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10 pages, 2031 KiB

Open AccessArticle

High-Pressure Vibrational and Structural Studies of the Chemically Engineered Ferroelectric Phase of Sodium Niobate

by Sanjay Kumar Mishra, Nandini Garg, Smita Gohil, Ranjan Mittal and Samrath Lal Chaplot

Crystals 2023, 13(8), 1181; https://doi.org/10.3390/cryst13081181 - 29 Jul 2023

Viewed by 1319

Abstract

Pure NaNbO₃ has an antiferroelectric phase at ambient pressure. The structural behaviour of the chemically engineered ferroelectric phase of sodium niobate, NNBT05: [(0.95) NaNbO₃-(0.05) BaTiO₃], under high-pressure has been studied using Raman scattering and angle-dispersive synchrotron X-ray diffraction [...] Read more.

Pure NaNbO₃ has an antiferroelectric phase at ambient pressure. The structural behaviour of the chemically engineered ferroelectric phase of sodium niobate, NNBT05: [(0.95) NaNbO₃-(0.05) BaTiO₃], under high-pressure has been studied using Raman scattering and angle-dispersive synchrotron X-ray diffraction techniques. At pressure > 1 GPa, noticeable changes in the Raman spectra can be seen in the low wavenumber modes (150–300 cm⁻¹). Large changes in the positions and intensities of the Raman bands as a function of pressure provide evidence for structural phase transition. The results indicate significant changes in the bond-lengths and the orientation of the NbO₆ octahedra at ~1 GPa, and a transition to the paraelectric phase at ~5 GPa, which are at lower pressures than previously found in pure NaNbO₃. The powder X-ray diffraction pattern shows an appreciable change in the peak profile in terms of position and width on increasing pressure. The pressure dependences of the structural parameters show that the response of the lattice parameters to pressure is strongly anisotropic. By fitting the pressure–volume data using the Birch–Murnaghan equation of state, the isothermal bulk modulus was estimated. The experimental results suggest that on doping BaTiO₃ in NaNbO₃, the bulk modulus increases. The bulk modulus of NNBT05 has been estimated to be 164.5 GPa, which is fairly close to 157.5 GPa, as previously observed in NaNbO₃. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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11 pages, 2301 KiB

Open AccessArticle

Pressure-Induced Structural Phase Transition of Co-Doped SnO₂ Nanocrystals

by Vinod Panchal, Laura Pampillo, Sergio Ferrari, Vitaliy Bilovol, Catalin Popescu and Daniel Errandonea

Crystals 2023, 13(6), 900; https://doi.org/10.3390/cryst13060900 - 31 May 2023

Cited by 2 | Viewed by 1421

Abstract

Co-doped SnO₂ nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P4₂/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 [...] Read more.

Co-doped SnO₂ nanocrystals (with a particle size of 10 nm) with a tetragonal rutile-type (space group P4₂/mnm) structure have been investigated for their use in in situ high-pressure synchrotron angle dispersive powder X-ray diffraction up to 20.9 GPa and at an ambient temperature. An analysis of experimental results based on Rietveld refinements suggests that rutile-type Co-doped SnO₂ undergoes a structural phase transition at 14.2 GPa to an orthorhombic CaCl₂-type phase (space group Pnnm), with no phase coexistence during the phase transition. No further phase transition is observed until 20.9 GPa, which is the highest pressure covered by the experiments. The low-pressure and high-pressure phases are related via a group/subgroup relationship. However, a discontinuous change in the unit-cell volume is detected at the phase transition; thus, the phase transition can be classified as a first-order type. Upon decompression, the transition has been found to be reversible. The results are compared with previous high-pressure studies on doped and un-doped SnO₂. The compressibility of different phases will be discussed. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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21 pages, 4754 KiB

Open AccessArticle

Theoretical Study of Pressure-Induced Phase Transitions in Sb₂S₃, Bi₂S₃, and Sb₂Se₃

by Estelina Lora da Silva, Mario C. Santos, Plácida Rodríguez-Hernández, Alfonso Muñoz and Francisco Javier Manjón

Crystals 2023, 13(3), 498; https://doi.org/10.3390/cryst13030498 - 14 Mar 2023

Cited by 6 | Viewed by 2330

Abstract

We report an ab initio study of Sb₂S₃, Sb₂Se₃, and Bi₂S₃ sesquichalcogenides at hydrostatic pressures of up to 60 GPa. We explore the possibility that the C2/m, C2/c, the disordered [...] Read more.

