Composition-induced Phase Transition Research Articles

Enhancing Surface Strain of Intermetallic Fuel Cell Catalysts by Composition-Induced Phase Transition.

The lattice parameter of platinum-based intermetallic compounds (IMCs), which correlates with the intrinsic activity of the oxygen reduction reaction (ORR), can be modulated by crystal phase engineering. However, the controlled preparation of IMCs with unconventional crystal structures remains highly challenging. Here, we demonstrate the synthesis of carbon-supported PtCu-based IMC catalysts with an unconventional L10 structure by a composition-regulated strategy. Experiment and machine learning reveal that the thermodynamically favorable structure changes from L11 to L10 when slight Cu atoms are substituted with Co. Benefiting from crystal-phase-induced strain enhancement, the prepared L10-type PtCu0.8Co0.2 catalyst exhibits much-enhanced mass and specific activities of 1.82 A mgPt-1 and 3.27 mA cmPt-2, which are 1.91 and 1.73 times higher than those of the L11-type PtCu catalyst, respectively. Our work highlights the important role of crystal phase in determining the surface strain of IMCs, and opens a promising avenue for the rational preparation of IMCs with different crystal phases by doping.

Effects of (KMg)3+ co-doping and sintering temperature on the negative thermal expansion property of Sc2W3O12 ceramics

(KMg)3+ co-doped Sc2W3O12 ceramics have been prepared via co-precipitation method. Effects of (KMg)3+ co-doping amount and sintering temperature on the phase constitution, microstructure, morphology, and negative thermal expansion (NTE) performance of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1.25) ceramics were studied. Results indicate that: Sc3+ cations can be substituted by (KMg)3+ dual-cations and a composition-induced structural phase transition was observed in KxMgxSc2-xW3O12. For x ≤ 0.25, orthorhombic Sc2W3O12 gradually changed into hexagonal KxMgxSc2-xW3O12 and coexisted. Single phase KxMgxSc2-xW3O12 with hexagonal symmetry can be prepared for 0.25 < x ≤ 1. When x ≥ 1.25, (KMg)3+ substitution reaches its solid solution limit in Sc2W3O12, and impurity MgWO4 formed in the samples. As co-doping amount x increased, the NTE performance of the KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics gradually enhanced. The coefficient of thermal expansion (CTE) of KxMgxSc2-xW3O12 (0 ≤ x ≤ 1) ceramics enhanced from −2.14 × 10−6 °C−1 to −19.68 × 10−6 °C−1 in 30–700 °C. With the increase of sintering temperature from 800 to 1100 °C, the grains grew up and the density of the K0.75Mg0.75Sc1.25W3O12 ceramics increased as well, and the NTE performance was also improved. The linear CTE of K0.75Mg0.75Sc1.25W3O12 ceramics enhances from −2.15 × 10−6 °C−1 to −14.70 × 10−6 °C−1 in 30–700 °C.

Large electric field-induced strain and excellent photoluminescence properties of Pr-modified 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 lead-free ferroelectric ceramics

The 0.94Bi0.5-xPrxNa0.5TiO3-0.06BaTiO3 (BNT-6BT-xPr) lead-free ceramics with doping content of x = 0, 0.01, 0.025, 0.04, 0.05, 0.075, and 0.1 were synthesized by the conventional solid-state reaction method. After doping with the Pr element, a composition-induced phase transition took place, which from the rhombohedral perovskite (R3c) and tetragonal perovskite (P4bm) phases coexisting into the pseudo-cubic phase. At the same time, the introduction of the Pr element had a beneficial effect on grain growth, and the average size of the crystal particle increased from 299 to 742 nm. The Curie temperature Tm showed no obvious change with the increasing Pr content, but the temperature TF-R of the ferroelectric-relaxor phase transition was gradually shifted toward lower temperatures. In addition, the depolarization temperature Td gradually decreased from 125 to 53 °C. In the BNT-6BT-0.025Pr ceramics, giant electric field-induced strain (Smax) and normalized piezoelectric coefficient (d33*) were obtained (Smax = 0.58%, d33* = 723 pm/V). Furthermore, a significant photoluminescence emission intensity was achieved, exhibiting a bright red color. Due to significant enhancement of the piezoelectric and the photoluminescence properties, the BNT-6BT-0.025Pr ceramics exhibited strong competitiveness in actuators and multifunctional devices, which may broaden their application fields.

