Mmm Structure Research Articles

The Effect of Yttrium Addition on Microstructure and Mechanical Properties of Refractory TiTaZrHfW High-Entropy Films

Refractory high-entropy films (RHEFs) are a new type of high-temperature material with great prospects for applications due to their superior properties. They have the potential to replace nickel-based superalloys in order to develop a new generation of materials that can be used under extreme conditions. (TiTaZrHf)100−xYx RHEFs are prepared using the magnetron sputtering technique. The yttrium (Y) content varies from 0 to 56 at.%. XRD analysis indicates the formation of an amorphous phase in Y-free films, while new phases are formed after the addition of Y. The results are confirmed by TEM analysis, revealing the formation of nano-grains with two phases L12 and Y-P6/mmm structure. With an increasing Y content, the grain size of the nano-grains increases, which has a significant effect on the mechanical properties of the films. Hardness decreases from 9.7 GPa to 5 GPa when the Y amount increases. A similar trend is observed for the Young’s modulus, ranging from 111.6 to 82 GPa. A smooth and featureless morphology is observed on the low Y content films, while those with a larger Y content appear columnar near the substrate. Furthermore, the phase evolution is evaluated by calculating the thermodynamic criteria ΔHmix, ΔSmix, Ω, and δ. The calculation results predict the formation of new phases and are then in good agreement with the experimental characterization.

Thermally induced planar defect formation in sputtered V1-xMoxB2-Δ films

This work reports on various thermally induced planar defects formed in ternary V1-xMoxB2-Δ coatings with different influence on mechanical properties. V0.58Mo0.42B1.9 and V0.37Mo0.63B1.2 films were prepared by magnetron sputtering and annealed in vacuum up to 1300 °C. Depending on the stoichiometry, different types of planar defects were identified by scanning transmission electron microscopy (STEM). In the slightly understoichiometric V0.58Mo0.42B1.9 system, anti-phase boundary defects (APB) were observed, which was accompanied by chemical separation of V and Mo. Accumulation of these defects led to formation of MoB-Cmcm orthorhombic phase in otherwise hexagonal VB2-P6/mmm structure, with no apparent effect on mechanical properties. On the other hand, in case of the highly understoichiometric V0.37Mo0.63B1.2 coating, no chemical separation was observed. Here, annealing resulted in crystallization from amorphous state into orthorhombic Cmcm phase with high density of twin lamellae, which was accompanied by a significant hardening.

The Study of the Fine Structure of Ti-Al Coatings on the Surface of Ti, Obtained by Mechanical Alloying

The work is devoted to the study of structural-phase transformations in composite coatings (Ti-Al)+Ti during mechanical alloying. The data on the structural-phase states of (Ti-Al)-Ti coatings after mechanical alloying have been obtained, confirming the mechanism of formation of the modified layer due to deformation compaction of powder particles on the titanium surface under mechanical action. As a result of mechanochemical fusion, a TiAl3 phase with a bcc lattice (I4=mmm structure) was detected, which corresponds to the stable state of the TiAl3 alloy. Under conditions of mechanical alloying of the structure, I4=mmm transforms into the L12 structure, which corresponds to the metastable state of TiAl3

Crystal structure and oxygen content of the double perovskites GdBaCo2−xFexO6−δ

The iron solubility limit, x, in GdBaCo2−xFexO6−δ determined by means of X-ray diffraction was found to be close to 0.65 in air. The crystal structure changes of the double perovskites GdBaCo2-xFexO6−δ (x=0–0.6) were studied by means of “in situ” X-ray diffraction in temperature range from 25 to 900°C in air. The oxygen content, 6−δ, was determined for these double perovskites in air as a function of temperature by means of thermogravimetric technique in range 25≤T, °C≤ 1100. The Pmmm-P4/mmm structure transition was found to occur in GdBaCo2-xFexO6-δ (0≤x≤0.4) with increasing temperature. This transition is observed at the same temperature for the compositions with 0≤x≤0.1 while the transition temperature reaches maximum for x=0.2 and that decreases linearly with further iron increase. The double perovskite GdBaCo1.4Fe0.6O6−δ was shown to have the tetragonal P4/mmm structure at room temperature. The P4/mmm-Pmmm structure transition occurs at temperature as low as 170°C for this double perovskite while reverse one is already observed at 290°C in air. The Pmmm-P4/mmm structure transition was found to be strongly related to the oxygen content for the undoped and slightly doped (x≤0.2) double perovskites while there is no such relation for the double perovskites enriched by iron (x≥0.2).

