Showing papers on "Orthorhombic crystal system published in 2001"
TL;DR: The phase transition behavior of the BaTiO3-BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques in this paper, where a single-phase perovskite structure was identified by the x-ray diffraction technique.
Abstract: In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...
314 citations
TL;DR: In this paper, it was shown that the alloys Co2Ni1−xGa1+x, x = 0.06, 0.09,0.12, and 0.15, are ferromagnetic shape memory alloys.
306 citations
TL;DR: In this article, a defect chemistry model has been formulated, which qualitatively describes the observed pyrochlore-fluorite structural transition, and a Raman spectrum of orthorhombic Y2TiO5.
291 citations
TL;DR: In this paper, an x-ray study of perovskite-type PbZn1−xTixO3 (PZT) and Pb(Zn 1/3Nb2/3)O3-PbTiO3 systems was conducted.
Abstract: Strong piezoelectricity in perovskite-type PbZn1−xTixO3 (PZT) and Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN–PT) systems is generally associated with the existence of a morphotropic phase boundary (MPB) separating regions with rhombohedral and tetragonal symmetry. An x-ray study of PZN–9% PT has revealed the presence of an orthorhombic phase at the MPB, and a near-vertical boundary between the rhombohedral and orthorhombic phases, similar to that found for PZT between the rhombohedral and monoclinic phases. We discuss the results in the light of a recent theoretical paper by Vanderbilt and Cohen [Phys. Rev. B 63, 94108 (2001)], which attributes these low-symmetry phases to the high anharmonicity in these oxide systems.
282 citations
TL;DR: In this paper, the charge-discharge reaction mechanism of the olivine-type Li x (Mn 0.6 Fe 0.4 )PO 4 (0 ≤ x ≤ 1.0), a possible 4 V class cathode material for lithium batteries, was investigated using equilibrium voltage measurements, X-ray diffraction.
Abstract: The charge-discharge reaction mechanism of the olivine-type Li x (Mn 0.6 Fe 0.4 )PO 4 (0 ≤ x ≤ 1.0), a possible 4 V class cathode material for lithium batteries, was investigated using equilibrium voltage measurements, X-ray diffraction. Mossbauer spectroscopy, and X-ray absorption spectroscopy. The flat two-phase region with an open-circuit voltage (OCV) of ca. 4.1 V (region 1: 0 ≤ x ≤ 0.6, Mn 3+ /Mn 2+ ) and the S-curved single-phase region with OCV 3.5 V (region 11: 0.6 ≤ x ≤ 1.0, Fe 3+ /Fe 2+ ) were clearly identified together with the corresponding change in the unit cell dimensions of the orthorhombic lattice. These features show significant differences from the reaction mechanism of Li x FePO 4 (0 ≤ x ≤ 1) ), in which the whole Fe 3+ /Fe 2+ reaction proceeds in a two-phase manner (LiFePO 4 -FePO 4 ) with a flat voltage profile at 3.4 V.
233 citations
TL;DR: In this paper, an orthorhombic ferroelectric phase was observed in 0.67Pb(Mg1/3Nb2/3)O3−0.33PbTiO3 single crystals.
Abstract: We report on the observation of an orthorhombic ferroelectric phase in 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals, whose polarization is along 〈011〉 direction and stability can be altered by poling conditions. We studied the piezoelectric properties on poled 〈011〉 crystals, in both monodomain and polydomain states, and found that the piezoelectric d32 coefficient, which is the piezoelectric response in perpendicular to the poling direction, is positive in both cases. Based on the phenomenological theory, we show that this is possible in a crystal with the electrostrictive coefficients Q11>Q44–Q12.
229 citations
TL;DR: In this article, the magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific...
Abstract: The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific ...
175 citations
TL;DR: In this article, the microstructure-property relationships for orthorhombic titanium aluminides reveal an attractive combination of low and high temperature loading capabilities, including high room-temperature ductility and good formability, high specific elevated temperature tensile and fatigue strength, reasonable roomtemperature fracture toughness and crack growth behavior.