Enhanced negative thermal expansion property in (KMg)xSc2-x(WO4)3 ceramics synthesized by solid–state reaction

Negative thermally expanded (NTE) (KMg)xSc2-x (WO4)3 (0 = x ≤ 1.5) ceramics have been prepared via solid-state reaction. X-ray diffraction (XRD) analysis reveals that (KMg)xSc2-x (WO4)3 undergoes a composition-induced phase transition as the (KMg)3+-substitution content increases. For x = 0.25, both hexagonal (KMg)xSc2-x (WO4)3 and orthorhombic Sc2(WO4)3 coexisted in the sample. For 0.25 < x ≤ 1, a single-phase, hexagonal (KMg)xSc2-x (WO4)3, was obtained. When x is further increased to 1.25, monoclinic MgWO4 impurity was detected in the sample. Meanwhile, as x increases, the grain size of (KMg)xSc2-x (WO4)3 ceramics grows up. Hexagonal (KMg)xSc2-x (WO4)3 ceramics show high densification and strong NTE property. Thermomechanical analysis indicates that the linear coefficients of thermal expansion (CTEs) of hexagonal (KMg)xSc2-x (WO4)3 ceramics decrease from −7.48 × 10−6 °C −1 to −14.06 × 10−6 °C −1 with the increase of x. High-temperature XRD analysis reveals that hexagonal (KMg)0.5Sc1.5(WO4)3 show an anisotropic NTE. The intrinsic linear CTEs are αa = αb = −6.74 × 10−6 °C −1, αc = 11.03 × 10−6 °C −1 and the volumetric CTE is αv = −2.52 × 10−6 °C −1.

Enhanced ferroelectric and piezoelectric properties of BCT-BZT at the morphotropic phase boundary driven by the coexistence of phases with different symmetries

The discovery of lead-free piezoelectric materials is crucial for future information and energy storage applications. Enhanced piezoelectric and other physical properties are commonly observed near the morphotropic phase boundary (MPB) composition of ferroelectric solid solutions. The $(1\ensuremath{-}x)\mathrm{Ba}({\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}){\mathrm{O}}_{3}\text{\ensuremath{-}}x({\mathrm{Ba}}_{0.7}{\mathrm{Ca}}_{0.3}){\mathrm{TiO}}_{3}$ $(\mathrm{BZT}\text{\ensuremath{-}}x\mathrm{BCT})$ system exhibits a large electromechanical response around its MPB region at $x=0.5$. We report experimental and theoretical results of $\mathrm{BZT}\text{\ensuremath{-}}x\mathrm{BCT}$ over a wide composition range $(0.3\ensuremath{\le}x\ensuremath{\le}1.0)$. X-ray diffraction and Raman spectroscopy studies indicate a composition-induced structural phase transition from a rhombohedral $(R3m)$ phase at $x\ensuremath{\le}0.4$ to a tetragonal $(P4mm)$ phase at $x\ensuremath{\ge}0.6$ through a multiphase coexistence region at 0.45 \ensuremath{\le} $x\ensuremath{\le}0.55$ involving orthorhombic + tetragonal (Amm2 + $P4mm$) phases. First-principles calculations elucidate the phase competition in the coexistence region. The critical composition $(x=0.5)$ displays enhanced dielectric, ferroelectric, and piezoelectric properties, where notably ${d}_{33}\ensuremath{\sim}320$ pC/N. This paper provides clear evidence of Amm2 + $P4mm$ crystallographic phases in the MPB region, which is responsible for the improved functional properties.