We report an ab initio study of Sb₂S₃, Sb₂Se₃, and Bi₂S₃ sesquichalcogenides at hydrostatic pressures of up to 60 GPa. We explore the possibility that the C2/m, C2/c, the disordered Im-3m, and the I4/mmm phases observed in sesquichalcogenides with heavier cations, viz. Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃, could also be formed in Sb₂S₃, Sb₂Se₃, and Bi₂S₃, as suggested from recent experiments. Our calculations show that the C2/c phase is not energetically favorable in any of the three compounds, up to 60 GPa. The C2/m system is also unfavorable for Sb₂S₃ and Bi₂S₃; however, it is energetically favorable with respect to the Pnma phase of Sb₂Se₃ above 10 GPa. Finally, the I4/mmm and the disordered body-centered cubic-type Im-3m structures are competitive in energy and are energetically more stable than the C2/m phase at pressures beyond 30 GPa. The dynamical stabilities of the Pnma, Im-3m, C2/m, and I4/mmm structural phases at high pressures are discussed for the three compounds. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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15 pages, 3447 KiB

Open AccessArticle

Pressure-Induced Monoclinic to Tetragonal Phase Transition in RTaO₄ (R = Nd, Sm): DFT-Based First Principles Studies

by Saheli Banerjee, Amit Tyagi and Alka B. Garg

Crystals 2023, 13(2), 254; https://doi.org/10.3390/cryst13020254 - 1 Feb 2023

Cited by 3 | Viewed by 1784

Abstract

In this manuscript, we report the density functional theory-based first principles study of the structural and vibrational properties of technologically relevant M′ fergusonite (P2/c)-structured NdTaO₄ and SmTaO₄ under compression. For NdTaO₄ and SmTaO_4, ambient unit [...] Read more.

In this manuscript, we report the density functional theory-based first principles study of the structural and vibrational properties of technologically relevant M′ fergusonite (P2/c)-structured NdTaO₄ and SmTaO₄ under compression. For NdTaO₄ and SmTaO_4, ambient unit cell parameters, along with constituent polyhedral volume and bond lengths, have been compared with earlier reported parameters for EuTaO₄ and GdTaO₄ for a better understanding of the role of lanthanide radii on the primitive unit cell. For both the compounds, our calculations show the presence of first-order monoclinic to tetragonal phase transition accompanied by nearly a 1.3% volume collapse and an increase in oxygen coordination around the tantalum (Ta) cation from ambient six to eight at phase transition. A lower bulk modulus obtained in the high-pressure tetragonal phase when compared to the ambient monoclinic phase is indicative of the more compressible unit cell under pressure. Phonon modes are calculated for the ambient and high-pressure phases with compression for both the compounds along with their pressure coefficients. One particular IR mode has been observed to show red shift in the ambient monoclinic phase, possibly leading to the instability in the compounds under compression. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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11 pages, 9939 KiB

Open AccessArticle

Strain-Rate Dependence of Plasticity and Phase Transition in [001]-Oriented Single-Crystal Iron

by Nourou Amadou, Abdoul Razak Ayouba Abdoulaye, Thibaut De Rességuier and André Dragon

Crystals 2023, 13(2), 250; https://doi.org/10.3390/cryst13020250 - 1 Feb 2023

Cited by 7 | Viewed by 1830

Abstract

Non-equilibrium molecular dynamics simulations have been used to investigate strain-rate dependence of plasticity and phase transition in [001]-oriented single-crystal iron under ramp compression. Here, plasticity is governed by deformation twinning, in which kinetics is tightly correlated with the loading rate. Over the investigated [...] Read more.