Abstract: Orthorhombic titanium aluminides represent the youngest class of alloys emerging out of the group of titanium aluminides. These new materials are based on the ordered orthorhombic phase Ti 2 AlNb, which was discovered for the first time in the late 1980s as a constituent in a Ti 3 Al-base alloy. In the 1990s primarily simple ternary Ti-Al-Nb orthorhombic alloys were investigated in countries such as the US, UK, India, France, Japan, and Germany. The drive was mainly provided by jet engine manufacturers and related research labs looking for a damage-tolerant, high-temperature, light-weight material. This follows the aim of further extending the use of lower density titanium-base materials in temperature regimes, where heavy nickel-base superalloys are the only alternative today. The present understanding of microstructure-property relationships for orthorhombic titanium aluminides reveals an attractive combination of low and high temperature loading capabilities. These involve high room-temperature ductility and good formability, high specific elevated temperature tensile and fatigue strength, reasonable room-temperature fracture toughness and crack growth behavior, good creep, oxidation, and ignition resistance combined with a low thermal expansion coefficient. This article reviews the aspects of composition-microstructure-property relationships in comparison to near-a titanium, TiAl, and nickel-base alloys. Special emphasis is also placed on the environmental degradation of the mechanical properties.
159 citations
TL;DR: This work discusses the changes of structural parameters with temperature and proposes a new structural model for the cubic phase of Sr 2 Fe 2 O 5 using neutron powder diffraction.
159 citations
TL;DR: In this paper, the authors synthesized BaCo2O5+x with various oxygen contents and investigated physical and structural properties, which can be classified into three parts of composition region in the structural properties.
158 citations
TL;DR: In this article, transition metal diselenides (MSe2, M=Ni, Co, Fe) were synthesized successfully through a simple solvothermal-reduction reaction at low temperatures.
Abstract: Transition metal diselenides (MSe2, M=Ni, Co, Fe) were synthesized successfully through a simple solvothermal-reduction reaction at low temperatures. EDXA and XPS techniques confirmed that the compositions of these diselenides were in good agreement with their stoichiometries. With increasing temperature, the obtained NiSe2 transformed from initial filament nanocrystallites to final octahedral crystals. The studies on the thermal stability of the as-prepared NiSe2 showed that NiSe acted in an intermediate role in the pyrolysis process. The Raman spectrum of NiSe2 presented the peak of the Se−Se stretching mode at 216.5 cm-1. XRPD techniques revealed that the as-prepared CoSe2 at higher temperature had an orthorhombic structure similar to that of FeSe2. At low temperature this orthorhombic phase CoSe2, which displayed rodlike shape with the growth direction along 〈121〉 , coexisted with a trace of the cubic phase CoSe2.
TL;DR: The intermediacy of the [Cu(imidazole)(2)CO(3)]-H(2)O species in the synthesis of the blue polymorph has been confirmed by spectroscopic and thermal analyses, and the correct Raman spectrum for copper(II) bisimidzolate, common for all polymorphs, has been definitely determined.
Abstract: Copper(II) bisimidazolate affords five different polymorphs; of these, one was structurally characterized 40 years ago by standard single-crystal X-ray diffraction (Jarvis, J. A. J.; Wells, A. F. Acta Crystallogr. 1960, 13, 1027), while the remaining four, selectively prepared as pure polycrystalline phases, have been now studied by X-ray powder diffraction (XRPD) methods. Of the four new (blue, green, olive-green, and pink) phases, three were solved by the ab initio XRPD technique and refined by the Rietveld method, and the fourth phase (pink) could not be structurally characterized. Crystal data for [Cu(imidazolate)(2)](n): blue phase, a = 27.559(3) A, c = 5.3870(9) A, trigonal, R3 macro, Z = 54; green phase, a = 21.139(1) A, b = 19.080(1) A, c = 9.2842(8) A, orthorhombic, Ccca, Z = 20; olive-green phase, a = 11.7556(8) A, b = 23.422(2) A, c = 9.0727(9) A, beta = 104.993(5) degrees, monoclinic, C2/c, Z = 12. All polymorphs contain four-coordinate CuN(4) chromophores and (N,N')-exobidentate imidazolate ligands, but show different spectroscopic and structural properties, the latter ranging from 2D to different 3D networks of the PtS, sodalite, and moganite archetypes. The intermediacy of the [Cu(imidazole)(2)CO(3)]-H(2)O species in the synthesis of the blue polymorph has been confirmed by spectroscopic and thermal analyses. FTIR, Raman, and electronic spectra were correlated with the structural features revealed in the present work, and used to gain insight into the coordination geometry of copper(II) ions of the pink polymorph. In addition, the correct Raman spectrum for copper(II) bisimidazolate, common for all polymorphs, has been definitely determined.
