2-type Structure Research Articles

Effects of C and Cr content on high-temperature microstructures of Fe–9Al–30Mn– xC– yCr alloys

This investigation elucidated the effects of C and Cr content on the high-temperature microstructures of Fe–9Al–30Mn– xC– yCr alloys by means of optical microscopy and transmission electron microscopy. With increasing Cr content, the phase transition sequence within the α phase was found to be α + B2 → α + B2 + DO 3 → α + DO 3. And with increasing C content, a γ → ( γ + κ) phase transition was observed within the γ phase. The κ phase carbides ((Fe,Mn) 3AlC x) had an ordered L′1 2-type structure with lattice parameter a = 0.368 nm and were formed by a spinodal decomposition during quenching. The amounts of Cr 7C 3 increased with the C and Cr content. Moreover, the Al and Mn content played important roles in expanding the ( α + γ) region. These features have not been previously reported in the Fe–Al–Mn–C–Cr alloy system.

Magnetocaloric effect of (Tb 1– xCe x)Co 2 alloys in low magnetic field

The phases in alloys (Tb 1– x Ce x )Co 2 with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 were investigated by X-ray diffraction analysis, and the magnetocaloric effect for x=0–0.4 was studied by magnetization measurement. The samples were almost single phase with MgCu 2-type cubic structure. The magnetization decreased with the increase of Ce. The Curie temperatures ( T c) of Tb 1– x Ce x Co 2 alloys with x from 0.1 to 0.4 were 180, 165, 160 and 152 K, respectively. For x=0.5 in the range from 100 K to 230 K, the point of magnetic transition was not obvious, and thus this article did not discuss the magnetocaloric effects for x=0.5. The maximum magnetic entropy change decreased with the increase of Ce content. The results of Arrott plots showed that the magnetic phase transition was second-order magnetic phase transition in those alloys.

Synthesis and characterization of (Pr 0.75Sr 0.25) 1 − xCr 0.5Mn 0.5O 3 − δ as anode for SOFCs

The complex perovskite (Pr 0.75Sr 0.25) 1 − x Cr 0.5Mn 0.5O 3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO 3-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr 0.75Sr 0.25Cr 0.5Mn 0.5O 3 − δ at 900 °C in air and 5% H 2/Ar are 9.6 and 0.14 S cm − 1 respectively. The conductivity of PSCM drops with decreasing Po 2 and is a p-type conductor at all studied Po 2. The average TEC of Pr 0.75Sr 0.25Cr 0.5Mn 0.5O 3 − δ is 9.3 × 10 − 6 K − 1 , in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H 2 is 1.31 Ω cm 2 at 910 °C and in wet CH 4 at 930 °C; the polarization resistance is 1.29 Ω cm 2. PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La 0.8Sr 0.2MnO 3 as anode and cathode respectively yielded a maximum power density of 0.18 W cm − 2 in wet 5%H 2/Ar at 910 °C and the corresponding current density was 0.44 A cm − 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H 2O) 5%H 2/Ar fuel is 85 kJ mol − 1 .

Crystal structure and energy gap of Ba 1− xSr xSi 2 ( x = 0.20 and 0.41)

The crystal structure and energy gap of Ba 1− x Sr x Si 2 ( x = 0.20 and 0.41), a promising material for solar cells, have been investigated. Ba 1− x Sr x Si 2 has the BaSi 2-type structure (orthorhombic, Pnma, Z = 8, a = 8.8258(7), b = 6.6736(4), c = 11.4364(8) Å for x = 0.20, a = 8.7466(4), b = 6.6278(3), c = 11.3553(5) Å for x = 0.41). The unit cell volume decreases with respect to the Sr content x. Ba atoms at a specific crystallographic site, A1 site, are substituted by Sr atoms preferentially. As a result, The Sr substitution deforms BaSi 2 inhomogeneously; the Sr substitution of 41% reduces the volume of coordination polyhedra of A1 and A2 atoms by 5.9 and 4.4%, respectively, while it reduces the volume of Si tetrahedron by a small amount, less than 1%. Diffuse reflectance spectroscopy has demonstrated that the energy gap increases with Sr content x, which is qualitatively consistent with the previous results obtained using Ba 1− x Sr x Si 2 epitaxal thin films.

Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi 2.6− xMn xCo 0.9 alloys

ReNi 2.6− x Mn x Co 0.9 ( x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce) 2Ni 7 phase with a Ce 2Ni 7-type structure, (Pr, Ce)Co 3 phase with a PuNi 3-type structure, and (La, Pr)Ni 5 phase with a CaCu 5-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni 5 phase disappeared with further increasing x ( x > 0.45). The hydrogen-storage capacity of the ReNi 2.6− x Mn x Co 0.9 ( x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi 2.6− x Mn x Co 0.9 ( x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g ( x = 0.0) to 352 mAh/g ( x = 0.45) and then decreased to 307 mAh/g ( x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi 2.15Mn 0.45Co 0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.

Development of high-performance cathodes for IT-SOFCs through beneficial interfacial reactions

We propose a new way to develop high-performance cathodes for IT-SOFCs by utilizing the interfacial reactions. SrCoO x was selected as the starting electrode material, which took a vacancy-ordered 2H BaNiO 3-type structure and showed negligible ionic conductivity and low electrical conductivity. Phase reactions between SrCoO x and Sm 0.2Ce 0.8O 1.9 happened at 900 °C or higher, resulting in the incorporation of Sm and Ce into its lattice structure. This promoted the phase transition to a cubic perovskite and led to substantial increase in the electrical conductivity and oxygen mobility of the electrode. By utilizing such phase reactions, the SrCoO x + Sm 0.2Ce 0.8O 1.9 composite was developed into a high performance electrode with a low area specific resistance of 0.08 Ω cm −2 at 650 °C. An anode-supported cell with such electrode delivered a peak power density of 795 mW cm −2 at 600 °C.

Magnetic properties and magnetocaloric effect of Nd(Mn 1− xFe x) 2Ge 2 compounds

Polycrystalline Nd(Mn 1− x Fe x ) 2Ge 2 (0 ≤ x ≤ 1) compounds have been prepared by arc-melting. X-ray powder diffraction analysis shows that all samples crystallize in the ThCr 2Si 2-type structure with the space group I4/ mmm. The substitution of Fe for Mn results in decreases of the lattice constants a, c and the unit-cell volume V. Magnetic properties and magnetocaloric effect of the compounds Nd(Mn 0.9Fe 0.1) 2Ge 2 and Nd(Mn 0.8Fe 0.2) 2Ge 2 have been studied by magnetic measurements. A spin reorientation transition at about 188 K is observed for Nd(Mn 0.9Fe 0.1) 2Ge 2. The Nd(Mn 0.8Fe 0.2) 2Ge 2 compound shows more complicated magnetic behavior, which is characterized by four magnetic ordering states below room temperature. The maximum values of the magnetic entropy change are 2.35 and 1.84 J kg −1 K −1 for x = 0.1 and 0.2, respectively, under the applied field changing from 0 to 5 T. The relative cooling powers of Nd(Mn 0.9Fe 0.1) 2Ge 2 and Nd(Mn 0.8Fe 0.2) 2Ge 2 are 145.9 and 133.8 J kg −1 under an applied field change of 5 T.

On the low-temperature properties of [formula omitted

Magnetic, electrical transport and heat capacity measurements have been performed on polycrystalline sample of the compound TmRu 2Si 2, which crystallizes with the tetragonal ThCr 2Si 2-type crystal structure. The results show paramagnetic behaviour down to 0.3 K, at variance with the literature data. On the basis of the collected data, possible origins of non-magnetic behaviour are discussed. Mixed valence of Tm ions or hybridisation between Tm 4f and Ru 4d states are the most probable reasons for paramagnetic behaviour of the TmRu 2Si 2.

Fe content effects on electrochemical properties of Fe-substituted Li 2MnO 3 positive electrode material

Fe-substituted Li 2MnO 3 including a monoclinic layered rock-salt structure ( C2/ m), (Li 1+ x (Fe y Mn 1− y ) 1− x O 2, 0 < x < 1/3, 0.1 ≤ y ≤ 0.5) was prepared by coprecipitation–hydrothermal–calcination method. The sample was assigned as two-phase composite structure consisting of the cubic rock-salt ( F m 3 ¯ m ) and monoclinic ones at high Fe content above 30% ( y ≥ 0.3), while the sample was assigned as a monoclinic phase ( C2 /m) at low Fe content less than 20%. In the monoclinic Li 2MnO 3-type structure, the Fe ion tends to substitute a Li (2 b) site, which corresponds to a center position of Mn 4+ hexagonal network in Mn–Li layer. The electrochemical properties including discharge characteristics under high current density (<3600 mA g −1 at 30 °C) and low temperature (<−20 °C at 40 mA g −1) were severely affected by chemical composition (Fe content and Li/(Fe + Mn) ratio), crystal structure (monoclinic phase content) and powder property (specific surface area). Under the optimized Fe content (0.2 < y < 0.4), the Li/sample cells showed high initial discharge capacity (240–300 mAh g −1) and energy density (700–950 mWh g −1) between 1.5 and 4.8 V under moderate current density, 40 mA g −1 at 30 °C. Results suggest that Fe-substituted Li 2MnO 3 would be a non-excludable 3 V positive electrode material.