Non-equilibrium molecular dynamics simulations have been used to investigate strain-rate dependence of plasticity and phase transition in [001]-oriented single-crystal iron under ramp compression. Here, plasticity is governed by deformation twinning, in which kinetics is tightly correlated with the loading rate. Over the investigated range of strain rates, a hardening-like effect is found to shift the onset of the structural bcc-to-hcp phase transformation to a high, almost constant stress during the ramp compression regime. However, when the ramp evolves into a shock wave, the bcc–hcp transition is triggered whenever the strain rate associated with the plastic deformation reaches some critical value, which depends on the loading rate, leading to a constitutive functional dependence of the transition onset stress on the plastic deformation rate, which is in overall consistence with the experimental data under laser compression. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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12 pages, 4364 KiB

Open AccessArticle

Phase Transformation Pathway of DyPO₄ to 21.5 GPa

by Jai Sharma, Henry Q. Afful and Corinne E. Packard

Crystals 2023, 13(2), 249; https://doi.org/10.3390/cryst13020249 - 1 Feb 2023

Cited by 3 | Viewed by 1989

Abstract

Interest in the deformation behavior and phase transformations of rare earth orthophosphates (REPO₄s) spans several fields of science—from geological impact analysis to ceramic matrix composite engineering. In this study, the phase behavior of polycrystalline, xenotime DyPO₄ is studied up to [...] Read more.

Interest in the deformation behavior and phase transformations of rare earth orthophosphates (REPO₄s) spans several fields of science—from geological impact analysis to ceramic matrix composite engineering. In this study, the phase behavior of polycrystalline, xenotime DyPO₄ is studied up to 21.5(16) GPa at ambient temperature using in situ diamond anvil cell synchrotron X-ray diffraction. This experiment reveals a large xenotime–monazite phase coexistence pressure range of 7.6(15) GPa and evidence for the onset of a post-monazite transformation at 13.9(10) GPa to scheelite. The identification of scheelite as the post-monazite phase of DyPO₄, though not definitive, is consistent with REPO₄ phase transformation pathways reported in both the experimental and the computational literature. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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19 pages, 2882 KiB

Open AccessArticle

Pressure-Induced Structural Phase Transitions in the Chromium Spinel LiInCr₄O₈ with Breathing Pyrochlore Lattice

by Meera Varma, Markus Krottenmüller, H. K. Poswal and C. A. Kuntscher

Crystals 2023, 13(2), 170; https://doi.org/10.3390/cryst13020170 - 18 Jan 2023

Cited by 1 | Viewed by 2033

Abstract

This study reports high-pressure structural and spectroscopic studies on polycrystalline cubic chromium spinel compound LiInCr₄O₈. According to pressure-dependent X-ray diffraction measurements, three structural phase transitions occur at ∼14 GPa, ∼19 GPa, and ∼36 GPa. The first high-pressure phase is [...] Read more.

This study reports high-pressure structural and spectroscopic studies on polycrystalline cubic chromium spinel compound LiInCr₄O₈. According to pressure-dependent X-ray diffraction measurements, three structural phase transitions occur at ∼14 GPa, ∼19 GPa, and ∼36 GPa. The first high-pressure phase is indexed to the low-temperature tetragonal phase of the system which coexists with the ambient phase before transforming to the second high-pressure phase at ∼19 GPa. The pressure-dependent Raman and infrared spectroscopic measurements show a blue-shift of the phonon modes and the crystal field excitations and an increase in the bandgap under compression. During pressure release, the sample reverts to its ambient cubic phase, even after undergoing multiple structural transitions at high pressures. The experimental findings are compared to the results of first principles based structural and phonon calculations. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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17 pages, 4952 KiB

Open AccessArticle

Ab Initio Theoretical Study of DyScO₃ at High Pressure

by Enrique Zanardi, Silvana Radescu, Andrés Mujica, Plácida Rodríguez-Hernández and Alfonso Muñoz

Crystals 2023, 13(2), 165; https://doi.org/10.3390/cryst13020165 - 17 Jan 2023

Cited by 1 | Viewed by 1699

Abstract

DyScO₃ is a member of a family of compounds (the rare-earth scandates) with exceptional properties and prospective applications in key technological areas. In this paper, we study theoretically the behavior of DyScO₃ perovskite under pressures up to about 65 GPa, including [...] Read more.