TL;DR: In this article, the results of X-ray diffraction, IR and Raman spectroscopy measurements of Li2CO3-BaCO3 system in a whole range of composition were presented.
TL;DR: In this article, the structure of Na4Co3(PO4)2(P2O7) has been determined by X-ray diffraction techniques using as the starting model the structure structure of the isostructural compound Na4M3(PPO4), and the ionic conductivity due to Na+ ions is measured for the three compounds.
Abstract: The new phases, Na4M3(PO4)2(P2O7) (M = Mn, Co, Ni), have been synthesized by solid-state reactions. Single crystals of Na4M3(PO4)2(P2O7) (M = Mn, Ni) have been isolated and their structure has been determined by X-ray diffraction techniques using as the starting model the structure of the isostructural compound Na4Co3(PO4)2(P2O7). These compounds crystallize in the orthorhombic noncentrosymmetric space group Pn21a with a = 17.991(3) A, b = 6.6483(1), and c = 10.765(2) A for the manganese compound and a = 17.999(2), b = 6.4986(6) A, and c = 10.4200(9) A for the nickel compound, with Z = 4. Magnetic measurements reveal the existence of antiferromagnetic interactions in the nickel compound. The manganese and cobalt compounds show canting antiferromagnetic behavior at low temperatures. Magnetic correlation is also studied from the analysis of possible superexchange pathways in the structure. The ionic conductivity, due to Na+ ions, is measured for the three compounds. The activation energy is nearly the same ...
TL;DR: In this paper, a Bi 2 S 3 nanorod with a diameter of 10 nm and a length of up to 300 nm was successfully prepared from a formaldehyde solution of bismuth nitrate and thiourea by microwave irradiation.
TL;DR: Four new quaternary selenides CsGdZnSe3, CsZrCuSe3?
Abstract: Four new quaternary selenides CsGdZnSe3, CsZrCuSe3, CsUCuSe3, and BaGdCuSe3 have been synthesized with the use of traditional high-temperature solid-state experimental methods. These compounds are isostructural with KZrCuS3, crystallizing with four formula units in the orthorhombic space group Cmcm. Cell constants (A) at 153 K are CsGdZnSe3 4.1684(7), 15.765(3), 11.0089(18); CsZrCuSe3 3.903(2), 15.841(10), 10.215(6); CsUCuSe3 4.1443(7), 15.786(3), 10.7188(18); and BaGdCuSe3 4.1839(6), 13.8935(19), 10.6692(15). The structure of these ALnMSe3 compounds (A = Cs, Ba; Ln = Zr, Gd, U; M = Cu, Zn) is composed of [LnMSe3n-] (n = 1, 2) layers separated by A atoms. The Ln atom is octahedrally coordinated to six Se atoms, the M atom is tetrahedrally coordinated to four Se atoms, and the A atom is coordinated to a bicapped trigonal prism of eight Se atoms. Because there are no Se−Se bonds in the structure, the oxidation state of A is 1+ (Cs) or 2+ (Ba), that of Ln is 3+ (Gd) or 4+ (Zr, U), and that of M is 1+ (Cu) or...
TL;DR: The high-temperature molar magnetic susceptibility of a powdered sample of 3 clearly confirms the presence of two d1 centers and Titration of VIV by CeIV revealed that 3 is the mixed-valent VIV2VV species.
