CsCl-type Crystal Structure Research Articles

Phase analysis of near equiatomic bulk FeRh alloys: X-ray versus neutron diffraction and magnetization measurements

The information obtained by X-ray diffraction (XRD) by impinging the X-ray beam onto the flat surface of bulk Fe–Rh alloys is contrasted with that provided by the thermomagnetic analysis curves measured under a magnetic field of 5 mT and 2 T, and the neutron diffraction (ND) patterns. The experiments were performed on slices cut from bulk Fe50Rh50 and Fe49Rh51 alloys prepared by induction melting followed by 48 h of thermal annealing at 1273 K. Whereas ND patterns and magnetization curves, directly and indirectly point out that the samples are single-phase with a CsCl-type crystal structure undergoing a magnetoelastic transition that changes the magnetic structure from antiferromagnetic to ferromagnetic on heating, multiple phases are identified in the XRD patterns, which modify upon polishing or etching the surface with a mixture of hydrochloric (HCl) and nitric acid (HNO3) at a volume ratio of 90:10 from 30 to 210 min. The limitation of XRD for accurately characterizing the phase constitution within the bulk of Fe–Rh alloys is underlined.

Theoretical Investigation of Pressure Dependent Structural and Elastic Properties of MnSi Compound

MnSi is a metallic compound that exhibits a structural phase transition as a function of pressure. At ambient conditions, MnSi has a cubic structure such as NaCl type crystal structure. However, at high pressures, it undergoes a structural phase transition to a CsCl type crystal structure. The pressure-dependent structural phase transition in MnSi has been studied using various experimental and theoretical techniques. In the present work, we have used an efficient inter-ionic potential approach to predict pressure dependent structural phase change and associated volume collapse in MnSi. Therein, the potential includes the long-range Coulomb, van der Waals (vdW) interaction, and the short-range repulsive interaction up to second neighbour ions. It has been demonstrated that the Hafemeister and Flygare approach successfully evaluates the equation of state for change in volume with respect to applied pressure. We have identified a structural phase transition from B1(NaCl) type structure to B2 (CsCl) type structure in this compound. The estimated value of phase transition pressure (Pt) is 42 GPa, which is consistent with the available reported data. The identified first order phase transformation showed a volume collapse of about 10% in the vicinity of transition. In addition, we have also investigated second order of elastic constants for MnSi compound.

Room-Temperature Solid-State Synthesis of Cs3Cu2I5 Thin Films and Formation Mechanism for Its Unique Local Structure.

Blue-emitting Cs3Cu2I5 has attracted attention owing to its near-unity PL quantum yield and applications in DUV photodetectors and scintillators. Its PL properties originate from the unique local structure around the luminescent center, the [Cu2I5]3- polyhedron iodocuprate anion consisting of the edge-shared CuI3 triangle and the CuI4 tetrahedron dimer, which is isolated by Cs+ ions. We found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form Cs3Cu2I5 and/or CsCu2I3 phases. High-quality thin films of these phases were obtained by the sequential deposition of CuI and CsI by thermal evaporation. We elucidated that the formation of interstitial Cu+ and the antisite of I- at the Cs+ site in the CsI crystal through Cu+ and I- diffusion results in the RT synthesis of Cs3Cu2I5. The unique structure formation of the luminescent center was revealed using a model based on the low packing density of the CsCl-type crystal structure, similar sizes of Cs+ and I- ions, and the high diffusivity of Cu+. The self-aligned patterning of the luminous regions on thin films was demonstrated.

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

The First Principles Approach to The Structural, Elastic, Electronic, Vibrational and Thermal Properties of CsCl type-ErAu Alloy

Lanthanide-gold binary alloys are very attracting attention in applications of electrical measuring technology based on resistance. Such materials are considered suitable for electrical circuits due to the large temperature stability. In this study, the structural, elastic, electronic, vibrational and thermal properties of the CsCl-type crystal structure of the ErAu binary alloy with two atoms in the unit cell are investigated in the framework of the first principles approach. The lattice parameter is found as 3.603 Å. Obtained structural parameters are consistent with the available studies. Considering of electronic properties, the electronic band structures, total and partial density of states of the ErAu alloy are determined. From these calculations, it has been decided that ErAu alloy is metallic in nature. The elastic constants are calculated using the stress-strain method. The elastic constants are also obtained for different pressure values. Elastic properties of the system present that ErAu alloy is mechanically stable at different pressure values. Phonon frequencies are calculated and the structure is determined as dynamically stable. To present the thermal properties of ErAu alloy, the free energy, entropy and heat capacity of the system are also obtained under increasing temperature values.

