NaCl-type Phase Research Articles

PBE-GGA predicts the B8↔B2 phase boundary of FeO at Earth’s core conditions

FeO is a crucial component of the Earth's core, and its thermodynamic properties are essential to developing more accurate core models. It is also a notorious correlated insulator in the NaCl-type (B1) phase at ambient conditions. It undergoes two polymorphic transitions at 300 K before it becomes metallic in the NiAs-type (B8) structure at ~100 GPa. Although its phase diagram is not fully mapped, it is well established that the B8 phase transforms to the CsCl-type (B2) phase at core pressures and temperatures. Here, we report a successful ab initio calculation of the B8↔B2 phase boundary in FeO at Earth's core pressures. We show that fully anharmonic free energies computed with the Perdew-Burke-Ernzerhof-generalized gradient approximation coupled with thermal electronic excitations reproduce the experimental phase boundary within uncertainties at P > 255 GPa, including the largely negative Clapeyron slope of -52 MPa/K. This study validates the applicability of a standard density functional theory functional to FeO under Earth's core conditions and demonstrates the theoretical framework that enables complex predictive studies of this region.

Glassy atomic vibrations and blurry electronic structures created by local structural disorders in high-entropy metal telluride superconductors

Recently, robustness of superconductivity to external pressure has been observed in high-entropy (HE) metal telluride (MTe). Here, we investigated the atomic displacement parameters (Uiso), atomic vibration characteristics, and electronic structure of MTe with various configurational entropies of mixing (ΔSmix) at the M site to understand the origin of the robustness of the superconductivity. Uiso clearly increased by M-site alloying with ΔSmix ≥ 1.1R, which is evidence of the created local disorder in the NaCl-type (low-pressure) phases. The simulated vibrational density of states (DOS) shows remarkable broadening with ΔSmix ≥ 1.1R, which indicates the glassy characteristics of atomic vibrations in the NaCl-type phases. In the calculated electronic structure for the CsCl-type (high-pressure) phases, a blurry electronic band structure appears with increasing ΔSmix, which indicates the evolution of blurry electronic states in HE MTe with the CsCl-type structure. The estimated electronic DOS at the Fermi energy cannot explain the changes in Tc for HE MTe when assuming conventional electron-phonon superconductivity, but the conventional explanation seems to work for PbTe. Therefore, the pairing mechanisms in HE MTe are affected by the glassy phonon and/or blurry electronic states, and the robustness of superconductivity possibly originates from the unique electron-phonon coupling.

Energy-efficient direct current arc plasma synthesis of tantalum carbide powder by advanced vacuum-free method

In this paper, we propose a method to manufacture tantalum carbide in open air by igniting a direct current arc discharge in a graphite crucible containing a mixture of raw materials: tantalum and carbon. Carbon burns to form carbon monoxide and carbon dioxide, which serve as a gaseous medium for the synthesis process. Using experimental and analytical procedures, we have obtained the necessary parameters — current, arcing time, and mass of the raw material — to produce the NaCl-type cubic phase of tantalum carbide. Micron- and submicron-sized tantalum carbide crystals are identified in the synthesis products. The combustion parameters of the resulting tantalum carbide are determined by differential thermal analysis. The proposed synthesis method is easier to implement, more energy-efficient, and less time-consuming than the current techniques.

Locally ordered nano-domains as novel microstructure defects suppressing the phonon transport in SnTe thermoelectrics

The phonon transport is intensively related to microstructure defects. Herein, a unique high-pressure sintering technology (GPa-level) was applied to modify the microstructure of pristine SnTe. There are many nano-domains exist in the high pressured NaCl-type SnTe binary compounds. The fast Fourier transform images show the domain phases have two sets of reflections with comparable intensity and exhibit doublet periodicities of special lattice planes, which demonstrates the domain phases are the CuPt and CuAu-I-type variants evolved from the NaCl-type matrix phase. An in-depth investigation indicates the constructions of the domain phase are related to locally off-stoichiometric of SnTe and the introduced strong inner-stress during high pressure sintering process. Additionally, a calculation of the phonon transport discloses the nano-domains restrict the phonon relaxation time by orders of magnitude over a wide phonon frequency range. As a result, a wide frequency phonon scattering network can be established to lower the lattice thermal conductivity.