Abstract: Complexes K11Na1[As2W18{Mn(H2O)}3O66]·27H2O (1) and Na12[As2W18{Co(H2O)}3O66]·34H2O (2) have been characterized. 1 crystallizes in the orthorhombic space group Pnma, with a = 30.6484(4) A, b = 14.9946(2) A, and c = 19.17080(10) A (Z = 4), while 2 crystallizes in the monoclinic space group C2/c, with a = 14.124(2) A, b = 23.294(3) A, c = 32.247(3) A, and β = 98.935(10)° (Z = 4). Structures of the anions of 1 and 2 are similar, the divalent metals adopting a square pyramidal environment. K11[As2W18(VO)3O66]·23H2O (3) crystallizes in the orthorhombic space group Pnma, with a = 30.6240(5) A, b = 14.9861(2) A, and c = 19.2651(3) A (Z = 4). The structure has revealed a disorder on two of the three metals linking the [α-AsW9O33]9- parts. For these two vanadium atoms, the VO bonds are directed alternatively toward the inside or the outside of the [α-AsW9O33]9- cavity. The remaining vanadium shows a VO bond always directed toward the outside of the cavity. Titration of VIV by CeIV revealed that 3 is the mixed-vale...
01 Jan 2001
TL;DR: In this article, a Bi2S3 nanorod with a diameter of 10 nm and a length of up to 300 nm was successfully prepared from a formaldehyde solution of bismuth nitrate and thiourea by microwave irradiation.
Abstract: Bi2S3 nanorods, with a diameter of ca. 10 nm and a length of up to ca. 300 nm, have been successfully prepared from a formaldehyde solution of bismuth nitrate and thiourea by microwave irradiation. Powder X-ray diffraction (XRD) pattern indicates that the product is pure orthorhombic Bi2S3 phase. The product is also characterized by the techniques of transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS). © 2001 Elsevier Science Ltd. All rights reserved.
TL;DR: A morphotropic phase boundary between (Bi1/2Na 1/2)TiO3 and KNbO3 was determined by using X-ray diffraction and dielectric measurements as discussed by the authors.
Abstract: A morphotropic phase boundary (MPB) between (Bi1/2Na1/2)TiO3 (rhombohedral system) and KNbO3 (orthorhombic system) was determined by using X-ray diffraction and dielectric measurements. A solid solution, (1-x)(Bi1/2Na1/2)TiO3–xKNbO3 [BNTK-100x], shows a rhombohedral symmetry and orthorhombic symmetry at a composition of around 0x0.95 and 0.96x1.00, respectively. The dielectric constant, es, at Curie temperature, Tc, was the largest value in the BNTK-96 system. From these results, the MPB of the BNTK system seems to exist in BNTK-96. The BNTK ceramics were confirmed to be ferroelectric by observing their hysteresis loops. Resistivities, ρ, of BNTK-100x (0.94x0.98) are 109–1012 Ωcm. An electromechanical coupling factor, k33, of BNTK-96 is 0.16 which is an unsaturated value.
TL;DR: In this article, X-ray diffraction patterns indicated that the products were pure orthorhombic Bi2S3 phase, cubic phase HgS, hexagonal phase CuS, cubic phases ZnS and cubic phase PbS, respectively.
Abstract: Nanocrystalline metal (Cu, Hg, Zn, Bi, Pb) sulfides with different shapes and different particle sizes have been successfully prepared in a formaldehyde solution of metal salt and thioacetamide by microwave irradiation. Powder X-ray diffraction patterns indicated that the products were pure orthorhombic Bi2S3 phase, cubic phase HgS, hexagonal phase CuS, cubic phase ZnS, cubic phase PbS, respectively. The products were also characterized by transmission electron microscopy. Ultraviolet reflection spectrum clearly indicates the presence of quantum size effects in ZnS.
TL;DR: In this paper, a series of samples of the mixed ferrite NiGd x Fe 2− x O 4 ( x = 0, 0.1, ǫ 0.5 and 0.7) were prepared by the usual ceramic technique.
