Congruent Melting Research Articles

Characterization of photorefractive BCT:Rh crystals at 1.06 µm by two-wave mixing

We present an experimental investigation of the photorefractive properties of rhodium-doped barium calcium titanate (BCT) crystals of the congruent melting composition Ba0.77Ca0.23TiO3. Considering the results previously obtained on these crystals in the visible region and at 850 nm wavelength, it should be a good alternative to BaTiO3:Rh. Besides, the rhodium doping has the effect of making this material sensitive up to the wavelength of 1.06 µm—a very technologically significant wavelength, because of the existence of high-power lasers. Therefore we present a study of rhodium-doped BCT crystals at 1.06 µm. These crystals are characterized by two-wave-mixing experiments. Two-beam coupling gain factors of up to 7.5 cm−1, with ordinary polarization and a response time of the order of 1 s at an incident power of 10 W cm−2, have been obtained. All properties are explained by a three-centre charge transport model, together with the existence of an iron impurity which masks the influence of rhodium at low doping concentrations.

Structure and physical properties of 1D magnetic chalcogenide, jamesonite (FePb4Sb6S14).

Monoclinic FePb(4)Sb(6)S(14) phase, jamesonite, which is a candidate material as a S = 2 Haldane compound, has been synthesized by the direct reaction of elements under dry conditions with sealed evacuated quartz tubes. The congruent melting point was determined at 592 degrees C by DTA measurements. Shiny metallic gray needle crystals grow on the surface of bulk heated at 550 degrees C. The elongated direction of each needle crystal is parallel to the c-axis. The crystal structure refinement (P2(1)/a, a = 15.750(6) A, b = 19.125(3) A, c = 4.030(4) A, beta = 91.68(8) degrees, V = 1213(1) A(3), Z = 2, D(c) = 5.651 g/cm(3), R(1) = 3.16%) reveals the presence of two rod substructures elongated parallel to the c-axis. One is the lozenge-shaped Bi(2)Te(3)-type (or called SnS archetype), [Pb(4)Sb(6)S(13)]. The other is the novel single magnetic one-dimensional (1D) straight chain, [FeS(6)]. This compound shows intrinsic semiconductor behavior in the electric conductivity measurements. The optical band gap, 0.48 eV, is estimated by near-IR diffuse reflectance measurements. In the magnetic susceptibility measurements, this compound shows 1D-Heisenberg antiferromagnetic behavior with a broad peak at approximately 33.5 K, where Fe(2+) takes the high-spin state, t(2g)(4)e(g)(2). A possibility for the S = 2 Haldane system is discussed.

Crystal Growth of Barium Aluminum (Gallium) Borates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural and thermodynamic aspects in the molybdenum melts of the system KF–K2MoO4–SiO2

The complex physico-chemical analysis of the system KF–K 2 MoO 4 –SiO 2 , based on the phase diagram, density, surface tension and viscosity measurements, was performed. The phase diagrams of the binary systems as well as that of the ternary system in the range up to 50% SiO 2 were measured using the thermal analysis method. The results proved the presence of the congruently melting compound K 3 FMoO 4 at 751 °C in the binary melt KF–K 2 MoO 4 . The strong positive deviation from ideal behaviour of the ternary system KF–K 2 MoO 4 –SiO 2 was ascribed to the formation of heteropolyanions [SiMo 12 O 40 ] 4− in the melt. In the investigated concentration range of the ternary system, no eutectic point has been found. It lies most probably beyond the investigated part of the system. The probable inaccuracy in the calculated ternary phase diagram is 5.9 °C. The density of the melts of the system KF–K 2 MoO 4 –SiO 2 was measured using the Archimedean method. Based on the obtained density data the molar volume, partial molar volume and excess molar volume of the melts were calculated in order to consider the possible chemical interactions of components. The degree of thermal dissociation of the additive compound K 3 FMoO 4 was calculated based on both the thermodynamic analysis of the phase diagram and the volume properties of the investigated system. It was found that the degree of the thermal dissociation of K 3 FMoO 4 at the melting point attains the value α o =0.81. The dissociation enthalpy, calculated on the basis of the density values is Δ H (dis, K 3 FMoO 4 )=18.8 kJ mol −1 . The surface tension of the molten system KF–K 2 MoO 4 –SiO 2 has been determined using the maximum bubble pressure method. Based on the obtained data the concentration dependence of the surface tension and of the surface tension excess of the investigated system was calculated. The viscosity of melts of the system KF–K 2 MoO 4 –SiO 2 has been measured using the computerised torsional pendulum method. The sum of viscosities of pure components in logarithm values multiplied by their mole fraction was used as an ‘additive’ behaviour. A multiple linear regression analysis was performed to obtain coefficients of the modified Redlich–Kister's type equation. Binary and ternary interactions of the particular components were found. X-ray powder diffraction patterns and IR spectra of the certain quenched powdered samples of both binary and ternary mixtures were recorded. By both techniques the presence of starting compounds and the presence of another compounds, as well, was detected. Some of these compounds were recognized based on the known data. The remaining data were attributed to the possible expected compounds.