Stability of the perovskite structure and possibility of the transition to the post-perovskite structure in CaSiO 3, FeSiO 3, MnSiO 3 and CoSiO 3

High pressure and high temperature experiments on CaSiO 3, FeSiO 3, MnSiO 3 and CoSiO 3 using a laser-heated diamond anvil cell combined with synchrotron X-ray diffraction were conducted to explore the perovskite structure of these compounds and the transition to the post-perovskite structure. The experimental results revealed that MnSiO 3 has a perovskite structure from relatively low pressure (ca. 20 GPa) similarly to CaSiO 3, while the stable forms of FeSiO 3 and CoSiO 3 are mixtures of mono-oxide (NaCl structure) + high pressure polymorph of SiO 2 even at very high pressure and temperature (149 GPa and 1800 K for FeSiO 3 and 79 GPa and 2000 K for CoSiO 3). This strongly suggests that the crystal field stabilization energy (CFSE) of Fe 2+ with six 3d electrons and Co 2+ with seven 3d electrons at the octahedral site of mono-oxides favors a mixture of mono-oxide + SiO 2 over perovskite where Fe 2+ and Co 2+ would occupy the distorted dodecahedral sites having a smaller CFSE (Mn 2+ has five 3d electrons and has no CFSE). The structural characteristics that the orthorhombic distortion of MnSiO 3 perovskite decreases with pressure and the tolerance factor of CaSiO 3 perovskite (0.99) is far from the orthorhombic range suggest that both MnSiO 3 and CaSiO 3 perovskites will not transform to the CaIrO 3-type post-perovskite structure even at the Earth's core–mantle boundary conditions, although CaSiO 3 perovskite has a potentiality to transform to the CaIrO 3-type post-perovskite structure at still higher pressure as long as another type of transformation does not occur.

Low temperature thermodynamical properties of ErCu 2Si 2

ErCu 2Si 2 crystallises in the tetragonal ThCr 2Si 2-type crystal structure. In this paper results of magnetometric, electrical transport, specific heat as well as neutron diffraction are reported. Results of electrical resistivity and specific heat measurements performed at low temperature yield existence of magnetic ordering roughly at 1.3 K. These results are in concert with neutron diffraction measurements, which reveal simple antiferromagnetic ordering between 0.47 and 1.00 K. At temperatures ranging from 1.00 up to 1.50 K an additional incommensurate magnetic structure was observed. The propagation vector k =(0;0;0.074) was proposed to describe magnetic reflections within the amplitude modulated magnetic structure. Basing on specific heat studies the crystal field levels splitting scheme and magnetic entropy were calculated.

Crystal structure of ternary germanides SrM 2Ge 2 (M = Ni and Ir)

Ternary intermetallics SrM 2Ge 2 (M = Ni and Ir) have been prepared by arc melting. The crystal structure of these intermetallics was determined by single-crystal X-ray diffraction. They are isostructural with the recently discovered superconductors (Ba,Sr) 1− x (K,Cs) x Fe 2As 2 with ThCr 2Si 2-type structure with the space group I4/ mmm (No. 139). The lattice parameters of SrNi 2Ge 2 and SrIr 2Ge 2 are a = 0.41801(1) and c = 1.02409(6) nm, and a = 0.41916(2) and c = 1.0674(1) nm, respectively. Neither of them shows superconductivity down to 1.8 K.

Microstructure and electrochemical investigations of La 0.76− xCe xMg 0.24Ni 3.15Co 0.245Al 0.105 ( x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys

The effects of substitution of Ce for La on the microstructure and electrochemical performance of La 0.76− x Ce x Mg 0.24Ni 3.15Co 0.245Al 0.105 ( x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses showed that the main phases of the alloys consist of (La, Mg)Ni 3 phase (PuNi 3-type rhombohedral structure), LaNi 5 phase (CaCu 5-type hexagonal structure) and (La, Mg) 2Ni 7 phase (Ce 2Ni 7-type hexagonal structure). The cell volume of the (La, Mg)Ni 3 phase, (La, Mg) 2Ni 7 phase and LaNi 5 phase decreased monotonously with increasing Ce content. Electrochemical investigations showed a decrease in the discharge capacity, while high rate dischargeability (HRD) first increased and then decreased with increasing Ce content. The Ce substitution for La slightly enhanced the cyclic stability of the alloy electrodes. The pressure–composition ( P– C) isotherms showed that the plateau region was broadened with Ce content increased in the alloys, meanwhile, two plateaus appeared and pressure of the hydrogen absorption and desorption increased accordingly.