Linzhiite, FeSi2, a redefined and revalidated new mineral species from Luobusha, Tibet, China

“Ferdisilicite” (FeSi 2 ) was first described as a member of the Fe–Si alloy mineral series by Gevork’yan (1969), without CNMMN approval. It is redefined here as a new mineral, revalidated with the new name linzhiite with a CNMNC approval (IMA 2010-011). The type sample was found in a podiform chromitite from the Luobusha ophiolite in Tibet, People’s Republic of China. It occurs as irregular grains ranging from 0.04 to 0.5 mm in diameter and intergrowths with zangboite and native silicon. Linzhiite is steel grey in colour, opaque, with a metallic lustre and a greyish-black streak. The mineral is brittle and has a conchoidal fracture. Cleavage was not observed. The Mohs hardness is 6½, and the calculated density is 4.972 g/cm 3 . Reflectance values of the material in air ( R %) are: 26.3 at 470 nm, 30.3 at 546 nm, 32.9 at 589 nm, and 36.3 at 650 nm. The composition (mean of 12 EMP analyses) is Fe 49.09, Si 50.00, Al 0.64 and Mn 0.28 wt%, sum 100.01 wt%. The ideal formula is FeSi 2 . The crystal structure was refined from single-crystal X-ray diffraction data to R = 0.043. The mineral has a tetragonal P 4/ mmm structure, with a = 2.696 (1) A, b = 2.696 (1) A, c = 5.147 (6), V = 37.41 (14) A 3 , Z = 1. The five strongest X-ray powder diffraction lines [ d -spacing in A ( I / I 0 )] are: 5.136 (96), 2.374 (81), 1.896 (55), 1.849 (100), and 1.086 (36). Linzhiite formed in a strongly reducing environment and possibly occurs as xenocrysts derived from mantle sources.

Phase Transition Domains in Ca‐based Complex Perovskite Dielectric Ceramics

Phase transition‐induced lamellar domain structures together with the phase transition process in Ca [( Mg 1/3 Ta 2/3)0.9 Ti 0.1] O 3 complex perovskite ceramics have been investigated by high‐resolution transmission electron microscopy. As the orthorhombic and monoclinic structures coexist in the present ceramics and even in a single grain, groups of lamellar domains have been observed and analyzed in the three characteristic zone axes of the parent cubic perovskite structure: 〈100〉, 〈110〉, and 〈111〉. zone axes. The antiparallel displacement of A‐site cations, in‐phase, and antiphase oxygen octahedra tilting and B‐site 1:2 cation ordering cause superstructure reflections to destroy the symmetry of the parent cubic structure and subsequently induce domain structures. Groups of (010), (100) reflection twin domains and 90° rotation domains along the [100], [010], and [001] directions in the 〈100〉 and 〈110〉 zone axes are refined to be caused by the loss of reflection planes and fourfold rotation symmetry, whereas the loss of threefold rotoinversion and gain of twofold rotation symmetry lead to 120° and 180° rotation domains in the [111] zone axis. Rhombohedral m and tetragonal 4/mmm structures are revealed to act as the bridging intermediate phase to make the first‐order phase transitions from the parent cubic m m structure to the orthorhombic mmm structure and monoclinic 2/m structure without group‐subgroup relations continuous and diffusionless, and thus could occur via second order to form domain structures.

Synthesis, structure and characterisation of the n=4 Aurivillius phase Bi 5Ti 3CrO 15

The n=4 Aurivillius phase, Bi 5Ti 3CrO 15, was synthesised by solid state reaction. Rietveld analysis of high resolution neutron diffraction data demonstrated this material to adopt the polar space group A2 1 am at room temperature, transforming to the aristotype I4/ mmm structure above 650 °C. This phase transition is coincident with an anomaly in DSC signal and relative permittivity, which are characteristic of a ferroelectric–paraelectric phase transition. Bi 5Ti 3CrO 15 exhibits paramagnetic behaviour at low temperature, with short range antiferromagnetic interactions, but no evidence for long range magnetic ordering. This is considered a consequence of significant disorder of Ti and Cr over the available octahedral sites, as demonstrated by analysis of neutron diffraction data.