Structural and thermodynamic aspects in the molybdenum melts of the system KF–K 2MoO 4–SiO 2

The complex physico-chemical analysis of the system KF–K 2MoO 4–SiO 2, based on the phase diagram, density, surface tension and viscosity measurements, was performed. The phase diagrams of the binary systems as well as that of the ternary system in the range up to 50% SiO 2 were measured using the thermal analysis method. The results proved the presence of the congruently melting compound K 3FMoO 4 at 751 °C in the binary melt KF–K 2MoO 4. The strong positive deviation from ideal behaviour of the ternary system KF–K 2MoO 4–SiO 2 was ascribed to the formation of heteropolyanions [SiMo 12O 40] 4− in the melt. In the investigated concentration range of the ternary system, no eutectic point has been found. It lies most probably beyond the investigated part of the system. The probable inaccuracy in the calculated ternary phase diagram is 5.9 °C. The density of the melts of the system KF–K 2MoO 4–SiO 2 was measured using the Archimedean method. Based on the obtained density data the molar volume, partial molar volume and excess molar volume of the melts were calculated in order to consider the possible chemical interactions of components. The degree of thermal dissociation of the additive compound K 3FMoO 4 was calculated based on both the thermodynamic analysis of the phase diagram and the volume properties of the investigated system. It was found that the degree of the thermal dissociation of K 3FMoO 4 at the melting point attains the value α o=0.81. The dissociation enthalpy, calculated on the basis of the density values is Δ H(dis, K 3FMoO 4)=18.8 kJ mol −1. The surface tension of the molten system KF–K 2MoO 4–SiO 2 has been determined using the maximum bubble pressure method. Based on the obtained data the concentration dependence of the surface tension and of the surface tension excess of the investigated system was calculated. The viscosity of melts of the system KF–K 2MoO 4–SiO 2 has been measured using the computerised torsional pendulum method. The sum of viscosities of pure components in logarithm values multiplied by their mole fraction was used as an ‘additive’ behaviour. A multiple linear regression analysis was performed to obtain coefficients of the modified Redlich–Kister's type equation. Binary and ternary interactions of the particular components were found. X-ray powder diffraction patterns and IR spectra of the certain quenched powdered samples of both binary and ternary mixtures were recorded. By both techniques the presence of starting compounds and the presence of another compounds, as well, was detected. Some of these compounds were recognized based on the known data. The remaining data were attributed to the possible expected compounds.

Crystal growth of barium aluminum (gallium) borates

AbstractCrystals listed in the JCPDS card in the BaO‐B2O3‐Al2O3 and BaO‐B2O3‐Ga2O3 systems are of interest from a viewpoint of the Second Harmonic Generation (SHG). Ba5Al4B12O29 and Ba5Ga4B12O29 are promising new crystals with SHG activity and with congruent melting mode. These crystals can be grown by the Czochralski method with a size of 3 × 3 × 0.1‐1mm. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of a pseudo-binary SrSe–Ga 2Se 3 phase diagram and single crystal growth of SrGa 2Se 4 compounds

We have investigated chemical reaction processes and thermal characteristics of IIa–III 2–VI 4 compounds in order to grow their single bulk crystals. Up to now, single crystals of Ca and Sr thiogallates have been successfully grown by the melt growth method based on their pseudo-binary phase diagrams. Here, a similar diagram of the SrSe–Ga 2Se 3 system has been constructed for the first time, where a eutectic reaction is found in the range of excess Ga 2Se 3 concentration, and it is shown that the SrGa 2Se 4 compound has a congruent melting point (1110 °C) suitable for the melt growth. A single crystal is grown from the melt by the horizontal Bridgman method. A trial is also made to grow a high-quality single crystal of CaGa 2S 4 already known as being grown easily.