Spin configuration and magnetostriction in Nd 0.9Tb 0.1(Fe 1− xCo x) 1.9 (0 ≤ x ≤ 0.55) alloys

Polycrystalline magnetostrictive alloys Nd 0.9Tb 0.1(Fe 1− x Co x ) 1.9 (0 ≤ x ≤ 0.55) were synthesized by high-pressure annealing. Measurements of crystal structure, Curie temperature, magnetization, spin-reorientation temperature and magnetostriction were made on the alloys. X-ray diffraction results show that all the samples exhibit cubic Laves phase with MgCu 2-type structure. The variations of Curie temperature and saturation magnetization are interpretable due to 3d band filling. The spin configuration diagram of Nd 0.9Tb 0.1(Fe 1− x Co x ) 1.9 was constructed. The spin-reorientation temperatures of the samples first increase and then decrease with the substitution of Co for Fe. The anisotropy minimum and low-filed large magnetostriction is realized around x = 0.05.

Structure and magnetostrictive properties of melt-spun Pr(Fe 0.4Co 0.6) 1.93 alloys

Pr(Fe 0.4Co 0.6) 1.93 ribbons were prepared by a melt-spinning method. Their structure and magnetic properties are investigated as functions of wheel speed and annealing temperature. The as-spun ribbon consists of a Pr(Fe, Co) 2 cubic Laves phase and an amorphous phase at a wheel speed of v≥35 m/s, while the non-cubic phases of PuNi 3-type and rare earth appear when the speed lower than 30 m/s. A single Pr(Fe, Co) 2 phase with MgCu 2-type structure has been synthesized by the process for the wheel speed of v≥35 m/s and subsequent annealing at 500 °C for 30 min. The epoxy/Pr(Fe 0.4Co 0.6) 1.93 composite has been produced by a cold isostatic pressing technique, and the magnetic properties have been investigated. The composite rod sample possesses good magnetostrictive properties, i.e., a large magnetostriction ( λ a= λ ∥− λ ⊥) of 710 ppm at 800 kA/m and a dynamic coefficient d 33 of 0.67 nm/A at 100 kA/m, and is of practical value.

Syntheses, crystal structures, and thermal analyses of solvent-free Ca(AlD 4) 2 and CaAlD 5

Solvent-free Ca(AlD 4) 2 and CaAlD 5 were synthesized directly by mechanochemical milling of CaD 2 and AlD 3 in the molar ratios of 1:2 and 1:1, respectively, under deuterium gas pressure of 0.3 MPa. The products were characterized by high-resolution synchrotron radiation powder X-ray diffraction (SR-PXD), powder neutron diffraction (PND), Raman spectroscopy, and thermogravimetry (TG) analysis. The crystal structure of Ca(AlD 4) 2 was determined and refined with the SR-PXD and PND data. It takes an orthorhombic Ca(BF 4) 2-type structure with a = 13.4491(27) Å, b = 9.5334(19) Å, and c = 9.0203(20) Å in the space group Pbca (No. 61) and Z = 8. The Raman spectrum shows bending and stretching modes of the AlD 4 tetrahedron at 480–650 cm −1 and 1290–1340 cm −1, respectively. TG analysis showed that Ca(AlD 4) 2 releases 4.8 wt.% of deuterium gas at the onset-temperature of 427 K. The crystal structure of CaAlD 5 was determined and refined from the PND data and was found to be a monoclinic α-SrAlF 5-type structure with a = 9.8000(19) Å, b = 6.9081(13) Å, c = 12.4503(23) Å, and β = 137.936(4)° in the space group P2 1/ c (No. 14) and Z = 8. The Raman spectrum of CaAlD 5 shows bending and stretching modes of the AlD 6 octahedron at 725 and 1060–1170 cm −1, respectively. From TG analysis, CaAlD 5 releases 7.1 wt.% of deuterium gas at the onset-temperature of 554 K.