DyScO₃ is a member of a family of compounds (the rare-earth scandates) with exceptional properties and prospective applications in key technological areas. In this paper, we study theoretically the behavior of DyScO₃ perovskite under pressures up to about 65 GPa, including its structural and vibrational properties (with an analysis of the Raman and infrared activity), elastic response, and stability. We have worked within the ab initio framework of the density functional theory, using projector-augmented wave potentials and a generalized gradient approximation form to the exchange-correlation functional, including dispersive corrections. We compare our results with existing theoretical and experimental published data and extend the range of previous studies. We also propose a candidate high-pressure phase for this material. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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19 pages, 5288 KiB

Open AccessArticle

Structural and Luminescence Properties of Cu(I)X-Quinoxaline under High Pressure (X = Br, I)

by Javier Gonzalez-Platas, Ulises R. Rodriguez-Mendoza, Amagoia Aguirrechu-Comeron, Rita R. Hernandez-Molina, Robin Turnbull, Placida Rodriguez-Hernandez and Alfonso Muñoz

Crystals 2023, 13(1), 100; https://doi.org/10.3390/cryst13010100 - 5 Jan 2023

Cited by 1 | Viewed by 1837

Abstract

A study of high-pressure single-crystal X-ray diffraction and luminescence experiments together with ab initio simulations based on the density functional theory has been performed for two isomorphous copper(I) halide compounds with the empirical formula [C₈H₆Cu₂X₂N [...] Read more.

A study of high-pressure single-crystal X-ray diffraction and luminescence experiments together with ab initio simulations based on the density functional theory has been performed for two isomorphous copper(I) halide compounds with the empirical formula [C₈H₆Cu₂X₂N₂] (X = Br, I) up to 4.62(4) and 7.00(4) GPa for X-ray diffraction and 6.3(4) and 11.6(4) GPa for luminescence, respectively. An exhaustive study of compressibility has been completed by means of determination of the isothermal equations of state and structural changes with pressure at room temperature, giving bulk moduli of K₀ = 14.4(5) GPa and K′₀ = 7.7(6) for the bromide compound and K₀ = 13.0(2) GPa and K′₀ = 7.4(2) for the iodide compound. Both cases exhibited a phase transition of second order around 3.3 GPa that was also detected in luminescence experiments under the same high-pressure conditions, wherein redshifts of the emission bands with increasing pressure were observed due to shortening of the Cu–Cu distances. Additionally, ab initio studies were carried out which confirmed the results obtained experimentally, although unfortunately, the phase transition was not predicted. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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17 pages, 5947 KiB

Open AccessArticle

First-Principle Study of Ca₃Y₂Ge₃O₁₂ Garnet: Dynamical, Elastic Properties and Stability under Pressure

by Alfonso Muñoz and Plácida Rodríguez-Hernández

Crystals 2023, 13(1), 29; https://doi.org/10.3390/cryst13010029 - 24 Dec 2022

Viewed by 1961

Abstract

We present here an ab initio study under the framework of the Density Functional Theory of the Ca₃Y₂Ge₃O₁₂ garnet. Our study focuses on the analysis of the structural, electronic, dynamic, and elastic properties of this material [...] Read more.