TL;DR: In this paper, powder neutron diffraction at ambient temperature, 800, and 1000°C was used to obtain a distorted perovskite (orthorhombic, pure: Pnma, doped: Imma ) for both phases.
TL;DR: In this article, the uranyl iodates, UO2(IO3)2 (AU1−8), and UO 2(H2O) (AU2−8) have been prepared from the reaction of UO3 with I2O5 under hydrothermal conditions.
Abstract: The uranyl iodates, UO2(IO3)2 (AU1−8) and UO2(IO3)2(H2O) (AU2−8) have been prepared from the reaction of UO3 with I2O5 under hydrothermal conditions. Equilibrium between AU1−8 and AU2−8 is established after 3 days at 400 °C, resulting in the isolation of an equimolar ratio of these compounds. Lowering of the reaction temperature to 180 °C or shortening of the reaction duration to 1 day allows for the isolation of AU2−8 in pure form. At 327 °C, AU2−8 undergoes dehydration and structural rearrangement to AU1−8, which is a reversible process. Single-crystal X-ray diffraction, bond valence sum calculations, EDX, DSC, TGA, and vibrational and fluorescence spectroscopy have been used to characterize these compounds. Crystallographic data are as follows: AU1−8, monoclinic, space group P21/n, a = 4.2454(8) A, b = 16.636(5) A, c = 5.284(1) A, β = 107.57(2)°, and Z = 2; AU2−8, orthorhombic, space group Pbcn, a = 8.452(2) A, b = 7.707(2) A, c = 12.271(3), and Z = 4. The structure of AU1−8 is pseudo-three-dimensiona...
TL;DR: Structural phase transitions of a 3d transition element, titanium, have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique and the delta-gamma transition could be explained as a rearrangement of the packing between the zigzag chains.
Abstract: Structural phase transitions of a 3d transition element, titanium, have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique. At 140 GPa, the hexagonal (w) phase was transformed into an orthorhombic (δ) phase with a distorted bcc structure via an intermediate (y) phase, which has recently been proposed by Vohra and Spencer [Phys. Rev. Lett. 86, 3068 (2001)]. Both the δ and the y phases had a unique zigzag-chain-like structure, which resulted from an orthorhombic distortion. The δ-γ transition could be explained as a rearrangement of the packing between the zigzag chains.
TL;DR: A series of manganese α,ω-dicarboxylates, Mn2(H2O)[O2C(CH2)nCO2]2 (n = 3−12), were synthesized as single crystals or polycrystallines by the hydrothermal reaction of MnCl2 with the corresponding diccarboxylic acid in the presence of base as discussed by the authors.
Abstract: A series of novel manganese α,ω-dicarboxylates, Mn2(H2O)[O2C(CH2)nCO2]2 (n = 3−12), were synthesized as single crystals or polycrystallines by the hydrothermal reaction of MnCl2 with the corresponding dicarboxylic acid in the presence of base. Mn2(H2O)[O2C(CH2)3CO2]2 and Mn2(H2O)[O2C(CH2)5CO2]2 (n = 3 and 5) crystallized in the orthorhombic space group Pbcm (No. 15), with a = 7.6701(6) A, b = 9.1156(4) A, c = 17.975(1) A, and Z = 4 and with a = 7.6089(8) A, b = 9.167(2) A, c = 23.070(3) A, and Z = 4, respectively. Using various α,ω-dicarboxylates with different numbers of carbons in alkyl chains, the interlayer spacings of the manganese(II) compounds have been varied from 8.99 to 20.42 A. Their organic chains between two terminal carboxylates are close-packed and self-assembled between Mn−O layers. The relative conformations of two terminal carboxylate groups determine the spacing groups of crystal structures. All the prepared manganese dicarboxylate compounds involve the replication of the local crystal ...