Highly enhanced discharged energy density and superior cyclic stability of Bi0.5Na0.5TiO3-based ceramics by introducing Sr0.7Ca0.3TiO3 component

The composition-induced phase transition from ferroelectric to relaxor and cycling stability is critical for energy storage application of Bi0.5Na0.5TiO3 (BNT) ceramics. The paraelectric Sr0.7Ca0.3TiO3 (SCT) component is selected to modify the ferroelectric phases of BNT ceramics and improve its energy storage properties. A tiny amount of manganese is used to reduce leakage current. High breakdown field of 210 kV/cm, large recoverable energy density of 2.64 J/cm3, and high power density of 61.89 MW/cm3 are achieved simultaneously in the SCT modified BNT ceramics. Meanwhile, an improved discharged energy density for practical application is obtained within the temperature range of 20–120 °C. By deviating from the critical composition with giant strain, the discharged energy density of 1.11 J/cm3 with excellent stability of cycling performance after 7100 cycles can be achieved. Therefore, the as-proposed solution can solve the ceramic cracking originating from high electric field-induced giant strain, and promote the practical capacitor application.

Dielectric properties and relaxor ferroelectric behavior of (1–y)Ba(Zr0.1Ti0.9)O3–yBa(Zn1/3Nb2/3)O3 ceramics

Abstract The microstructure, dielectric and ferroelectric properties of (1–y)Ba(Zr0.1Ti0.9)O3–yBa(Zn1/3Nb2/3)O3 (y=0–0.05) ceramics prepared by traditional solid state method were investigated by X-ray diffractometer, scanning electron microscope, electric parameter testing system and ferroelectric tester. It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y. The permittivity maximum er,max is suppressed from 8948 to 1611 at 1 kHz with increasing y, and the ferroelectric–paraelectric phase transition temperature Tm decreases from 93 to –89 °C at 1 kHz as y increases. The composition-induced diffuse phase transition is enhanced with increasing y. The relaxor-like ferroelectric behavior with a strong frequency dispersion of Tm and permittivity at T

Mid-temperature thermoelectric performance of zone-melted Sb2(Te,Se)3 alloys near phase transition boundary

(Bi,Sb)2(Te,Se)3 alloys are widely used commercial thermoelectric (TE) materials for solid-state refrigeration around room temperature. The composition-induced structural phase transition could be realized by varying the compositions in these alloys, which may largely alter the electronic structure and phonon dispersion. Among them, the Se-alloyed Sb2Te3 accompanied with structural transition is seldom reported. Herein, the interrelations of Se-alloying induced changes in structural phase transition, band structure and TE properties of p-type zone-melted Sb2Te3-xSex (x = 1.5–2.4) alloys near phase transition boundary are systematically investigated. The results demonstrate that Sb2Te3-xSex shows a structural transition from a rhombohedral phase to mixed structure at x = 2.0. The carrier concentration and bandgap at room temperature of Sb2Te3-xSex (x = 1.5–2.4) constantly decrease with increasing Se contents x. The zT peak of the Sb2TeSe2 matrix is improved and shifted to higher temperature by optimizing carrier concentration via Ag doping. A maximum zT of ∼0.4 is obtained at 680 K in Sb1.97Ag0.03TeSe2 alloy, about 100% enhancement compared with the undoped sample.

Sol-gel processing and nanoscale characterization of (Bi0.5Na0.5)TiO3-SrTiO3 lead-free piezoelectric thin films