The isothermal section of the Nd–Fe–Ga ternary system at 773 K

The isothermal section of the Nd–Fe–Ga ternary system at 773 K was constructed using X-ray diffraction analysis. Ten binary compounds, Nd 2Fe 17, Nd 5Fe 17, Nd 9Ga 4, Nd 5Ga 3, NdGa, NdGa 2, FeGa 3, Fe 3Ga 4, α-Fe 6Ga 5 and α-Fe 3Ga, and three ternary compounds, NdFe 2Ga 8, NdFe 5Ga 7 and Nd 6Fe 13Ga were existed in this isothermal section. The solid solutions Nd 2Fe 17.00–15.57Ga 0–1.43 (Th 2Zn 17-type structure, space group R 3 ¯ m ), NdFe 5.46–4.65Ga 6.54–7.35 (ThMn 12-type structure, space group I4/ mmm) and Nd 6Fe 13.0–12.0Ga 1.0–2.0 (La 6Co 11Ga 3-type structure, space group I4/ mcm) were formed by substitution of Ga for Fe in the compounds Nd 2Fe 17, NdFe 5Ga 7 and Nd 6Fe 13Ga at 773 K, respectively. The ternary compound NdFe 2Ga 8 is CaCo 2Al 8-type (space group Pbam) with a = 1.43742 (4), b = 1.24601 (4), c = 0.40479 (1)). The homogeneity range of NdGa 2 is from 66.7 to 74.7 at.% Ga in Nd–Ga side but the solid solubility of Fe in this compound is very small. The homogeneity ranges of Nd 6Fe 13Ga and NdFe 5Ga 7 are from 5.0 to 10.0 and from 50.3 to 56.5 at.% Ga, respectively. The maximum solid solubilities of Ga in Nd 2Fe 17 is 7.5. The homogeneity ranges for the other compounds are small.

Phase relationships and crystallography of annealed alloys in the Ce 5Si 4–Ce 5Ge 4 pseudobinary system

The phase relationships of annealed alloys in the Ce 5Si 4− x Ge x system were determined by X-ray powder diffraction (XRD). Two structurally distinct terminal phase regions were observed in this system: the Ce 5Si 4-based solid solution (0 ≤ x < 2.85) crystallizing in the Zr 5Si 4-type tetragonal structure with space group P4 12 12, and the Ce 5Ge 4-based solid solution (3.35 < x ≤ 4) crystallizing in the Sm 5Ge 4-type orthorhombic structure with space group Pnma. An intermediate phase, which has a narrow composition range with the monoclinic Gd 5Si 2Ge 2-type structure, space group P112 1/ a, was found to exist at x = 2.95 ± 0.05. The Rietveld powder diffraction profile fitting technique was used to refine the crystal structures, lattice parameters, and the atomic positions. The phase relationships of the Ce 5Si 4− x Ge x pseudobinary system after heat treatment were established from these data.

Magnetic properties of RCr 2Si 2 compounds (R=Tb, Er)

We report on magnetic properties of RCr 2Si 2 compounds with R=Tb, Er. The polycrystalline samples were characterized by powder X-ray diffraction and found to be isostructurally crystallizing in the ThCr 2Si 2-type tetragonal structure. The samples were further investigated by specific heat, AC-susceptibility and magnetization methods in the temperature range 2–900 K and in magnetic fields up to 9 T. The magnetic measurements revealed the magnetic ordering of the rare-earth moments below about 2 K, whilst no high-temperature ordering of the Cr moments was observed. The evidence of for Cr magnetism is corroborated by results of first-principles calculations based on the density functional theory (DFT).

Superconductivity induced by doping Ru in SrFe 2−xRu xAs 2

Using one-step solid state reaction method, we have successfully synthesized the superconductor SrFe 1− x Ru x As. X-ray diffraction indicates that the material has formed the ThCr 2Si 2-type structure with a space group I4/mmm. The systematic evolution of the lattice constants demonstrates that the Fe ions are successfully replaced by the Ru. By increasing the doping content of Ru, the spin-density-wave (SDW) transition in the parent compound is suppressed and superconductivity emerges. The maximum superconducting transition temperature is found at 13.5 K with the doping level of x = 0.7. The temperature dependence of DC magnetization confirms superconducting transitions at around 12 K. Our results indicate that similar to non-isoelectronic substitution, isoelectronic substitution contributes to changes in both the carrier concentration and internal pressure, and superconductivity could be induced by isoelectronic substitution.