Multicomponent TixNbCrAl nitride films deposited by dc and high-power impulse magnetron sputtering

Multicomponent TixNbCrAl nitride films were deposited on Si(100) substrates by reactive direct current magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS) in the absence of substrate heating and bias. Three single Ti, Nb, and Cr50Al50 targets were either driven by three dc or three HiPIMS power supplies. The Ti content in the films was varied by tuning the power applied to the Ti target. The composition was determined by ion beam analysis. The nitrogen content is nearly stoichiometric (48–50 at.%) in the HiPIMS series, while the dcMS are understoichiometric (39–45 at.%). The crystal structure, stress and density of the studied film were investigated by X-ray techniques and the microstructure was examined by scanning electron microscopy. All the Ti-containing films for both series exhibit an fcc NaCl-type phase structure. In particular, the dcMS series shows a (111) preferred orientation, resulting in a faceted surface morphology compared to a dense and smooth microstructure of the HiPIMS films. The compressive stress of the HiPIMS series (> 2.0 GPa) is significantly larger than the values of the dcMS series (<0.5 GPa). Nanoindentation measurements show a maximum hardness of 29.9 GPa and Young's modulus of 304 GPa were obtained in the HiPIMS series. The results may promote HiPIMS techniques for the synthesis of complex multicomponent films for the application aspect to protective and hard coatings.

Designing highly incompressible transition metal nitrides: A new class of W0.5Al0.5N phases

Herein, we used first-principles calculations and the particle swarm optimization technique to predict a highly incompressible W0.5Al0.5N phase with the space group R3¯m(166). Our results reveal that this phase, which was characterized by a negative formation enthalpy, is thermodynamically and dynamically stable, as revealed by the absence of imaginary modes in the phonon spectra. Furthermore, its energetic stability at a pressure of 15 GPa indicates a feasible strategy for experimental synthesis. The high performance stems from the optimized octahedral coordination between N and W/Al. Additionally, the good elastic parameters with BH of 310 GPa, GH of 206 GPa, and HV of 27 GPa confirm that it has preferable mechanical behaviors among the various W0.5Al0.5N phases and is even superior to those of the experimentally well-established NaCl-type phase. Based on the recently developed strain–stress method, it is shown that the ideal indentation strength of R3¯m(166) is about 32.7 GPa in the (1 1¯ 0) [0 0 1] direction, which is in excellent agreement with estimated HV. Therefore, our findings open the possibility for producing a new class of transition metal aluminum nitrides that have a broad range of applications.

High-pressure behavior of disordered kesterite-type Cu2ZnSnS4

We have investigated the high-pressure structural and vibrational behavior of the disordered kesterite-type Cu2ZnSnS4 compound at ambient temperature. Our experimental and theoretical investigations have revealed a clear structural transition to a GeSb-type phase close to 15 GPa, a tetragonally distorted variant of the NaCl-type phase. The latter transformation is accompanied by a cationic coordination increase from fourfold to sixfold with respect to the sulfur anions. In addition, a change in the compressibility rate was detected at about 8 GPa within the pressure stability range of the disordered kesterite-type phase. Upon decompression, a disordered zinc blende/sphalerite structure is recovered. We discuss our findings in close conjunction with our recent high-pressure work on the ordered Cu2ZnSnS4 modification.

Colossal enhancement of the thermoelectric power factor in stress-released orthorhombic phase of SnTe

At normal conditions, tin telluride (SnTe) adopts a cubic NaCl-type structure, but under applied pressure above 1.5–2 GPa, it transforms to a distorted crystal structure with an orthorhombic symmetry. Electronic properties of this high-pressure phase, including potential thermoelectricity, remain unexplored to date. Here, we measure the thermoelectric power (the Seebeck coefficient) and electrical resistivity of undoped single crystals of SnTe under applied high pressure up to 9 GPa, i.e., across the above phase transition. We establish that the high-pressure polymorph of SnTe is a p-type semiconductor and estimate its bandgap value at 3 GPa as Eg ∼ 65 meV. In contrast to the NaCl-type phase, the orthorhombic phase is stable in a much wider pressure range up to about 20 GPa, and its energy gap only insignificantly decreases with pressure with a coefficient of dEg/dP ∼ −4 meV/GPa. We find that the thermoelectric power factor of SnTe can be significantly improved in its orthorhombic phase due to the enhancement of the Seebeck coefficient. Furthermore, we show that the high-pressure phase preserves on the pressure releasing down to 0.3 GPa, and its thermopower grows progressively up to about 100 μV/K due to the bandgap expansion to Eg ∼ 105 meV. This results in a colossal rising of the thermoelectric power factor to about 8 mW/(K2m). Probably, this enhancement is contributed by structural distortions in the orthorhombic phase. We discuss how one could fabricate and optimize the orthorhombic polymorph of SnTe for potential use in various technologies, including thermoelectric applications.