(1-x)(Bi0.5Na0.5)TiO3-xSrTiO3 (BNT-xST) (0 ≤ x ≤ 0.4) thin films were fabricated using a sol-gel technique on Pt(111)/Ti/SiO2/Si(100) substrates, which were investigated by piezoresponse force microscopy (PFM) and Raman spectroscopy. The composition-induced phase transition was analyzed by acquiring structural variations and the domain distribution on a local scale. Raman spectra showed phonon anomalies with peak broadening and shifting when increasing SrTiO3 (ST) concentrations were used. Changes in the domain morphology with changes in the composition were observed, and grains smaller than 0.5µm were observed at lower concentrations of x = 0–0.25, while larger grains appeared with increasing ST contents. The switching spectroscopy PFM (SS-PFM) results supported a ferroelectric (FE) to relaxor ferroelectric (RFE) phase transition at approximately x ≈ 0.3 by means of analyzing the parameters as a function of the composition including the piezoresponse parameters of hysteresis loops (Dmax, Drem) and amplitude butterfly loops (Stotal, Sneg). Hence, these results demonstrated that the composition-driven FE to RFE phase transition behavior, which is consistent with the localized response behavior, is dependent on the ST content in bulk BNT-xST ceramics.

Evolution of relaxor behavior and high-field strain responses in Bi(Mg1/2Ti1/2)O3-PbTiO3-Pb(Ni1/3Nb2/3)O3 ferroelectric ceramics

A composition-induced phase transition from a diffuse-type ferroelectric to a relaxor ferroelectric and then to a weak relaxor ferroelectric was observed in a new (0.68-x)Bi(Mg1/2Ti1/2)O3-0.32PbTiO3-xPb(Ni1/3Nb2/3)O3 (BMT-PT-xPNN) ternary system. Meanwhile, a gradual evolution process of nonergodic-ergodic-nonergodic realxor states was identified at room temperature from the BMT-rich side to the PNN-rich side. Two strain maxima of ∼0.42% and ∼0.29% under 7 kV/mm were obtained in the x = 0.2 and x = 0.65 samples, respectively. It is worthy to note that the high-field strain of the x = 0.2 composition was thermally stable but seriously hysteretic. By comparison, the x = 0.65 composition exhibited a weakly hysteretic strain and a relatively small threshold electric field. This phenomenon was attributed to larger sizes and slower dynamics of polar nanoregions (PNRs) in BMT-rich compositions than those in PNN-rich compositions. Moreover, it was believed that the strain hysteresis of BMT-PT-xPNN relaxor ferroelectrics would be dominated by the growth process of PNRs into ferroelectric microdomains during loading and their subsequent dissociation process during unloading.

Control of the white-light emission in the mixed two-dimensional hybrid perovskites (C6H11NH3)2[PbBr4−xIx

The control of the composition of mixed organic-inorganic hybrid perovskites by solid-state alloying is a very efficient tool to tune the band gap of the material, leading to enhanced optical performances. At this end, we have elaborated a series of mixed-halide two-dimensional hybrid perovskites (C6H11NH3)2[PbBr4−xIx], with 0 ≤ x ≤ 4, for which we studied the composition-dependence of the structural parameters and their effect on the white-light luminescence properties at room temperature. The structural analysis revealed the existence of a composition-induced structural phase transition occurring in the range 2 < x < 2.4, associated to a change between Cmc21 and Pbca space groups. The optical properties, investigated using optical absorption and photoluminescence measurements, have shown that bromine (Br) to iodine (I) halogen ion substitution redshifts the excitonic optical absorption band, as a result of the hybridization of the valence band structure, built from np orbitals of Br and/or I halogens and 6s orbitals of lead. When iodine was predominant (x > 2), the photoluminescence spectra showed a sharp peak with a relatively small Stokes shift (less than 0.2 eV) attributed to free or bound excitons emissions. In contrast, when the bromide is majority (x < 2), the photoluminescence response consists in a broadband white-light emission with a very large Stokes shift (between 1.2 eV and 0.3 eV), attributed to self-trapped excitons activated by a strong structural distortion of the inorganic sheets. Confronting optical and structural data highlighted the effect of the structure in monitoring the optical properties, since the intensity of the white-light emission increases with the bromine content and was found to excellently correlate with the change of the angular distortion of inorganic octahedra PbX6 (X = I/Br).