Dielectric and Infrared Response of Ba0.77Ca0.23TiO3

Low-frequency (100 Hz–1 MHz) dielectric permittivity and infrared reflectivity studies were performed for barium-calcium-titanate single crystals of the congruently melting composition Ba0.77Ca0.23TiO3, nominally pure (BCT77/23) and doped by Cr (70 ppm, BCT77/23:Cr). Both the materials reveal a sharp ϵ′(T) maximum at 375 K (cooling run) with a pronounced temperature hysteresis, evidencing a first-order ferroelectric phase transition (PT) O h → C 4v. The high-temperature wing of the ϵ′(T) maximum reveals a weak dispersion of a non-relaxation nature. The low-temperature wing contains a reproducible step in the ϵ′(T) dependence. Another low-temperature PT was found at ∼41 K. Cr doping strongly influences this PT shifting the position of the ϵ′(T) maximum to ∼20 K. Polarised IR reflectivity shows a huge A 1 − E splitting of TO 1 polar phonons below the ferroelectric PT as in the pure BaTiO3. IR spectra indicate a displacive and diffuse character for the low temperature PT, but leave open the question about the predominance of displacive or order-disorder character of the high temperature PT.

Effects of laser sintering processing parameters on the microstructure and densification of iron powder

The densification behavior and the attendant microstructural features of iron powder processed by direct laser sintering were investigated. The effects of processing parameters such as laser power, scan rate, scan line spacing, thickness of layer, scanning geometry and sintering atmosphere were studied. A specific energy input ( ψ) was defined using the “energy conservation” rule to explore the effects of the processing condition on the density and the attendant microstructure of laser sintered iron. It was found that the sintered density increased sharply with increasing the specific energy input until a critical energy input had been reached ( ψ∼0.2 kJ mm −3). The microstructure consists of large pores (>0.5 mm) and elongated ferrite grains parallel to the building direction. The increase in the sintered density was followed with further increasing the specific energy, but at slower rate. Intensifying the energy input over 0.8 kJ mm −3 leads to the formation of horizontally elongated pores while the sintered density remains almost constant. The inter-agglomerates are fully dense and consist of elongated ferrite grains which are oriented parallel to the building direction. The iron powder was used as a model material so the outcomes are generic and can be applied to other material systems with congruent melting point or systems which melting/solidification approach is the mechanism feasible for the rapid bonding of metal powders in direct laser sintering.

Differential drop calorimetry for the determination of enthalpy of fusion

Drop calorimetry was modified in order to adapt it to the measurement of the enthalpy of fusion of materials with above-ambient fusion temperatures, T F, such as salt hydrates suitable as phase change materials. In the modified form, differential drop calorimetry, the energy delivered to the molten sample at temperatures above T F and by the crystalline sample at temperatures below T F are extrapolated to T F to give the enthalpy of freezing. The calorimetric system was calibrated electrically and with acetamide ( T F=353.3 K) and applied to the congruently melting magnesium nitrate hexahydrate, cadmium nitrate tertrahydrate, ammonium alum, and several hydrated salt mixtures.

Partial Melting of Metapelitic Rocks Beneath the Bushveld Complex, South Africa

Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high- and low-aH2O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H2O-rich volatile phase derived from underlying, dehydrating rocks. At lower grade (T < 700°C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (<1 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3–4%), preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H2O. Subhorizontal volatile-phase flow was concentrated within thin (∼10 mm) metapsammite horizons that are preserved as stromatic quartz–sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz–sillimanite veins and along the axial planar schistosity. Closer to the contact (T > 725°C), volatile-phase-absent, biotite-consuming melting and melt extraction produced coarse-grained garnet–cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.