Influence of Power Pulse Parameters on the Microstructure and Properties of the AlCrN Coatings by a Modulated Pulsed Power Magnetron Sputtering

In this study, AlCrN coatings were deposited using modulated pulsed power magnetron sputtering (MPPMS) with different power pulse parameters by varying modulated pulsed power (MPP) charge voltages (350 to 550 V). The influence of power pulse parameters on the microstructure, mechanical properties and thermal stability of the coatings was investigated. The results indicated that all the AlCrN coatings exhibited a dense columnar microstructure. Higher charge voltage could facilitate a denser coating microstructure. As the charge voltage increased up to 450 V or higher, the microvoids along the column boundaries disappeared and the coatings became fully dense. The main phase in the AlCrN coatings was the c-(Al, Cr)N solid solution phase with NaCl-type phase structure. A diffraction peak of the h-AlN phase was detected at a 2θ of around 33°, when the charge voltage was higher than 500 V. The hardness of the AlCrN coatings varied as a function of charge voltage. The maximum value of the hardness (30.8 GPa) was obtained at 450 V. All the coatings showed good thermal stability and maintained their structure and mechanical properties unchanged up to 800 °C during vacuum annealing. However, further increasing the annealing temperature to 1000 °C resulted in apparent change in the microstructure and decrease in the hardness. The charge voltages also showed a significant influence on the high-temperature tribological behavior of the coatings. The coating deposited at the charge voltage of 550 V exhibited excellent tribological properties with a low friction coefficient.

Theoretical investigation on the high-pressure physical properties of ZnN in cubic zinc blende, rock salt, and cesium chloride structures

In the current work, the aim is to report systematic results from first-principles calculations with density functional theory (DFT) on three cubic structures, rock salt (NaCl-type), zinc blende (ZnS-type), and cesium chloride (CsCl-type), of ZnN under high pressure. From the enthalpy versus pressure relations, we find that the NaCl-type phase of ZnN is more stable than the ZnS-type phase when the pressure higher than 2.55 GPa and high-pressure NaCl-type phase will stabilize up to 150 GPa. Through the careful evaluation with the quasi-harmonic Debye model, a complete set of thermodynamic data up to 2000 K, including PVT equation of state, isothermal bulk modulus, Debye temperature, Grüneisen parameter, thermal expansivity, heat capacity, and entropy for the ZnN with high-pressure NaCl-type structure is achieved. This set of data is considered as the useful information to understand the high-temperature and high-pressure properties of ZnN.

Domain structure analysis of (AgSbTe2)15(GeTe)85 thermoelectric compounds fabricated by rapid solidification process (RSP) and spark plasma sintering (SPS)

The domain structure of the thermoelectric compound (AgSbTe2)15(GeTe)85 was analyzed using high resolution transmission electron microscopy. Numerous coherent domain boundaries of two different types were observed, formed by the polar phase transition from the cubic NaCl-type phase to a rhombohedral structure with an angular distortion of 0.5–1.5°. One type has twin boundaries parallel to [001] and [110] directions, the other has inversion domain boundaries parallel to [111] and [112] directions. This high density of internal interfaces could reduce lattice thermal conductivity due to phonon scattering on the interfaces, and less deleterious on electronic transport processes, because they are coherent.

Plastic Crystals with Polar Halochromate Anion: Thermosensitive Dielectrics Based upon Plastic Transition and Dipole Rotation.

Plastic crystals functioning with rotatable components offer new opportunities in areas such as modern optoelectronic materials. Here, by taking advantage of controllable rotation of the polar component within the ion-pair plastic-crystal system, we present two such crystals, namely, (Et4N)(CrO3X) (X = Cl or Br), which are unusual examples exhibiting two-staged thermosensitive dielectric responses above room temperature. The frequency-dependent response in the first stage is due to the structural phase transitions, whereas that in the second stage is induced by dynamic rotation of the polar halochromate anions in their NaCl-type plastic-crystal phases. The intrinsic mechanisms were also explicated by molecular dynamics simulations, providing a direct insight into the dynamic characteristics of these two compounds. These studies show that ionic plastic crystals functioning with polar groups are an attractive candidate as sensitive thermoresponsive dielectric materials.