Temperature and composition induced phase transitions in Sr2−xCa1+xTeO6 (0 ≤ x ≤ 2) double perovskite oxides

Structures of Sr2−xCa1+xTeO6 double perovskites have been studied by the profile analysis of X-ray diffraction data and Raman spectroscopy at room temperature. This series adopts a monoclinic symmetry for the compositions (0 ≤ x ≤ 0.5) with P21/n as space group and a triclinic system with P1¯ space group for the compositions (0.5 < x ≤ 2). These results were confirmed by the observed tolerance factor calculated from the distances obtained from the Rietveld refinements which indicates that the true tilt system for the compositions range (0.5 < x ≤ 2) is the triclinic tilt system. Clear changes in the Raman modes centered at 600, 610 and 620 cm−1 and the FWHM of OTeO bending vibrations, centered at 738 cm−1 confirmed that the triclinic symmetry takes place between the compositions x = 0.5 and x = 1. Furthermore, Raman spectroscopy studies at high temperature were done for Ca3TeO6. For this compound, considerable changes in the temperature dependence of the modes were well illustrated.

Structural and magnetic properties of morphotropic phase boundary involved Tb1−xGdxFe2 compounds

In the present paper, structural, magnetic and magnetostrictive properties of Tb1−xGdxFe2 (0 ≤ x ≤ 1.0) were studied. Synchrotron x-ray diffraction (XRD) results show the non-cubic symmetry of Tb1−xGdxFe2 at room temperature and composition-induced crystallographic phase transition from rhombohedral phase to tetragonal phase. The Gd concentration dependent lattice parameters, lattice distortion and change of easy magnetic direction were detected by synchrotron XRD. With the Gd concentration increases, Curie temperature Tc increases while room temperature magnetization and magnetostriction coefficient λ111 and the anisotropy of TbFe2 decrease. The decrease in spontaneous magnetostriction coefficient λ111 with increasing Gd substitution can be understood on the basis of the single-ion model; the corresponding decrease of magnetostriction for Tb1−xGdxFe2, and the large magnetostriction value occurs on the Tb-rich side, are ascribed to decrease of λ111.

Crystal structure and microwave dielectric characteristics of Zr-substituted CoTiNb2O8 ceramics

Co(Ti1-xZrx)Nb2O8 microwave dielectric ceramics were synthesized via the conventional solid-state reaction route. The dependence of microwave dielectric properties on the crystal structure was discussed. The phase transitions were analyzed using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. A series of composition-induced phase transitions were confirmed via the sequence: tetragonal rutile structure→coexistence of rutile and wolframite phase→monoclinic wolframite structure. For the Co(Ti1-xZrx)Nb2O8 compounds, Zr substitutions from 0 to 1 led to a decrease in the εr from 62.4 to 23.8. In contrast to εr, Q×f increased considerably, which was explained in terms of packing fraction. The τf was correlated with oxygen octahedral distortion and B-site bond valence. A near zero τf of 4.4ppm/°C was obtained in the CoTi0.4Zr0.6Nb2O8 ceramics with an εr of 29.9 and a high Q×f of 72,833GHz.

Antagonism between Spin-Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH3NH3Sn1-xPbxI3.

Halide perovskite solar cells are a recent ground-breaking development achieving power conversion efficiencies exceeding 18%. This has become possible owing to the remarkable properties of the AMX3 perovskites, which exhibit unique semiconducting properties. The most efficient solar cells utilize the CH3NH3PbI3 perovskite whose band gap, Eg, is 1.55 eV. Even higher efficiencies are anticipated, however, if the band gap of the perovskite can be pushed deeper in the near-infrared region, as in the case of CH3NH3SnI3 (Eg = 1.3 eV). A remarkable way to improve further comes from the CH3NH3Sn1-xPbxI3 solid solution, which displays an anomalous trend in the evolution of the band gap with the compositions approaching x = 0.5 displaying lower band gaps (Eg ≈ 1.1 eV) than that of the lowest of the end member, CH3NH3SnI3. Here we use first-principles calculations to show that the competition between the spin-orbit coupling (SOC) and the lattice distortion is responsible for the anomalous behavior of the band gap in CH3NH3Sn1-xPbxI3. SOC causes a linear reduction as x increases, while the lattice distortion causes a nonlinear increase due to a composition-induced phase transition near x = 0.5. Our results suggest that electronic structure engineering can have a crucial role in optimizing the photovoltaic performance.

