High Temperature Phase Diagram Research Articles

Phase diagram of ruthenium characterized In Situ by synchrotron X-ray diffraction and Ab Initio simulations

In this work, an in situ characterization of the high-pressure and high-temperature phase diagram of ruthenium was carried out. This experiment was performed using a combination of laser-heated diamond anvil cell (LH-DAC) and X-ray diffraction (XRD) techniques at the beamline ID27 of the European Synchrotron Radiation Facility (ESRF). XRD patterns were collected in the range of 15 GPa to 110 GPa and from ambient temperature up to 6600 K. While the hcp-fcc transition, predicted to occur at elevated temperatures was not observed, this study has produced the first experimental observation of a solid–liquid phase transition of Ru at HP conditions. A P-V-T equation of state valid up to 100 GPa and 3000 K is reported.

Recension of boron nitride phase diagram based on high-pressure and high-temperature experiments

Abstract Cubic boron nitride and hexagonal boron nitride are the two predominant crystalline structures of boron nitride. They can interconvert under varying pressure and temperature conditions. However, this transformation requires overcoming significant potential barriers in dynamics, which poses great difficulty in determining the c-BN/h-BN phase boundary. This study used high-pressure in situ differential thermal measurements to ascertain the temperature of h-BN/c-BN conversion within the commonly used pressure range (3–6 GPa) for the industrial synthesis of c-BN to constrain the P–T phase boundary of h-BN/c-BN in the pressure–temperature range as much as possible. Based on the analysis of the experimental data, it is determined that the relationship between pressure and temperature conforms to the following equation: P = a + 1 b T . Here, P denotes the pressure (GPa) and T is the temperature (K). The coefficients are a = −3.8±0.8 GPa and b = 229.8±17.1 GPa/K. These findings call into question existing high-pressure and high-temperature phase diagrams of boron nitride, which seem to overstate the phase boundary temperature between c-BN and h-BN. The BN phase diagram obtained from this study can provide critical temperature and pressure condition guidance for the industrial synthesis of c-BN, thus optimizing synthesis efficiency and product performance.

High-pressure–high-temperature phase diagram of tin

High-pressure–high-temperature phase diagram of tin

High-Temperature Mechanical and Dynamical Properties of γ-(U,Zr) Alloys

High-temperature body-centered cubic (BCC) γ-U is effectively stablized by γ-(U,Zr) alloys that also make it feasible to use it as a nuclear fuel. However, relatively little research has focused on γ-(U,Zr) alloys due to their instability at room temperature. The effect of Zr composition on its mechanical properties is not clear yet. Herein, we perform molecular dynamics simulations to investigate the mechanical and dynamical stabilities of γ-(U,Zr) alloys under high temperatures, and we calculate the corresponding lattice constants, various elastic moduli, Vickers hardness, Debye temperature, and dynamical structure factor. The results showed that γ-U, β-Zr, and γ-(U,Zr) are all mechanically and dynamically stable at 1200 K, which is in good agreement with the previously reported high-temperature phase diagram of U-Zr alloys. We found that the alloying treatment on γ-U with Zr can effectively improve its mechanical strength and melting points, such as Vickers hardness and Debye temperature, making it more suitable for nuclear reactors. Furthermore, the Zr concentrations in γ-(U,Zr) alloys have an excellent effect on these properties. In addition, the dynamical structure factor reveals that γ-U shows different structural features after alloying with Zr. The present simulation data and insights could be significant for understanding the structures and properties of UZr alloy under high temperatures.

Characterization of the high-pressure and high-temperature phase diagram and equation of state of chromium

The high-pressure and high-temperature phase diagram of chromium has been investigated both experimentally (in situ), using a laser-heated diamond-anvil cell technique coupled with synchrotron powder X-ray diffraction, and theoretically, using ab initio density-functional theory simulations. In the pressure–temperature range covered experimentally (up to 90 GPa and 4500 K, respectively) only the solid body-centred-cubic and liquid phases of chromium have been observed. Experiments and computer calculations give melting curves in agreement with each other that can both be described by the Simon–Glatzel equation T_{m}(P) = 2136K (1 + P/25.9)^{0.41}. In addition, a quasi-hydrostatic equation of state at ambient temperature has been experimentally characterized up to 131 GPa and compared with the present simulations. Both methods give very similar third-order Birch–Murnaghan equations of state with bulk moduli of 182–185 GPa and respective pressure derivatives of 4.74–5.15. According to the present calculations, the obtained melting curve and equation of state are valid up to at least 815 GPa, at which pressure the melting temperature is 9310 K. Finally, from the obtained results, it was possible to determine a thermal equation of state of chromium valid up to 65 GPa and 2100 K.

