase transitions are very rare and have attracted increasing attention in recent years. In this paper is described the successful synthesis of a novel high-temperature host-guest inclusion ferroelectric: [(C₆H₅NF₃)(18-crown-6)][BF₄] ( 1 ) that shows a pair of reversible peaks at 348 K (heating) and 331 K (cooling) with a heat hysteresis about 17 K by differential scanning calorimetry measurements, thus indicating that 1 undergoes a reversible structural phase transition. Variable-temperature PXRD and temperature-dependent dielectric measurements further prove the phase-transition behavior of 1 . The second harmonic response demonstrates that 1 belongs to a non-centrosymmetric space group at room temperature and is a good nonlinear optical material. In its semiconducting properties, 1 shows a wide optical band gap of about 4.43 eV that comes chiefly from the C, H and O atoms of the crystals. In particular, the ferroelectric measurements of 1 exhibit a typical polarization-electric hysteresis loop with a large spontaneous polarization (P_s) of about 4.06 μC/cm². This finding offers an alternative pathway for designing new ferroelectric-dielectric and nonlinear optical materials and related physical properties in organic-inorganic and other hybrid crystals.     

Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO7 (Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln₃MO₇ (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln₃MoO₇ (Ln=La-Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2₁2₁2₁, in which Ln³⁺ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2₁2₁2₁ to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd₃MoO₇ shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm₃MoO₇ and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln₃MoO₇ were compared with the magnetic properties and structural transitions of Ln₃MO₇ (M=Nb, Ru, Sb, Ta, Re, Os, or Ir).     

Crystal structure and magnetic properties of the solid-solution phase Ca3Co2-vMnvO6

The homogeneity range of the Ca₃Co_2-vMn_vO₆ solid-solution phase covers the entire composition interval from v=0 to 1. A systematic powder X-ray and neutron diffraction, magnetic susceptibility, and magnetization study has been carried out to investigate effects of the Mn-for-Co substitution on structural and magnetic properties. The Mn substitution concerns primarily only the octahedral Co1 site of the Ca₃Co1Co2O₆ crystal structure, whereas the trigonal-prismatic Co2 site structurally is left essentially unaffected. The Ca₃Co_2-vMn_vO₆ crystal structure belongs to space group with unit-cell dimensions (in hexagonal setting) 9.084?a?9.134 Å and 10.448?c?10.583 Å. A cut through the magnetic phase diagram at 10 K shows a ferrimagnetic domain for 0?v<∼0.3 and an antiferromagnetic domain for ∼0.50<v<∼1. The magnetic ordering temperatures are quite low (<∼25/18 K), and even so further magnetic transitions appear to take place at still lower temperature. The legitimity and reliability of the different indicators used to establish the magnetic transitions, their individual accuracy, and mutual consistency are briefly discussed. Variable parameters of the crystal and magnetic structures of Ca₃Co1_1-vMn_vCo2O₆ are determined and their variation with v is briefly discussed in relation to chemical bonding. The magnetic structure in the ferrimagnetic region is essentially the same as that of the pristine v=0 phase, but since the moments at the Co2 site decrease and those at the (Co1,Mn) site increase with increasing v; characteristic traits of ferrimagnetism in magnetic susceptibility and magnetization gradually disappear. The magnetic arrangement in the antiferromagnetic region is characterized by differently sized moments at the (Co1,Mn) and Co2 sites, moments at adjacent sites in each of these sublattices being oppositely oriented along [001].     

Crystal Structure,Thermal and Electric Behaviour of [(CH3)2NH2]3[As2Cl9] and [(CH3)2NH2][AsOCl2]

Single crystals of the organic‐inorganic arsenate(III): [(CH₃)₂NH₂]₃[As₂Cl₉] and [(CH₃)₂NH₂][AsOCl₂] have been grown from aqueous hydrochloric acid solution. The crystals [(CH₃)₂NH₂]₃[As₂Cl₉] have been investigated by X‐ray diffraction (at 253 K), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dilatometric and dielectric methods. They undergo two structural phase transitions of first order at 228/235 and 298/307 K (on cooling/heating), respectively, which are classified as an "order‐disorder" type. The trigonal [(CH₃)₂NH₂]₃[As₂Cl₉] structure (at 253 K, intermediate phase (II)) refined in the space group R3c, consists of isolated [As₂Cl₉]^3‐ bioctahedral units and dimethylammonium cations hydrogen bonded to the bridging Cl atoms of the anions. The crystals of [(CH₃)₂NH₂][AsOCl₂] at 100 K are orthorhombic, space group Cmca. The structure contains one‐dimensional chains formed by strong distorted [AsO₂Cl₄] octahedra. The dimethylammonium cations reveal distinct disorder.