Investigation of tetragonal ReN 2 and WN 2 with high shear moduli from first-principles calculations

Two new transition metal dinitrides, ReN 2 and WN 2, with the P 4 / mmm structure are investigated by the first-principles calculations. The computed shear moduli of 327 GPa for ReN 2 and 334 GPa for WN 2 exceed those of all transition metal dinitrides previously reported. The estimated theoretical hardness are 46.3 GPa for ReN 2 and 47.9 GPa for WN 2, respectively. The calculated high shear moduli and hardness indicate that they are potential ultrahard materials. It is important to note that the computed hardness of the weakest bond are 34.7 GPa (W–N) for WN 2 and 33.1 GPa (Re–N) for ReN 2, much higher than that of 21.1 GPa (Re–B) for ReB 2, which suggests that tetragonal ReN 2 and WN 2 are probably harder than ReB 2. The total and partial electron density of states and the electron localization function for ReN 2 and WN 2 are analyzed. We attribute the high bulk modulus, shear modulus, and hardness to a three-dimensional covalently bonded framework in tetragonal ReN 2 and WN 2. Our calculations show that tetragonal ReN 2 is expected to be synthesized above 62.7 GPa and tetragonal WN 2 may be hard to be synthesized.

Structural, mechanical, and electronic properties of [formula omitted], TaB, [formula omitted], and IrB: First-principle calculations

First-principle calculations were performed to investigate the structural, elastic, and electronic properties of TaB 2 , TaB, IrB 2 , and IrB. The calculated equilibrium structural parameters, shear modulus, and Young's modulus of TaB 2 are well consistent with the available experimental data, and TaB 2 with P 6 / mmm space group has stronger directional bonding between ions than WB 2 , OsB 2 , IrN 2 , and PtN 2 . For TaB 2 , the hexagonal P 6 / mmm structure is more stable than the orthorhombic Pmmn one, while for IrB 2 the orthorhombic Pmmn structure is the most stable one. The high shear modulus of P 6 / mmm phase TaB 2 is mainly due to the strong covalent π -bonding of B-hexagon in the (0001) plane. Such a B-hexagon network can strongly resist against an applied [ 11 2 ¯ 0 ] (0001) shear deformation. Correlation between the hardness and the elastic constants of TaB 2 was discussed. The band structure shows that P 6 / mmm phase TaB 2 and Pmmn phase IrB 2 are both metallic. The calculations show that both TaB and IrB are elastically stable with the hexagonal P 6 3 / mmc structure.

SrCo 1− xSb xO 3− δ perovskite oxides as cathode materials in solid oxide fuel cells

The SrCo 1− x Sb x O 3− δ ( x = 0.05, 0.1, 0.15 and 0.2) system was tested as possible cathode for solid oxide fuel cells (SOFCs). X-ray diffraction results show the stabilization of a tetragonal P4/ mmm structure with Sb contents between x = 0.05 and x = 0.15. At x = 0.2 a phase transition takes place and the material is defined in the cubic Pm-3m space group. In comparison with the undoped hexagonal SrCoO 3 phase, the obtained compounds present high thermal stability without abrupt changes in the expansion coefficient. In addition, a great enhancement of the electrical conductivity was observed at low and intermediate temperatures ( T ≤ 800 °C). The sample with x = 0.05 displays the highest conductivity value that reaches 500 S cm −1 at 400 °C and is over 160 S cm −1 in the usual working conditions of a cathode in SOFC (650–900 °C). Moreover, the impedance spectra of the SrCo 1− x Sb x O 3− δ /Ce 0.8Nd 0.2O 2− δ /SrCo 1− x Sb x O 3− δ ( x ≥ 0.05) symmetrical cells reveal polarization resistances below 0.09 Ω cm 2 at 750 °C which are much smaller than that displayed by the pristine SrCoO 3− δ sample. The composition with x = 0.05 shows the lowest ASR values ranging from 0.009 to 0.23 Ω cm 2 in the 900–600 °C temperature interval with an activation energy of 0.82 eV.