High temperature and composition induced phase transitions in LiZnV1−xAsxO4 phenacites: Crystal structure and Raman spectroscopy studies

In this work, using techniques of X-ray diffraction and Raman spectroscopy, we report the composition and high-temperature induced phase transition in the system LiZnV1−xAsxO4 (0⩽x⩽1). Both techniques showed that the increase of arsenic amount induced a structural transition from R-3 LiZnVO4 type to LiZnAsO4 type belonging to R3 space group, the transition occurring between x=0.7 and x=0.8. Furthermore, increasing temperature for the compositions (0.8⩽x⩽1) manifests a transition from the LiZnAsO4 structural type with R3 space group to the R-3 LiZnVO4 structural type. For this series, the transition from the space group R3 to the centro-symmetric space group R-3 shows considerable changes in the compositional and temperature dependencies of the bands: spectral positions of all the observed Raman bands exhibit shifts linearly proportional to the temperature increase, with points of shift-rate changes revealing a symmetry change. The Raman-spectra based temperature-composition phase diagram confirms the results obtained using the method of Rietveld refinements, thus showing the R-3 to R3 transition occurring between x=0.7 and 0.8.

Diffused phase transitions in Pb(Zr0.65Ti0.35)O3-Pb(Fe2/3W1/3)O3 multiferroics

The solid solutions of (1−x)Pb(Zr0.65Ti0.35)O3-xPb(Fe2/3W1/3)O3 in different ratios were fabricated by a high-temperature solid-state reaction method using high-purity oxides for possible multi-ferroic applications. Structural analysis using x-ray diffraction powder patterns of the system by Rietveld refinement method exhibits the formation of rhombohedral phase with R3c space group. Detailed studies of dielectric permittivity as a function of temperature of the systems show that the frequency independent dielectric maximum temperature shifts toward room temperature on increasing x. In addition to this, the degree of diffuseness of the permittivity anomaly is more pronounced for higher content of Pb(Fe2/3W1/3)O3, implying the existence of a composition-induced diffuse phase transition for the limited range of compositional ratios. Detailed impedance spectroscopy analysis shows the contributions of grain, grain boundary, and interfacial polarizations in the resistive characteristics and conduction mechanism of the materials. The weak ferromagnetic and saturated ferroelectric loops indicate that system with x = 0.1, 0.2 have good multiferroic characteristics, and may be useful for future spintronic devices.

Structural phase transformation and microwave dielectric studies of SmNb(1-x)(Si(1/2)Mo(1/2))(x)O(4) compounds with fergusonite structure.

Temperature- and composition-induced phase transition in SmNbO4 was studied by differential scanning calorimetry, Raman spectroscopy and high-temperature powder X-ray diffraction measurements. In situ X-ray diffraction studies revealed that SmNbO4 possesses a monoclinic fergusonite crystal structure at ambient temperature and transforms to a tetragonal scheelite structure above the transition temperature (To ≥ 800 °C). The second-order nature of this transition was confirmed by observing a linear relationship between the spontaneous strain (es) of SmNbO4 and the Landau order parameter (η) around the phase transition temperature. We stabilized this high-temperature tetragonal scheelite phase at ambient temperature by substituting Si(4+) and Mo(6+) into the Nb site of SmNbO4. The SmNb1-x(Si1/2Mo1/2)xO4 (x = 0.0-0.69) ceramic compositions were prepared by the conventional solid-state reaction method. Rietveld refinement was carried out on all the compositions to examine the phase purity, and the compositions where x < 0.06 all formed a monoclinic fergusonite structure (I2/a space group, Z = 2). Both the X-ray diffraction and Raman spectroscopy measurements revealed that increasing the concentration of x transformed the structure from monoclinic fergusonite to tetragonal scheelite (I41/a space group, Z = 4) at a critical concentration (xc). Both the monoclinic and tetragonal phases coexisted in the composition range of 0.06 ≤ x < xc. The Hakki-Coleman and reflection cavity techniques were used to measure the dielectric constant and quality factor of these stabilized phases, respectively. The temperature coefficient of the resonant frequency was measured by using an invar cavity attached to a programmable hot plate. The high-density samples possessed good microwave dielectric properties.