TL;DR: In this article, the authors used DTA, XRD, BET, SEM, Mossbauer spectroscopy and tested for the catalytic combustion of methane using perovskite catalysts, assuming that the excess of iron oxide in the e > 0 samples corresponds to the presence of surface of layers of FeO 5 polyhedra, coordinatively unsaturated.
Abstract: La (1− e ) FeO (3−1.5 e ) perovskite catalysts ( e =0, 0.1, 0.2, 0.3) are synthetized by low-temperature thermal decomposition of La and Fe nitrates and further heating to 873 K in air. They are characterized by DTA, XRD, BET, SEM, Mossbauer spectroscopy and tested for the catalytic combustion of methane. All oxides are monophasic; for e =0 the structure is orthorhombic, whereas for 0 e ≤0.3 a pseudocubic symmetry is evident. The Mossbauer spectra are characterized by antiferromagnetic sextuplets; for e =0.3 a doublet is also present. The catalytic activity toward CH 4 oxidation appears to be strongly enhanced in the samples with e >0; the most active composition corresponds to e =0.1, while for e =0.2 and 0.3 the catalytic activity is slightly decreased. The results are explained assuming that the excess of iron oxide in the e >0 samples corresponds to the presence of surface of layers of FeO 5 polyhedra, coordinatively unsaturated, according to proposed structural model.
TL;DR: In this paper, the phase transitions and microwave dielectric properties in the (1−x)Ca(Al 0.5Nb0.5)O3-xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectrics measurements at microwave frequencies (2-8 GHz).
Abstract: Phase transitions and microwave dielectric properties in the (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al0.5Nb0.5)O3, tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO3 and Ca(Al0.5Nb0.5)O3 are described by orthorhombic Pbnm and monoclinic P21/n symmetries, respectively, with similar lattice parameters, √2ac×√2ac×2ac (where ac is the lattice parameter of cubic perovskite). The (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system features both cation ordering and octahedral tilting phase transitions. The Ca(Al0.5Nb0.5)O3 structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with increasing Ti content, which correlates with a progressive increase of cation disorder in the specimens. A disordered structure is attained at x=0.5. For the “solid solutions,” the nonlinear dependence of both permittivity e and the temperature coefficient of the resonant frequency τf on Ti content corresponds to a linear dependence of the macroscopic polarizability on composition; that is, the oxide additivity rule was closely obeyed. Therefore, this rule can be used to predict e and τf for any intermediate composition from the permittivities and temperature coefficients of permittivity of the end compounds. A zero temperature coefficient of the resonant frequency occurs at the composition x≈0.5 with a relative permittivity of 50 and a Qf value of approximately 30 000 GHz (@4 GHz).
TL;DR: A new transformation pathway, based on a Pmm2 orthorhombic intermediate state with two SiC units per cell, is found to be energetically favored over the traditional R3m mechanism.
Abstract: The mechanism of the B3/B1 phase transition of SiC has been investigated by periodic LCAO-DFT least-enthalpy calculations. A new transformation pathway, based on a Pmm2 orthorhombic intermediate state with two SiC units per cell, is found to be energetically favored over the traditional R3m mechanism. The computed activation enthalpy is 0.75 eV/SiC unit at the predicted transition pressure of 92 GPa (B3LYP functional). Activation enthalpy and activation volume vs pressure are analyzed to characterize the kinetic aspects of the transformation.
TL;DR: Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)(4)[A(I)M(III)(C(2)O(4))(3)].