High-temperature diffraction experiments and phase diagram of ZrO2 and ZrSiO4

Abstract High-temperature X-ray diffraction (XRD) experiments up to T = 2710 °C have been performed on ZrSiO4 and ZrO2 powders, using the container-less levitation technique. A two-dimensional imaging plate (IP) detector was used for short-time observation. The diffraction data in a wide area was projected in one dimension. The unit cell parameters, thermal expansions, and c/a ratios for ZrSiO4 (space group I41/amd and Z = 4), tetragonal ZrO2 (space group P42/nmc and Z = 2) and cubic ZrO2 (space group F m 3 ‾ m $Fm3‾{}m$ and Z = 4) were measured to understand the high-temperature behaviors. The transition temperature between tetragonal and cubic ZrO2 was specified to be between 2430 and 2540 °C. The pre-transitional behavior was observed around 2200 °C. As no clear change in unit cell volume is evident, the phase boundary between the tetragonal and the cubic phase has been shown to be a positive slope. The ZrO2 and ZrO2–SiO2 phase diagrams are proposed based on the chemical composition and the crystal structure.

Charge density waves in cuprate superconductors beyond the critical doping

The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of xc ≈ 0.19. Charge density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value xc, leading to questions regarding its essential relevance. Here, we use X-ray diffraction to demonstrate that CDW correlations in La2−xSrxCuO4 persist up to a doping of at least x = 0.21. The correlations show strong changes through the superconducting transition, but no obvious discontinuity through xc ≈ 0.19, despite changes in Fermi surface topology and electronic transport at this doping. These results demonstrate the interaction between CDWs and superconductivity even in overdoped cuprates and prompt a reconsideration of the role of CDW correlations in the high-temperature cuprate phase diagram.

All-electrical detection of skyrmion lattice state and chiral surface twists

We study the high-temperature phase diagram of the chiral magnetic insulator Cu$_2$OSeO$_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk Cu$_2$OSeO$_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in non-collinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

The high-pressure, high-temperature phase diagram of cerium

We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to ∼22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic oC4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the α-cF4–oC4–tI2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the oC4 phase, and 1–2 GPa lower than previously reported. We find the α-cF4 → tI2 phase boundary to have a positive gradient of 280 K (GPa)−1, less steep than the 670 K (GPa)−1 reported previously, and find the oC4 → tI2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2–3 GPa in the transition pressures at which the oC4 → tI2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the α-cF4 → tI2 is not second order at all temperatures up to 820 K.

Thermodynamics, crystal chemistry and structural complexity of the Fe(SO4)(OH)(H2O) x phases: Fe(SO4)(OH), metahohmannite, butlerite, parabutlerite, amarantite, hohmannite, and fibroferrite

Using combination of acid-solution, high-temperature oxide-melt, relaxation, and differential scanning calorimetry, we have determined the thermodynamic properties of all phases in the system Fe(SO 4 )(OH)–H 2 O (estimated values in parentheses): Using these data, phase diagrams for low-temperature (25 °C) systems in contact with aqueous solutions predict that these phases should precipitate from extremely concentrated, low-pH solutions. In a relative humidity–temperature space, only Fe(SO 4 )(OH), parabutlerite, and amarantite have stability fields; the higher hydrates would require unrealistically high air humidities to form as stable phases. High-temperature high-pressure phase diagrams produce reasonable topologies, although the details of the phase relations between Fe(SO 4 )(OH), Fe 2 (SO 4 ) 3 , and hydronium jarosite deserve more work. We also present a new structural model for amarantite, including the positions of the H atoms, and an analysis of the hydrogen bonding network in this mineral. Using the concept of information density in minerals, the Fe(SO 4 )(OH)(H 2 O) x phases were analyzed. This analysis lends some validity to the premise that more complex structures are also the more stable ones, but other systems should be investigated to test these trends.

CaCO3 phase diagram studied with Raman spectroscopy at pressures up to 50 GPa and high temperatures and DFT modeling

We revise the high pressure, high temperature phase diagram of CaCO3 using Raman spectroscopy in conjunction with laser heated diamond anvil cell experiments. We confirm numerous aspects of earlier studies, including a recent X-ray diffraction study about the stability field of CaCO3-VII. Our Raman results show that CaCO3-VII is stable in the lower mantle at a depth of 690–1010 km. Our DFT calculations show that the phase transition from aragonite to CaCO3-VII at ≈25 GPa and from CaCO3-VII to post-aragonite at ≈40 GPa are accompanied by density changes of 2% and 3.5%, respectively. Shear sound velocities change by 9% and −12% across the transitions, respectively. Hence, a sufficient amount, at least locally, of CaCO3 in the Earth’s mantle can be detectable by an increase of the shear velocity at 690 km and a decrease of the shear velocity at 1010 km depths.