Soft chemical synthesis and structural characterization of Y2HfxTi2−xO7

A novel soft chemistry route was developed to synthesize Y2HfxTi2−xO7 (0≤x≤2.0) oxide solid solution. An aqueous solution containing reactants was produced to ensure the combustion reaction taking place at the molecular level. A suite of characterization techniques, including X-ray diffraction, Raman, transmission electron spectroscopy, as well as X-ray absorption near edge structure (XANES), is employed to investigate the structural and phase changes of the series. Both X-ray and electron diffraction patterns show that the Y2HfxTi2−xO7 system undergoes a clear composition-induced phase transition from ordered pyrochlore to disordered defect-fluorite at x~1.5. On the other hand, Raman and XANES spectra reveal a gradual evolution of the local structure with the substitution of Hf for Ti.

Phase relations in the system FeCO3-CaCO3 at 6 GPa and 900-1700 C and its relation to the system CaCO3-FeCO3-MgCO3

The subsolidus and melting phase relations in the CaCO 3 -siderite system have been studied in multianvil experiments using graphite capsules at pressure of 6 GPa and temperatures of 900-1700 °C. At low temperatures, the presence of ankerite splits the system into two partial binaries: siderite + ankerite at 900 °C and ankerite + aragonite up to 1000 °C. Extrapolated solvus curves intersect near 50 mol% just below 900 °C. At 1100 and 1200 °C, the components appear to form single-phase solid solutions with space group symmetry R 3 c , while CaCO 3 maintains aragonite structure up to 1600 °C and 6 GPa. The FeCO 3 solubility in aragonite does not exceed 1.0 and 3.5 mol% at 900-1000 and 1600 °C, respectively. An increase of FeCO 3 content above the solubility limit at T &gt; 1000 °C, leads to composition-induced phase transition in CaCO 3 from aragonite, Pmcn , to calcite, R 3 c , structure, i.e., the presence of FeCO 3 widens the calcite stability field down to the P-T conditions of sub-cratonic mantle. The siderite-CaCO3 diagram resembles a minimum type of solid solutions. The melting loop for the FeCO 3 -CaCO 3 join extends from 1580 °C (FeCO 3 ) to 1670 °C (CaCO 3 ) through a liquidus minimum near 1280 ± 20 °C and 56 ± 3 mol% CaCO 3 . At X (Ca) = 0-30 mol%, 6 GPa and 1500-1700 °C, siderite melts and dissolves incongruently according to the reaction: siderite = liquid + fluid. The apparent temperature and X (Ca) range of siderite incongruent dissolution would be determined by the solubility of molecular CO 2 in (Fe,Ca)CO 3 melt. The compositions of carbonate crystals and melts from the experiments in the low-alkali carbonated eclogite (Hammouda 2003; Yaxley and Brey 2004) and peridotite (Dasgupta and Hirschmann 2007; Brey et al. 2008) systems are broadly consistent with the topology of the melting loop in the CaCO 3 - MgCO 3 -FeCO 3 system at 6 GPa pressure: a Ca-rich dolomite-ankerite melt coexists with Mg-Fe-calcite in eclogites at CaO/MgO &gt; 1 and Mg-dolomite melt coexists with magnesite in peridotites at CaO/MgO &lt; 1. However, in fact, the compositions of near solidus peridotite-derived melts and carbonates are more magnesian than predicted from the (Ca,Mg,Fe)CO3 phase relations.