Hydrogen in nonstoichiometric cubic niobium carbides: Neutron vibrational spectroscopy and neutron diffraction studies

The vibrational spectra and positions of H(D) atoms in NbC 1− y H x (D x ) (0.19 ≤ y ≤ 0.29, 0.04 ≤ x ≤ 0.30) have been studied by inelastic neutron scattering (INS) and neutron diffraction. The analysis of the neutron diffraction data for NbC 0.76H x (D x ) and NbC 0.71H x (D x ) has revealed a number of different structures depending on the carbon concentration and the presence of absorbed H(D) atoms: the partially ordered cubic P m 3 ¯ m structure for NbC 0.76, the partially ordered orthorhombic Pmmm structure for NbC 0.76D 0.17 and NbC 0.76H 0.18, the disordered cubic F m 3 ¯ m structure for NbC 0.71 and NbC 0.71D 0.30, and the disordered tetragonal I4/ mmm structure for NbC 0.71H 0.28. The INS spectra of NbC 0.81H x and NbC 0.76H x (D x ) in the energy transfer range 40–140 meV are found to consist of a single fundamental peak due to hydrogen optical vibrations (centered at 98 meV for H and at 65 meV for D) and a single peak due to carbon optical vibrations (centered at 78 meV). In addition to these peaks, the INS spectrum of NbC 0.71H 0.28 exhibits a peak at 130 meV, suggesting that H atoms in this compound occupy the sites displaced from the centers of carbon vacancies.

Crystal structures and magnetic properties of CeAu 4Si 2 and CeAu 2Si 2

Single crystals of CeAu 4Si 2 and CeAu 2Si 2 have been grown out of ternary fluxes rich in Au, and the former, also by sintering the stoichiometric composition at 750 °C. The single-crystal X-ray refinement result for CeAu 4Si 2 is orthorhombic, Cmmm (No. 65, Z=2), different from a tetragonal result found from an X-ray powder diffraction refinement [H. Nakashima, et al., J. Alloys Compds. 424 (2006) 7]. For CeAu 2Si 2, this is the first report of the stoichiometric crystalline phase, in the known tetragonal I4/ mmm structure. The anisotropic field- and temperature-dependent magnetizations, as well as specific heat and resistivity data are compared. Although both compounds have related structural packing, they present unique magnetic features. CeAu 2Si 2 is a typical antiferromagnet with T N=8.8(1) K and CeAu 4Si 2 features a ferromagnetic component below T c=3.3(1) K. Both phases have effective moments close in value to that of free Ce 3+.

Structural phase transition and magnetic properties of the Sr 2FeRe 1− xBx′O 6 ( B′=Nb, Ta; x=0, 0.1) double perovskites

The double perovskites, Sr 2FeReO 6 and Sr 2FeRe 0.9 M 0.1O 6 ( M=Nb, Ta) have been obtained by soft synthesis methods which yield homogeneous particles of micrometric grain size. The materials have been studied by X-ray and neutron powder diffraction, scanning electron microscopy and magnetic measurements. Rietveld refinements show that the compounds adopt a tetragonal I4/ mmm structure at high temperatures and monoclinic P2 1/ n below the transition temperature. The magnetic structures were determined by neutron powder diffraction at 100 and 300 K for the Sr 2FeReO 6, Sr 2FeRe 0.9Nb 0.1O 6 and Sr 2FeRe 0.9Ta 0.1O 6 phases, respectively. Evidence for a ferrimagnetic coupling between the Fe 3+ and Re 5+ sublattices has been observed. Magnetic measurements yield magnetic moments lower than the theoretical ones being in accord with the antisite disorder of 25% in the B– B′ positions.