The Structural, Elastic, Electronic, Thermodynamic and Vibrational Properties of Protactinium Monocarbide (PaC) from First-Principles Calculations

The structural, elastic, electronic, thermodynamic, and vibrational properties of protactinium monocarbide (PaC), which crystallizes in NaCl-type phase (B1), were studied by performing ab-initio calculations based on density functional theory with in the generalized gradient approximation (GGA) using the Vienna Ab-initio Simulation Package (VASP). The calculated structural parameters, such as lattice constant, bulk modulus and its pressure derivative, formation energy were presented for B1 and B2 structures. This compound exhibits crystallographic phase transition from NaCl-type (B1) to CsCl-type (B2) structure at 42.7 GPa pressure. In order to gain further information, we investigated the elastic properties such as, Zener anisotropy factor, Poisson's ratio, Young's modulus, and isotropic shear modulus; the thermodynamic properties such as, the pressure and temperature dependent behavior of the normalized volume, bulk modulus, thermal expansion coefficient, heat capacity, Debye temperature, Gruneisen parameter, and entropy over a pressure range of 0-40 GPa and a temperature range of 0-2000 K. The electronic band structure, total density of states, phonon dispersion curves and one-phonon density of states of B1 phase were also presented.

Structural phase transition, electronic and elastic properties of rocksalt structure SnAs and SnSb

Abstract Pressure induced structural phase transitions in SnAs and SnSb have been studied using ab-initio density functional theory. The phase transition from NaCl to CsCl structure occurs at 29.8 GPa for SnAs, which agrees well with experimental data, while the same for SnSb is found to be 10.6 GPa, reported for the first time. The calculated ground state properties are in good agreement with available experimental and theoretical results. The electronic and bonding properties have also been analyzed. The elastic constants along with other secondary elasticity properties in B1 (NaCl-type) phase are also estimated at ambient as well as high pressure.

Atomic structure and chemistry of dislocation cores at low-angle tilt grain boundary in SrTiO3 bicrystals

Dislocations in perovskite oxides have been found to have important impacts on their electronic and ionic transportation properties, which are believed to be related to the structure and chemistry of dislocation cores. For dislocation cores at a 6° low-angle [001] tilt grain boundary in SrTiO3, an embedded TiOx rocksalt-like structure has been suggested, consistent with a deficiency of Sr. However, direct evidence supporting these suggestions has not been obtained up to now. In this work, we reveal the atomic structure and chemistry of edge dislocation cores at a low-angle [001] symmetric tilt-boundary in SrTiO3 bicrystals by imaging techniques of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) with an FEI Titan cube3 60–300 (PICO) microscope operated at 80kV. The experimental results demonstrate direct evidence for a local coordination of edge-sharing TiO6 octahedra at the dislocation cores, which can be understood as the result of strain. The local coordination of edge-sharing TiO6 octahedra is associated with the face-centered cubic (FCC) NaCl-type TiO phase. The present study therefore provides a solid structural and chemical basis for understanding the properties of dislocations.

(GeTe)nSbInTe3 (n≤3)—Element distribution and thermal behavior

Antimony in germanium antimony tellurides (GeTe)n(Sb2Te3) can be substituted by indium. Homogeneous bulk samples of GeSbInTe4 (R3̄m, Z=3, a=4.21324(5)Å, c=41.0348(10)Å) and Ge2SbInTe5 (P3̄m1, Z=1, a=4.20204(6)Å, c=17.2076(4)Å) were obtained; their structures were refined with the Rietveld method. Single-crystal X-ray diffraction using synchrotron radiation at the K edges of Sb and Te (exploiting anomalous dispersion) yields precise information on the element distribution in the trigonal layered structure of Ge3SbInTe6 (R3̄m, Z=3, a=4.19789(4)Å, c=62.1620(11)Å). The structure is characterized by van der Waals gaps between distorted rocksalt-type slabs of alternating cation and anion layers. The cation concentration is commensurately modulated with Sb preferring the positions near the gaps. In contrast to unsubstituted Ge3Sb2Te6, quenching the NaCl-type high-temperature phase (stable above ~510°C) easily yields a pseudocubic modification that is metastable at ambient conditions. Temperature-dependent powder diffraction reveals a broader stability range of the cubic high-temperature modification of Ge3SbInTe6 compared to the ternary phases. In-containing samples partially decompose at ca. 300°C but become homogeneous again when the high-temperature phase is formed.