Scanning tunneling microscopy study of the early stages of epitaxial growth of CoSi2 and CoSi films on Si(111) substrate: Surface and interface analysis

We report scanning tunneling microscopy observations of solid-phase epitaxial growth of ultra-thin CoSi2 and CoSi films at heating temperatures in the range of 350–600°C using a disordered ‘(1×1)’ phase of Co/Si(111) as a pre-existing template. It is found that depending on the growth conditions two types of epitaxial cobalt silicide films can be formed. At coverages below ~2 ML (1 ML=7.83×1014atoms/cm2) of Co, CoSi2 phase grows in the form of flat two-layer patches spreading over the surface at elevation of the temperature. It consists of precisely two the completely filled Co layers and four Si layers above the interface. At coverages higher than ~2 ML of Co, the layer-by-layer growth of CoSi phase with the CsCl-type crystal structure also takes place. The CoSi phase consists of one or more Co-Si double layers, which are located on top of the second Si-Co-Si triple layer of CoSi2, i.e., it has а coherent double interface CoSi/CoSi2/Si(111). According to experimental findings and ab initio total-energy calculations, the structures with the eightfold coordination of interfacial Co atoms and lateral unit cells of the silicide layer and substrate mutually rotated by 180° prove to be the most stable ones.

Elastic properties of ternary (Alx,Mg1−x)Sc random alloys from first principles methods

Alloys in the (Alx,Mg1−x)Sc system were studied computationally in order to investigate the compositional dependence of the elastic properties. The CsCl-type crystal structure was considered, and the Special Quasirandom Structures method was used to generate random models with minimal periodicity error. These models were studied using density functional perturbation theory implemented with planewaves and pseudopotentials to compute the elastic tensors for different compositions. It was found that addition of Mg to the alloy up to 20at% preserves the Young’s modulus, while further addition of Mg led to lower densities but also decrease of elastic moduli and increased brittleness.

Effect of Cu or Ti Substitution in MgNi on Crystal Structure and Hydrogen Absorption-Desorption Properties

Our previous study showed the noble MgNi intermetallic compound with a CuTi-type crystal structure could be synthesized from amorphous MgNi precursor under more than 5 GPa by high pressure synthesis and could be hydrogenated into novel hydride with a CsCl-type crystal structure at 355 K under 2 MPa of hydrogen. In this study, hydrogen absorption-desorption properties of the MgNi compound with CuTitype structure were investigated. It was found that the MgNi with CuTi-type structure absorbed and desorbed hydrogen reversibly at 473 K. Effects of substitution of Ni by Cu or Ti in the MgNi compounds were also investigated. Ternary MgNi1� xCux (x ¼ 0:2; 0:5) and MgNi0:9Ti0:1 compounds were synthesized from amorphous precursors by high pressure synthesis. The MgNi1� xCux (x ¼ 0:2; 0:5) and MgNi0:9Ti0:1 have CsCl-type crystal structure. The lattice parameter of MgNi1� xCux increased with increasing of Cu content. MgNi1� xCux (x ¼ 0:2; 0:5) and MgNi0:9Ti0:1 could be hydrogenated under hydrogen pressure of 2 MPa and the crystal structure of these hydrides were CsCl-type structure. The hydrogen desorption temperature of these hydrides decreased with increasing of amount of Cu or Ti substitution of Ni. The substitution of Ni by Ti in MgNi improved the hydrogen absorption capacity from 1.65 to 2.06 mass%. [doi:10.2320/matertrans.M2009064]

Elastic constants and high-pressure structural transitions in lanthanum monochalcogenides from experiment and theory

The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density-functional calculations of LaS, LaSe, and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray-diffraction investigations of LaS and LaSe up to a maximum pressure of $41\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. A structural phase transition from the NaCl-type to CsCl-type crystal structure is found to occur in all cases. The experimental transition pressures are 27--28 and $19\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for LaS and LaSe, respectively, while the calculated transition pressures are 29, 21, and $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for LaS, LaSe, and LaTe, respectively. The calculated ground-state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.