High-Pressure Phase Diagram and Superionicity of Alkaline Earth Metal Difluorides

We study the high-pressure–high-temperature phase diagram and superionicity of alkaline earth metal (AEM) difluorides (AF2, A = Ca, Sr, Ba) with first-principles simulation methods. We find that the superionic behavior of SrF2 and BaF2 at high pressures differ appreciably from that previously reported for CaF2 [Phys. Rev. Lett. 2014, 113, 235902]. Specifically, the critical superionic temperature of SrF2 and BaF2 in the low-pressure cubic fluorite phase is not reduced by effect of compression, and the corresponding high-pressure orthorhombic contunnite phases become superionic at elevated temperatures. We get valuable microscopic insights into the superionic features of AEM difluorides in both the cubic fluorite and orthorhombic contunnite phases by means of ab initio molecular dynamics simulations. We rationalize our findings on the structural and superionic behavior of AF2 compounds in terms of simple ionic radii arguments and generalize them across the whole series of AEM dihalides (AB2, A = Mg, Ca, Sr...

High pressure-high temperature phase diagram of an energetic crystal: Dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50)

Raman spectroscopy was used to examine the high pressure-high temperature structural and chemical stability of an insensitive, high-performance energetic crystal – dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50). The phase diagram was determined over 8GPa and (293–760)K. Under isobaric heating, the melting/decomposition of TKX-50 is preceded by a transformation to two consecutive high-temperature intermediates; a lower-temperature intermediate – diammonium 5,5′-bistetrazole-1,1′-diolate, and a higher-temperature intermediate – dihydroxylammonium 5,5′-bistetrazolate and/or diammonium 5,5′-bistetrazolate. Pressure strongly increases the transition temperatures for these transformations and subsequent decomposition. Significant increase in the chemical stability of TKX-50 and intermediates with pressure was attributed to a suppression of hydrogen-transfer.

High temperature and low field regime vortex phase diagram of in-situ prepared stainless steel sheathed MgB2 tapes

A magnetic field-temperature vortex phase diagram has been proposed for MgB2 on the basis of magnetoresistance studies performed on an in-situ prepared stainless steel sheathed MgB2 tapes. Thermally activated flux flow (TAFF) behavior is analyzed using Arrhenius relation. Low dissipative part of magnetoresistance with temperature is well described by vortex glass theory. Above critical region, the resistive behavior is influenced by fluctuation conductivity. In addition, a considerable vortex liquid region below HC2 is also observed but TAFF region is found to be quite narrow.

Unexpected coordination number and phase diagram of niobium diselenide under compression.

We discovered several new energetically competitive structures of NbSe2 using the multi-algorithm collaborative (MAC) crystal structure prediction algorithm combined with the density functional theory. It was found that the coordination number of Nb in NbSe2 is increased from 6 to 7, and then to 8 with increasing pressure. Furthermore, it was unexpected that an Se atom would be squeezed to the center of a cage formed by 12 other Se atoms and then have 12-fold coordination when the pressure was increased to 130.4 GPa. The 12-coordination metalloid atom has never been discovered in other transition metal dichalcogenides. The new C2/m, I4/mmm, and P4/mmm NbSe2 were verified to be stable under both dynamically and mechanically stabile conditions. It is especially noteworthy that the new C2/m-NbSe2 was predicted to be potentially synthesized at high pressure and recovered under ambient conditions. A detailed high-pressure and high-temperature phase diagram was constructed based on the quasi-harmonic approximation up to 200 GPa, and the synthesis conditions of different new NbSe2 materials were also analyzed. All the discoveries in this study will guide the future synthesis of new NbSe2 materials at specific pressure and under temperature conditions and also help to further understand other transition metal dichalcogenides.