La xSr 2− xFe yRu 1− yO 4± δ: a new family of K 2NiF 4 type oxides

The phases La x Sr 2− x Fe y Ru 1− y O 4± δ ( x=0.2–0.8; y=0.6–0.9) have been synthesized by solid-state techniques and yield tetragonal structures with I4 /mmm symmetry. The oxygen stoichiometry and high-temperature structures have been examined using diffraction techniques and in situ Mössbauer spectroscopy at temperatures up to ∼600°C. Furthermore, new reduced phases that adopt structures with Immm symmetry have been discovered. Unusual coordination numbers have been determined for the most highly reduced samples with square planar coordination evident for the B site cations. The reduced orthorhombic Immm phases were found to readily reoxidize in air to the tetragonal I4 /mmm structure at relatively low temperatures of only ∼500°C.

Characterisation of La 2NiO 4+ δ using in-situ high temperature neutron powder diffraction

La 2NiO 4+ δ , has been studied using in-situ high temperature neutron diffraction over a temperature range of 25–800 °C in vacuum. The behaviour of this material, and in particular the oxygen interstitial content, is discussed and quite remarkable bond length changes observed. It is observed that at temperatures above 150 °C La 2NiO 4+ δ transforms to the tetragonal I4/ mmm structure and maintains this over the entire temperature range on both heating and cooling. The loss of the interstitial oxygen was observed over the low temperature region of the study and significant changes in both lattice constant and bond lengths found to mirror these changes, indicating the structural importance of the interstitial oxygen.

Optical study of the ferroelastic phase transition of (N(CH3)4)2ZnBr4

Abstract (N(CH3)4)2ZnBr4 undergoes a ferroelastic phase transition from an orthorhombic mmm structure to a monoclinic 2/m phase. Birefringence and optical indicatrix rotation about the monoclinic axis have been measured in the ferroelastic phase and their temperature dependences have been related to the order parameter of the transition. The spontaneous strain has also been determined by means of X-ray diffraction. It is shown that in the temperature range covered, this quantity is proportional to the optical polarization coefficient B6, which becomes spontaneous at the transition. This fact has allowed us to obtain a numerical estimate of the p66 elastooptic coefficient at the transition point.

Phase transformation of AnX compounds under high pressure (An ≡ Np, Pu; X ≡ Sb, Te)

High pressure X-ray diffraction studies were performed on AnX compounds (An ≡ Np, Pu; X ≡ Sb, Te) in the pressure range up to about 57 GPa, at room temperature, using a diamond anvil cell in an energy-dispersive X-ray diffraction facility. These AnX compounds have an f.c.c., NaCl-type structure at ambient pressure and undergo first-order phase transitions under pressure. Whereas NpTe and PuTe transform to a primitive cubic cell of the CsCl type at 13 GPa and 15 GPa respectively, a tetragonal structure, space group P4/ mmm, is observed for NpSb above 10 GPa. This tetragonal structure can be regarded as a “distorted” CsCl-type structure, the atomic positions being the same as in the cubic CsCl cell. In PuSb two transitions are detected: the CsCl-type structure is observed above 17 GPa; then PuSb transforms to the tetragonal P4/ mmm structure at about 42 GPa. In addition, the compressibility of the low pressure phase was determined from the V( p) data for each compound.

Magnetic order and hyperfine interactions in RFe 6Al 6 ( R = rare earth)

The systems RFe 6Al 6( R = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) crystallize in the tetragonal body centered I4/ mmm structure. In striking contrast to the magnetic behaviour of RFe 4Al 8 (weakly coupled R and Fe sublattices, complicated magnetic structure, low T c ~ 130 K), in the RFe 6Al 6 systems all magnetic sublattices order simultaneously at a relatively high temperature. The magnetization curves start with low values at low temperatures and rise to very high values at T max ~ 230 K and then drop to 0 at T c ~ 330 K. All samples show strong hysteresis effects at temperatures just below T max. Mossbauer studies of 57Fe in the ( f) and ( j) sites and 151Eu, 155Gd, 161Dy, 166Er and 170Yb in the ( a) site yield all hyperfine interaction parameters and temperature dependence of the local magnetic moments. All Mossbauer and magnetization experimental results can be explained in a self consistent way with a simple molecular field model. The Fe in the ( j) site plays the dominant role in its strong intrasublattice ferromagnetic exchange and its strong antiferromagnetic exchange with the rare earth site. The Fe in the ( f) site have an antiferromagnetic intrasublattice exchange, they have a canted strcuture with the ferromagnetic component parallel to the ( j) sublattice magnetization.