Effects of Pressure on the Magnetic and Structural Properties of GdCu

The electrical resistivity down to 4.2 K and X-ray at room temperature have been measured under pressure in GdCu. At ambient pressure, this material undergoes a martensitic transformation when the temperature is decreased below T MS = 235 K. We found that at room temperature, the CsCl-type crystal structure remains stable up to 13 GPa. However, the martensitic transformation is very sensitive to pressure, being suppressed above 0.6 GPa.

Mechanical properties of single crystal YAg

YAg, a rare earth-precious metal “line compound”, is one member of the family of B2 rare earth intermetallic compounds that exhibit high ductilities. Tensile tests of polycrystalline YAg specimens have produced elongations as high as 27% before failure. In the present work, single crystal specimens of YAg with the B2, CsCl-type crystal structure were tensile tested at room temperature. Specimens with a tensile axis orientation of [0 1 1 ̄ ] displayed slip lines on the specimen faces corresponding to slip on the {1 1 0}〈0 1 0〉 with a critical resolved shear stress of 13 MPa. A specimen with a tensile axis orientation of [1 0 0] showed no slip lines and began to crack at a stress of 300 MPa. The test specimens also displayed some slip lines whose position corresponded to slip on the {1 0 0}〈0 1 0〉 ; these slip lines were found near intersections of {1 1 0}〈0 1 0〉 slip lines, which suggests that the {1 0 0}〈0 1 0〉 may be a secondary slip system in YAg. Transmission electron microscope (TEM) examination of the crystals was performed after tensile testing and the dislocations observed were analyzed by g · b=0 out of contrast analysis. This TEM analysis indicated that the predominant Burgers vector for the dislocations present was 〈1 1 1〉 with some 〈0 1 1〉 dislocations also being observed. This finding is inconsistent with the 〈0 1 0〉 slip direction determined by slip line analysis, and possible explanations for this surprising finding are presented.

Origin of the Absorption Bands of Cesium Halides Doped with Tl+ Ions

We have systematically investigated the absorption spectra of CsX:Tl + (X = Cl, Br, I) and their mixed crystals with CsCl-type crystal structure. The lowest energy absorption bands, which are peaked at 5.18, 4.85, and 4.27 eV for CsCl:Tl + , CsBr:Tl + , and CsI:Tl + , respectively, exhibit the similar temperature dependence as the A band of Tl + centres in NaCI-type alkali halides. In the higher energy region, several absorption bands are observed with no extremely different absorption intensity. The temperature evolution of the higher energy absorption bands is not characteristic of the Tl + centres in NaCI-type alkali halides. Absorption bands in CsBr 1-x CsI x :Tl + mixed crystals can be classified into two groups: the rapidly disappearing and gradually fading absorption bands with increasing composition of the hetero-halogen ions. The origin of the higher absorption bands is discussed on the basis of the spectral changes in the CsBr 1-x CsI x :Tl (0.0≤ x ≤1.0) mixed crystals.

Characterization of the age-hardening behavior of a precipitation-hardenable austenitic steel

This paper describes the age-hardening behavior and microstructural features of a precipitation-hardenable austenitic steel. It has been found that the optimum heat treatment for the current alloy, in terms of strength, involves solution treatment at 1473 K for 1 h followed by aging at 973 K for 5 h. The precipitating phase, leading to significant strengthening in this alloy, has been identified as NiAl. The NiAl precipitates are ordered with a CsCl-type crystal structure and exhibit a plate-like morphology. These precipitates have been found distributed in two orientations and show a Kurdjmov–Sachs orientation relationship with the matrix. The alloy retains high strength up to about 700 K, beyond which there is a significant drop in strength.