Study of the thermal stability of phases in the Mg-Al system

A part of the Al-Mg phase diagram was studied in the range of composition between 48 and 61 at.% Al by DSC and SEM/EDS methods. The temperature ranges of stability of the β, γ, and ɛ phases were considered. It was found that the congruent melting temperature of the β phase was 450±1 °C. The Al content in that phase was determined to be 61±1 at.% Al at 420 °C. The upper temperature limit of the stability of the ɛ phase was established to be 427±1 °C. The Al content changed from 54 at.% at 390 °C to 56 at.% at 420 °C. The lower temperature limit of the ɛ phase formation was not determined, as a result of a slow ɛ=β+γ reaction. The hypothetical λ or ζ phases were not found, but it was observed that decomposition of oxides might produce extra thermal effects.

Experimental and petrological constraints on lunar differentiation from the Apollo 15 green picritic glasses

Abstract— Phase equilibrium experiments on the most magnesian Apollo 15C green picritic glass composition indicate a multiple saturation point with olivine and orthopyroxene at 1520°C and 1.3 GPa (about 260 km depth in the moon). This composition has the highest Mg# of any lunar picritic glass and the shallowest multiple saturation point. Experiments on an Apollo 15A composition indicate a multiple saturation point with olivine and orthopyroxene at 1520°C and 2.2 GPa (about 440 km depth in the moon). The importance of the distinctive compositional trends of the Apollo 15 groups A, B, and C picritic glasses merits the reanalysis of NASA slide 15426,72 with modern electron microprobe techniques. We confirm the compositional trends reported by Delano (1979, 1986) in the major element oxides SiO2, TiO2, Al2O3, Cr2O3, FeO, MnO, MgO, and CaO, and we also obtained data for the trace elements P2O5, K2O, Na2O, NiO, S, Cu, Cl, Zn, and F.Petrogenetic modeling demonstrates that the Apollo 15 A‐B‐C glass trends could not have been formed by fractional crystallization or any continuous assimilation/fractional crystallization (AFC) process. The B and C glass compositional trends could not have been formed by batch or incremental melting of an olivine + orthopyroxene source or any other homogeneous source, though the A glasses may have been formed by congruent melting over a small pressure range at depth. The B compositional trend is well modeled by starting with an intermediate A composition and assimilating a shallower, melted cumulate, and the C compositional trend is well modeled by a second assimilation event. The assimilation process envisioned is one in which heat and mass transfer were separated in space and time. In an initial intrusive event, a picritic magma crystallized and provided heat to melt magma ocean cumulates. In a later replenishment event, the picritic magma incrementally mixed with the melted cumulate (creating the compositional trends in the green glass data set), ascended to the lunar surface, and erupted as a fire fountain. A barometer created from multiple saturation points provides a depth estimate of other glasses in the A‐B‐C trend and of the depths of assimilation. This barometer demonstrates that the Apollo 15 A‐B‐C trend originated over a depth range of ˜460 km to ˜260 km within the moon.

The nature of low-frequency Raman scattering in congruent melting crystals of lithium niobate

Raman spectra of congruent melting crystals of lithium niobate are studied. It is shown that a band near ∼100 cm−1 corresponds to first-order Raman scattering and is due to nonstoichiometry of the crystal. The adequacy of various models for description of this band is discussed.

Aqueous nonelectrolyte solutions. Part XIX. Congruent dissociation melting point and the formula of structure II methane hydrate

Twenty-four equilibrium pressures, P(h2l1g), of structure II methane hydrate h2 with water l1 between 27.0 and 46.9°C are well represented by a four-parameter equation, which indicates a standard error (SE) of 1.95% on a single pressure measurement. Forty equilibrium constants Kp(h2[Formula: see text]l1g) for dissociation of structure II methane hydrate into water and methane between 27.0 and 47.7°C and at pressures up to 784 MPa at 45.0°C are best represented by a three-parameter thermodynamic equation, which indicates an SE 1.25% on a single Kp(h2[Formula: see text]l1g) determination. The congruent dissociation melting point C(h2l1gxm) of structure II methane hydrate is at 47.71°C with SE 0.03°C and at pressure 533 MPa with SE 5 MPa. The congruent Kp(h2[Formula: see text]l1g) is 102.9 with SE 0.3 MPa, ΔH°t(h2[Formula: see text]l1g) is 61 531 with SE 244 J mol–1, and the congruent formula is CH4·5.670H2O with SE 0.061H2O. At congruent point ΔV(h2[Formula: see text]l1g) = 0 and its estimate is 1.0 with SE 1.6 cm3 mol–1. Stability range of structure II methane hydrate with water extends from quadruple point Q(h1h2l1g) at 26.7°C and 55.5 MPa up to quadruple point Q(h2h3l1g) at 47.3°C and 620 MPa. The metastability range of structure I methane hydrate with water is discussed.Key words: methane hydrate, clathrate structure II, stability range, dissociation equilibrium constant, formula, congruent melting point, metastability of structure I hydrate.