Metallic transition of the colossal magnetoresistance material Fe x Mn1−x S (x= 0.18) under high pressure

A pressure-induced phase transition in the colossal magnetoresistance (CMR) material Fe0.18Mn0.82S was studied by using infrared (IR) reflection and X-ray diffraction (XRD) at pressures up to 40 GPa at room temperature. XRD shows that the crystal structure of this sample is a NaCl-type structure at ambient pressure, that a structural change starts around 17 GPa, and that a mixed phase mixed between the NaCl-type low-pressure phase and an unknown structure high pressure phase continues up to around 25 GPa. On the other hand, the IR reflectivity increases with increasing pressure from 15 GPa and becomes remarkably high around 20 GPa. The spectra do not show any changes from 30 GPa. From these results, we conclude that the phase transition of Fe0.18Mn0.82S at room temperature starts around 15 GPa and is completed around 30 GPa and that the high-pressure phase is not a band-overlapping semimetal but a true metal.

High pressure-induced phase transitions in CdS up to 1 Mbar

The pressure-induced structural transformations of CdS have been investigated using synchrotron x-ray diffraction in a diamond anvil cell up to 104 GPa and the density functional theory calculations. The x-ray diffraction experiments show that CdS is stable with the wurtzite-type structure at ambient conditions. The wurtzite-type phase transforms to NaCl-type structure at 3 GPa, which is followed by a second phase transition at 52.3 GPa. In the diffraction patterns, the peak-splitting is observed, indicating that the high-pressure phase appearing at 52.3 GPa is the Pmmn structure, rather than the Cmcm phase reported earlier. With increasing pressure, the lattice parameter a of the Pmmn phase increases abnormally, contrary to decrease of b and c axes. Our calculations reveal that the abnormal change of the a-axis could be related to the pressure-induced crystal structural change. The bulk modulus (B0), is 64.3(9) GPa for wurzite-type phase, 105(2) GPa for NaCl-type phase, and 54(4) GPa for the Pmmn phase, respectively.

Epitaxial Growth of Chromium Oxynitride Thin Films on Magnesium Oxide (100) Substrates and Their Oxidation Behavior

Epitaxially grown Cr(N,O) thin films were prepared on MgO substrates, with a misfit of 11.7% with respect to CrN, using pulsed laser deposition. X-ray diffraction patterns showed the peak for the (200) reflection of Cr(N,O) around the peak for the (200) reflection of MgO. The X-ray diffraction pattern of the ¤ scan for the (111) reflection of Cr(N,O) showed a narrow peak appearing every 90 degrees. From microstructural observations, grain boundaries in the thin films could not be confirmed. In order to evaluate the oxidation behavior of Cr(N,O) thin films, oxidation tests in air were carried out. After the oxidation tests, a Cr2O3 phase was formed at 873K and the B1 (NaCl-type) phase disappeared at 1173K. In the thin film oxidized at 1073K, a Cr2O3 layer on the surface of the thin film as well as a compositional gradient of oxygen were observed. This indicates that Cr(N,O) hard coatings can form oxidation barrier layers to extend the lifetime of cutting tools. [doi:10.2320/matertrans.MAW201310]

Effect of electrical field on crystallization and ferroelectric properties of Ge:Sb:Te films

AbstractIn this work impedance spectroscopy has been used to investigate of the NaCl type‐hexagonal transition in stoichiometric Ge2Sb2Te5 and in non‐stoichiometric (Ge24Sb15Te61) films. The temperature dependence of capacitance in all films shows an abrupt change (about 4‐6 times) at the temperature corresponding to the end of the NaCl type‐hexagonal transition. Additional, impedance measurements were carried out in films which have been crystallized with an external DC electric field. Comparing the results in the films crystallized with and without the external electric field, it is observed that in films crystallized with an external electric field, the capacitance increases during the NaCl type‐hexagonal transition for about 9‐15 times and that the piezoresponse force microscopy measurements have shown ferroelectric domains in the NaCl type phase. External electrical field changes significantly the onset of amorphous‐NaCl type crystallization temperature which could be related with an increase in atomic diffusion, promoting the growth of the crystalline phase. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)