Identifying the Distribution of Al3+ in LiNi0.8Co0.15Al0.05O2

The doping of Al into layered Li transition metal (TM) oxide cathode materials, LiTMO2, is known to improve the structural and thermal stability, although the origin of the enhanced properties is not well understood. The effect of aluminum doping on layer stabilization has been investigated using a combination of techniques to measure the aluminum distribution in layered LiNi0.8Co0.15Al0.05O2 (NCA) over multiple length scales with 27Al and 7Li MAS NMR, local electrode atom probe (APT) tomography, X-ray and neutron diffraction, DFT, and SQUID magnetic susceptibility measurements. APT ion maps show a homogeneous distribution of Ni, Co, Al, and O2 throughout the structure at the single particle level in agreement with the high-temperature phase diagram. 7Li and 27Al NMR indicates that the Ni3+ ions undergo a dynamic Jahn–Teller (JT) distortion. 27Al NMR spectra indicate that the Al reduces the strain associated with the JT distortion, by preferential electronic ordering of the JT lengthened bonds directed to...

Pressure and temperature induced phase transition in WB4: A first principles study

Using an unbiased structure search method based on particle-swarm optimization algorithms in combination with density functional theory calculations, we investigate the phase stability and electronic properties of WB4 under high pressures. Through structural searching as implemented in the CALYPSO code, we obtained the most state hexagonal phase (P63/mmc, named hp10) at ambient condition and a new monoclinic phase (C2/m structure, mp5) at high pressures for WB4. The P63/mmc structure transforms to the C2/m phase at about 71 GPa. Surprisingly, the two phases are both dynamically and mechanically stable at ambient conditions. The high bulk and shear modulus, low Poisson’s ratio for both phases in WB4, place WB4 as a promising low compressible material. Both phases with high hardness are stable due to the strong covalent bonding nature from electronic density of states and electron localization function analysis. To guide experimental finding of more metastable superhard materials, the high pressure high temperature phase diagram of WB4 is established within the quasi-harmonic approximation.

Phase Diagram and Decomposition of 1,1-Diamino-2,2-dinitroethene Single Crystals at High Pressures and Temperatures

The high-pressure–high-temperature (HP-HT) phase diagram and decomposition of 1,1-diamino-2,2-dinitroethene (FOX-7), central to understanding its stability and reactivity, were determined using optical spectroscopy and imaging measurements in hydrostatically compressed and heated single crystals. Boundaries between various FOX-7 phases (α, α′, β, γ, and e) and melting/decomposition curves were established up to 10 GPa and 750 K. The main findings are (i) a triple point is observed between α, β, and γ phases ∼0.6 GPa and ∼535 K; (ii) the previously suggested δ phase is not a new phase but is a partly decomposed γ phase. (iii) the α–α′ transition takes place along an isobar, whereas the α′−e transition pressure decreases with increasing temperature; and (iv) melting/decomposition temperatures increase rapidly with pressure, with an increase in the slope at the onset of the α′−e transition. Our results differ from the recently reported HP-HT phase diagram for nonhydrostatically compressed polycrystalline FOX...

High-pressure high-temperature phase diagram of gadolinium studied using a boron-doped heater anvil

A boron-doped designer heater anvil is used in conjunction with powder x-ray diffraction to collect structural information on a sample of quasi-hydrostatically loaded gadolinium metal up to pressures above 8 GPa and 600 K. The heater anvil consists of a natural diamond anvil that has been surface modified with a homoepitaxially grown chemical-vapor-deposited layer of conducting boron-doped diamond, and is used as a DC heating element. Internally insulating both diamond anvils with sapphire support seats allows for heating and cooling of the high-pressure area on the order of a few tens of seconds. This device is then used to scan the phase diagram of the sample by oscillating the temperature while continuously increasing the externally applied pressure and collecting in situ time-resolved powder diffraction images. In the pressure-temperature range covered in this experiment, the gadolinium sample is observed in its hcp, αSm, and dhcp phases. Under this temperature cycling, the hcp → αSm transition proceeds in discontinuous steps at points along the expected phase boundary. From these measurements (representing only one hour of synchrotron x-ray collection time), a single-experiment equation of state and phase diagram of each phase of gadolinium is presented for the range of 0–10 GPa and 300–650 K.

High-pressure high-temperature phase diagram of organic crystal paracetamol

High-pressure high-temperature (HPHT) Raman spectroscopy studies have been performed on the organic crystal paracetamol in a diamond anvil cell utilizing boron-doped heating diamond anvil. Isobaric measurements were conducted at pressures up to 8.5 GPa and temperature up to 520 K in five different experiments. Solid state phase transitions from monoclinic Form I → orthorhombic Form II were observed at various pressures and temperatures as well as transitions from Form II → unknown Form IV. The melting temperature for paracetamol was observed to increase with increasing pressures to 8.5 GPa. This new data is combined with previous ambient temperature high-pressure Raman and x-ray diffraction data to create the first HPHT phase diagram of paracetamol.