Alloys with the shape memory effect in systems of d-transition metals containing metals of the platinum group

Our own and literature data on the reactions in binary and ternary systems of transition metals, one component of which is a platinum group metal, and which contain phases which undergo a martensitic transformation, are reviewed. The transformation is responsible for the shape memory effect, which occurs in the alloys under consideration at temperatures above 300°C. In all equiatomic phases of the binary systems discussed (including quasibinary sections in the ternary systems Ti(Sc)–Ni(Co)–Me; Me=platinum group metal), in which the shape memory effect occurs, the transformation takes place from a matrix phase with a CsCl-type crystal structure. The properties of the parent and transformation phases, phase equilibria in the region of phase formation, and the effect of composition on these are described.

Hydrogenation characteristics of TiFe 1− xPd x (0.05≤ x≤0.30) alloys

Pseudobinary TiFe 1− x Pd x (0.05≤ x≤0.30) alloys have been investigated on their crystal structure and hydriding behavior. The lattice constant increased with an increase in the substituted amount of Pd with maintaining the CsCl type crystal structure. The partial substitution by Pd with superior catalytic and hydriding natures were effective for mitigation of their activation conditions to a considerable extent. The substitution resulted in lowering the plateau pressures ascribed to formation of the monohydride in the PCT diagrams.

Constitution of the Hf-Ni system up to 50 at.% Ni

The Hf-Ni system phase diagram has been reinvestigated in the hafnium-rich composition range up to 50 at.% Ni. Two intermetallic compounds Hf 2Ni and HfNi melting congruently at 1220 °C and 1505 °C, respectively, were observed. There are two eutectics: L ⇌ β-Hf + Hf 2Ni with about 29 at.% Ni at 1145°C and L ⇌ Hf 2Ni + HfNi with about 35 at.% Ni at 1175°C. Allotropic transformation in HfNi takes place at 1150°C. The cubic CsCl-type crystal structure of the high temperature modification of HfNi has been determined indirectly for the first time.

Field induced transitions in intermetallic compounds ErAg and HoAg

High field magnetism was investigated for antiferromagnetic intermetallic compounds ErAg and HoAg with the CsCl-type crystal structure. The measurements of magnetization were carried out under static magnetic fields up to 270 kOe. A three-stage field induced transition in ErAg and a five-stage transition in HoAg are observed. The temperature dependence of the critical fields is also examined and magnetic phase diagrams are presented.

Tight-binding calculation of the local moments in ordered FeV alloy

A tight-binding type self-consistent band calculation is used to study the local magnetic moments of the ordered FeV alloy with CsCl-type crystal structure within the Hartree-Fock approximation of the Hubbard model for the spin polarisation. It is shown that the reasonable choice of the parameters for the numbers of d-electrons and intra-atomic Coulomb integrals can account for the experimental values of the local moments if the hopping integrals between Fe and V are chosen to be about 6% smaller than the usual geometrical average of the hopping integrals for pure metals.

Pressure effect on the Néel temperature of GdCu, GdAg, TbCu and TbAg

The pressure effect on the Néel temperature was measured on the antiferromagnetic compounds GdCu, GdAg, TbCu and TbAg with a CsCl-type crystal structure. dTN/dP is obtained to be 0.27 K/kbar for GdCu, 1.1 K/kbar for GdAg, 0.06 K/kbar for TbCu and 0.38 K/kbar for TbAg.

High-pressure optical absorption and x-ray-diffraction studies in RbI and KI approaching the metallization transition

The pressure dependence of the fundamental optical absorption edges in RbI and KI crystals has been investigated up to 70 GPa at room temperature by using high-pressure apparatus of sapphire and diamond anvil types. The absorption edges in RbI and KI are found to show some complicated behaviors below \ensuremath{\sim}5 GPa within the uv region. Beyond \ensuremath{\sim}5 GPa, absorption edges decrease monotonically with increasing pressure up to 70 GPa. The x-ray-diffraction study shows that the CsCl-type crystal structure remains stable up to 67 GPa after the NaCl- to CsCl-type phase transition at 0.4 GPa for RbI and at 1.9 GPa for KI, respectively. The metallization pressures of RbI and KI are estimated to be approximately 85 and 115 GPa, respectively.