We present here an ab initio study under the framework of the Density Functional Theory of the Ca₃Y₂Ge₃O₁₂ garnet. Our study focuses on the analysis of the structural, electronic, dynamic, and elastic properties of this material under hydrostatic pressure. We report information regarding the equation of state, the compressibility, and the structural evolution of this compound. The dynamical properties and the evolution under pressure of infrared, silent, and Raman frequencies and their pressure coefficients are also presented. The dependence on the pressure of the elastic constants and the mechanical and elastic properties are analyzed. From our results, we conclude that this garnet becomes mechanically unstable at 45.7 GPa; moreover, we also find evidence of soft phonons at 34.4 GPa, showing the dynamical instability of this compound above this pressure. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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10 pages, 2532 KiB

Open AccessArticle

Phase Relations of Ni₂In-Type and CaC₂-Type Structures Relative to Fe₂P-Type Structure of Titania at High Pressure: A Comparative Study

by Khaldoun Tarawneh and Yahya Al-Khatatbeh

Crystals 2023, 13(1), 9; https://doi.org/10.3390/cryst13010009 - 21 Dec 2022

Viewed by 1722

Abstract

Density functional theory (DFT) based on first-principles calculations was used to study the high-pressure phase stability of various phases of titanium dioxide (TiO₂) at extreme pressures. We explored the phase relations among the following phases: the experimentally identified nine-fold hexagonal Fe [...] Read more.

Density functional theory (DFT) based on first-principles calculations was used to study the high-pressure phase stability of various phases of titanium dioxide (TiO₂) at extreme pressures. We explored the phase relations among the following phases: the experimentally identified nine-fold hexagonal Fe₂P-type phase, the previously predicted ten-fold tetragonal CaC₂-type phase of TiO₂, and the recently proposed eleven-fold hexagonal Ni₂In-type phase of the similar dioxides zirconia (ZrO₂) and hafnia (HfO₂). Our calculations, using the generalized gradient approximation (GGA), predicted the Fe₂P → Ni₂In transition to occur at 564 GPa and Fe₂P → CaC₂ at 664 GPa. These transitions were deeply investigated with reference to the volume reduction, coordination number decrease, and band gap narrowing to better determine the favorable post-Fe₂P phase. Furthermore, it was found that both transitions are mostly driven by the volume reduction across transitions in comparison with the small contribution of the electronic energy gain. Additionally, our computed Birch–Murnaghan equation of state for the three phases reveals that CaC₂ is the densest phase, while Ni₂In is the most compressible phase. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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13 pages, 1467 KiB

Open AccessArticle

Stability of FeVO₄-II under Pressure: A First-Principles Study

by Pricila Betbirai Romero-Vázquez, Sinhué López-Moreno and Daniel Errandonea

Crystals 2022, 12(12), 1835; https://doi.org/10.3390/cryst12121835 - 15 Dec 2022

Cited by 5 | Viewed by 2749

Abstract

In this work, we report first-principles calculations to study FeVO

_{4}

in the CrVO

_{4}

-type (phase II) structure under pressure. Total-energy calculations were performed in order to analyze the structural parameters, the electronic, elastic, mechanical, and vibrational properties of FeVO

_{4}

[...] Read more.

In this work, we report first-principles calculations to study FeVO

_{4}

in the CrVO

_{4}

-type (phase II) structure under pressure. Total-energy calculations were performed in order to analyze the structural parameters, the electronic, elastic, mechanical, and vibrational properties of FeVO

_{4}

-II up to 9.6 GPa for the first time. We found a good agreement in the structural parameters with the experimental results available in the literature. The electronic structure analysis was complemented with results obtained from the Laplacian of the charge density at the bond critical points within the Quantum Theory of Atoms in Molecules methodology. Our findings from the elastic, mechanic, and vibrational properties were correlated to determine the elastic and dynamic stability of FeVO

_{4}

-II under pressure. Calculations suggest that beyond the maximum pressure covered by our study, this phase could undergo a phase transition to a wolframite-type structure, such as in CrVO

_{4}

and InVO

_{4}

. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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13 pages, 1484 KiB

Open AccessArticle

Corresponding States for Volumes of Elemental Solids at Their Pressures of Polymorphic Transformations

by Oliver Tschauner

Crystals 2022, 12(12), 1698; https://doi.org/10.3390/cryst12121698 - 23 Nov 2022

Cited by 1 | Viewed by 1430

Abstract

Many non-molecular elemental solids exhibit common features in their structures over the range of 0 to 0.5 TPa that have been correlated with equivalent valence electron configurations. Here, it is shown that the pressures and volumes at polymorphic transitions obey corresponding states given [...] Read more.