Understanding congruent melting in binary solids: Molecular models of benzene–hexafluorobenzene mixtures

We present a Monte Carlo computer simulation study of solid–fluid and solid–solid equilibrium in molecular models representative of binary mixtures of benzene and hexafluorobenzene. The models considered treat the molecular shape with hard sphere interaction site potentials and the electrostatic interactions via quadrupole–quadrupole potentials. Monte Carlo simulations are used to calculate the free energy of the pure solids and the 1:1 solid compound that appear in the phase diagram, and also the solution thermodynamics in the fluid phase. In common with the real fluid phase mixtures of benzene and hexafluorobenzene the fluid phase of the model is close to an ideal mixture. The model also exhibits solid–fluid and solid–solid equilibrium similar to that seen experimentally.The results indicate that enhanced stability of the benzene-hexafluorobenzene 1:1 solid compound can be understood in terms of the cooperative effects of electrostatic interactions and the packing of the anisotropic repulsive cores of the molecules.

Nonstoichiometry and P–T–x diagrams of binary systems

A P–T–x three-dimensional space diagram forms the complete representation between pressure (P), temperature (T) and composition (x) of coexisting phases—solid (S), liquid (L) and vapor (V). It gives the number of nonstoichiometric compounds that may be formed by the components and the stability limits of phases which are in equilibrium. Definitions of stoichiometry and nonstoichiometry are given. Some features of P–T–x diagrams with nonstoichiometric compounds are considered: maximum (Tmmax) and congruent (Tmc) melting points, difference between the compositions of solid (xS), liquid (xL) and vapor (xV): xL≠xS≠xV at maximum melting point T=Tmmax, the width and position of the homogeneity range, and the nonstoichiometry caused by defects are also discussed. A new technique for the investigation of P–T–x diagrams is presented.

Phase Relations in the BaO · 2B 2 O 3 –La 2 O 3 · 3B 2 O 3 System

The BaO · 2B2O3–La2O3 · 3B2O3 join of the ternary system BaO–La2O3–B2O3 is studied using differential thermal analysis, x-ray diffraction, and density measurements. The join is shown to be pseudobinary, with eutectic phase relations. In the composition range 5–99.8 mol % La2O3 · 3B2O3 , glassy materials are obtained, which seems to be associated with the existence of the congruently melting compound BaO · La2O3 · 5B2O3 .

Crystallization and Phase Equilibria in the Tb2O3–Ga2O3 System

The gallium-rich portion of the Tb2O3–Ga2O3 system is studied experimentally. It is found that the compound Tb3 + xGa5 – xO12 (0 < x < 0.1) undergoes a congruent melting. The TbGaO3 compound exists only near the peritectic melting temperature and undergoes an incongruent melting. The liquidus curve is determined experimentally in the region of primary crystallization of Tb3Ga5O12. The conditions of growing Tb3Ga5O12 single crystals by the Czochralski method are analyzed.

Comments on “Phase equilibria in the pseudo-binary systems LiF–CaAlF 5 and LiF–SrAlF 5” [J. Crystal Growth 235 (2002) 596

Different quasi-binary sections, binary sub-systems, and ternary sub-systems of the ternary systems LiF–Me IIF 2–AlF 3 (Me II=Ca, Sr) were reported in a number of recent papers. From these data it is likely, that CaAlF 5 melts incongruently, but SrAlF 5 melts congruently. The present paper confirms the incongruent melting of CaAlF 5 by DTA measurements within the quasi-binary system CaF 2–AlF 3. Chen et al. construct an incorrect “quasi binary” section LiF–CaAlF 5, outgoing from the wrong assumption of congruent melting of CaAlF 5.