Abstract: Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)(4)[A(I)M(III)(C(2)O(4))(3)].PhCN, where A(I) = H(3)O, NH(4), K; M(III) = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene. Attempts to substitute M(III) with Ti, Ru, Rh, or Gd are also described. New compounds with M = Co and Al are prepared and detailed structural comparisons are made across the whole series. Compounds with A = H(3)O(+) and M = Cr, Fe are monoclinic (space group C2/c), at 150, 120 K a = 10.240(1) A, 10.232(12) A; b = 19.965(1) A, 20.04(3) A; c = 34.905(1) A, 34.97(2) A; beta = 93.69(1) degrees, 93.25(11) degrees, respectively, both with Z = 4. These salts are metallic at room temperature, becoming superconducting at 5.5(5) or 8.5(5) K, respectively. A polymorph with A = H(3)O(+) and M = Cr is orthorhombic (Pbcn) with a = 10.371(2) A, b = 19.518(3) A, c = 35.646(3) A, and Z = 4 at 150 K. When A = NH(4)(+), M = Fe, Co, Al, the compounds are also orthorhombic (Pbcn), with a = 10.370(5) A, 10.340(1) A, 10.318(7) A; b = 19.588(12) A, 19.502(1) A, 19.460(4) A; c = 35.790(8) A, 35.768(1) A, 35.808(8) A at 150 K, respectively, with Z = 4. All of the Pbcn phases are semiconducting with activation energies between 0.15 and 0.22 eV. For those compounds which are thought to contain H(3)O(+), Raman spectroscopy or C=C and C-S bond lengths of the BEDT-TTF molecules confirm the presence of H(3)O(+) rather than H(2)O. In the monoclinic compounds the BEDT-TTF molecules adopt a beta' ' packing motif while in the orthorhombic phases (BEDT-TTF)(2) dimers are surrounded by monomers. Raman spectra and bond length analysis for the latter confirm that each molecule of the dimer has a charge of +1 while the remaining donors are neutral. All of the compounds contain approximately hexagonal honeycomb layers of [AM(C(2)O(4))(3)] and PhCN, with the solvent occupying a cavity bounded by [M(C(2)O(4))(3)](3-) and A. In the monoclinic series each layer contains one enantiomeric conformation of the chiral [M(C(2)O(4))(3)](3-) anions with alternate layers having opposite chirality, whereas in the orthorhombic series the enantiomers form chains within each layer. Analysis of the supramolecular organization at the interface between the cation and anion layers shows that this difference is responsible for the two different BEDT-TTF packing motifs, as a consequence of weak H-bonding interactions between the terminal ethylene groups in the donor and the [M(C(2)O(4))(3)](3-) oxygen atoms.
TL;DR: In this article, a theoretical study of the temperature-dependent structural and thermodynamic properties of solid-phase Al-Sc alloys and compounds based upon first-principles calculations of electronic free energies and ionic vibrational spectra is presented.
Abstract: We present results of a theoretical study of the temperature-dependent structural and thermodynamic properties of solid-phase Al-Sc alloys and compounds based upon first-principles calculations of electronic free energies and ionic vibrational spectra. This work extends a previous first-principles study of the fcc portion of the Al-Sc phase diagram which demonstrated a large effect of vibrational free energy upon calculated Sc solid-solubility limits [V. Ozoli\ifmmode \mbox{\c{n}}\else \c{n}\fi{}\ifmmode \check{s}\else \v{s}\fi{} and M. Asta, Phys. Rev. Lett. 86, 448 (2001)]. Here the contributions of nonconfigurational (electronic and vibrational) entropies to the free energies of solid-phase Al-Sc alloys and compounds are analyzed in further detail, and the accuracy of the approximations employed in these calculations is assessed. For each of the reported intermetallic compounds in this system, calculated formation enthalpies agree to within 10% (0.05 eV/atom) of published calorimetry measurements. Large negative entropies of formation, equal to $\ensuremath{-}{0.77k}_{B}/\mathrm{atom},$ $\ensuremath{-}{0.58k}_{B}/\mathrm{atom},$ and $\ensuremath{-}{0.24k}_{B}/\mathrm{atom}$ are calculated for cubic ${\mathrm{Al}}_{3}\mathrm{Sc},$ cubic AlSc, and orthorhombic AlSc compounds, respectively, resulting primarily from the stiffening of nearest-neighbor Al-Sc bonds in the intermetallic phases relative to elemental Al and Sc. The net effects of nonconfigurational free energy contributions to the fcc portion of the Al-Sc phase diagram are 100 and 450 K decreases in the calculated Al solvus phase boundary temperatures associated with electronic and vibrational entropy, respectively, at the maximum measured Sc solid-solubility limit.