Many non-molecular elemental solids exhibit common features in their structures over the range of 0 to 0.5 TPa that have been correlated with equivalent valence electron configurations. Here, it is shown that the pressures and volumes at polymorphic transitions obey corresponding states given by a single, empirical universal step-function V_tr/L = −0.0208(3) · P_tr + N_i, where V_tr is the atomic volume in Å³ at a given transformation pressure P_tr in GPa, and L is the principal quantum number. N_i assumes discrete values of approximately 20, 30, 40, etc. times the cube of the Bohr radius, thus separating all 113 examined polymorphic elements into five discrete sets. The separation into these sets is not along L. Instead, strongly contractive polymorphic transformations of a given elemental solid involve changes to different sets. The rule of corresponding states allows for predicting atomic volumes of elemental polymorphs of hitherto unknown structures and the transitions from molecular into non-molecular phases such as for hydrogen. Though not an equation of state, this relation establishes a basic principle ruling over a vast range of simple and complex solid structures that confirms that effective single-electron-based calculations are good approximations for these materials and pressures The relation between transformation pressures and volumes paves the way to a quantitative assessment of the state of very dense matter intermediate between the terrestrial pressure regime and stellar matter. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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14 pages, 932 KiB

Open AccessArticle

Effects of Physical and Chemical Pressure on Charge Density Wave Transitions in LaAg_1−xAu_xSb₂ Single Crystals

by Li Xiang, Dominic H. Ryan, Paul C. Canfield and Sergey L. Bud’ko

Crystals 2022, 12(12), 1693; https://doi.org/10.3390/cryst12121693 - 23 Nov 2022

Cited by 1 | Viewed by 1562

Abstract

The structural characterization and electrical transport measurements at ambient and applied pressures of the compounds of the LaAg

_{1 - x}

Au

_{x}

Sb

_{2}

family are presented. Up to two charge density wave (CDW) transitions could be detected upon cooling from room [...] Read more.

The structural characterization and electrical transport measurements at ambient and applied pressures of the compounds of the LaAg

_{1 - x}

Au

_{x}

Sb

_{2}

family are presented. Up to two charge density wave (CDW) transitions could be detected upon cooling from room temperature and an equivalence of the effects of chemical and physical pressure on the CDW ordering temperatures was observed with the unit cell volume being a salient structural parameter. As such LaAg

_{1 - x}

Au

_{x}

Sb

_{2}

is a rare example of a non-cubic system that exhibits good agreement between the effects of applied, physical, pressure and changes in unit cell volume from steric changes induced by isovalent substitution. Additionally, for LaAg

_{0.54}

Au

_{0.46}

Sb

_{2}

anomalies in low temperature electrical transport were observed in the pressure range where the lower charge density wave is completely suppressed. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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11 pages, 3876 KiB

Open AccessArticle

Understanding the Semiconducting-to-Metallic Transition in the CF₂Si Monolayer under Shear Tensile Strain

by Tarik Ouahrani and Reda M. Boufatah

Crystals 2022, 12(10), 1476; https://doi.org/10.3390/cryst12101476 - 18 Oct 2022

Cited by 1 | Viewed by 1478

Abstract

With the ever-increasing interest in low-dimensional materials, it is urgent to understand the effect of strain on these kinds of structures. In this study, taking the CF

_{2}

Si monolayer as an example, a computational study was carried out to investigate the effect [...] Read more.

With the ever-increasing interest in low-dimensional materials, it is urgent to understand the effect of strain on these kinds of structures. In this study, taking the CF

_{2}

Si monolayer as an example, a computational study was carried out to investigate the effect of tensile shear strain on this compound. The structure was dynamically and thermodynamically stable under ambient conditions. By applying tensile shear, the structure showed a strain-driven transition from a semiconducting to a metallic behavior. This electronic transition’s nature was studied by means of the electron localization function index and an analysis of the noncovalent interactions. The result showed that the elongation of covalent bonds was not responsible for this metallization but rather noncovalent interactions governing the nonbonded bonds of the structure. This strain-tuned behavior might be capable of developing new devices with multiple properties involving the change in the nature of chemical bonding in low-dimensional structures. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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12 pages, 2499 KiB

Open AccessArticle

Chinese Colorless HPHT Synthetic Diamond Inclusion Features and Identification

by Ying Ma, Zhili Qiu, Xiaoqin Deng, Ting Ding, Huihuang Li, Taijin Lu, Zhonghua Song, Wenfang Zhu and Jinlin Wu

Crystals 2022, 12(9), 1266; https://doi.org/10.3390/cryst12091266 - 6 Sep 2022

Cited by 2 | Viewed by 3534

Abstract

Chinese HPHT diamonds have improved dramatically in recent years. However, this brings a challenge in identifying type IIa colorless diamonds. In this study, eleven HPHT and three natural, colorless, gem-quality IIa diamonds were analyzed using magnified observation, Raman, PL and chemical element analysis. [...] Read more.

Chinese HPHT diamonds have improved dramatically in recent years. However, this brings a challenge in identifying type IIa colorless diamonds. In this study, eleven HPHT and three natural, colorless, gem-quality IIa diamonds were analyzed using magnified observation, Raman, PL and chemical element analysis. The results show that only HPHT samples possessed kite-like inclusions and lichenoid inclusions, as verified by their complex Raman spectra (100–750 cm⁻¹). Through PL mapping, HPHT and natural IIa diamonds were distinguished by their growth environments, which were reflected by PL peaks at 503, 505, 575, 637, 693, 694 and 737 nm. The chemical components of HPHT IIa diamond carbide inclusions are mainly Fe, Co, Ni and Mn, but those of Natural IIa are mainly Fe and Ni. As a result, the chemical components can be used to distinguish a natural colorless IIa diamond from a synthetic diamond. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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16 pages, 1467 KiB

Open AccessArticle

On the Definition of Phase Diagram

by Kamil Filip Dziubek

Crystals 2022, 12(9), 1186; https://doi.org/10.3390/cryst12091186 - 23 Aug 2022

Cited by 2 | Viewed by 3383

Abstract

A phase diagram, which is understood as a graphical representation of the physical states of materials under varied temperature and pressure conditions, is one of the basic concepts employed in high-pressure research. Its general definition refers to the equilibrium state and stability limits [...] Read more.

A phase diagram, which is understood as a graphical representation of the physical states of materials under varied temperature and pressure conditions, is one of the basic concepts employed in high-pressure research. Its general definition refers to the equilibrium state and stability limits of particular phases, which set the stage for its terms of use. In the literature, however, a phase diagram often appears as an umbrella category for any pressure–temperature chart that presents not only equilibrium phases, but also metastable states. The current situation is confusing and may lead to severe misunderstandings. This opinion paper reviews the use of the “phase diagram” term in many aspects of scientific research and suggests some further clarifications. Moreover, this article can serve as a starting point for a discussion on the refined definition of the phase diagram, which is required in view of the paradigm shift driven by recent results obtained using emerging experimental techniques. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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10 pages, 2065 KiB

Open AccessArticle

Computational Investigation of the Stability of Di-p-Tolyl Disulfide “Hidden” and “Conventional” Polymorphs at High Pressures

by Valeriya Yu. Smirnova, Anna A. Iurchenkova and Denis A. Rychkov

Crystals 2022, 12(8), 1157; https://doi.org/10.3390/cryst12081157 - 17 Aug 2022

Cited by 3 | Viewed by 2069

Abstract

The investigation of molecular crystals at high pressure is a sought-after trend in crystallography, pharmaceutics, solid state chemistry, and materials sciences. The di-p-tolyl disulfide (CH₃−C₆H₄−S−)₂ system is a bright example of high-pressure polymorphism. It [...] Read more.

The investigation of molecular crystals at high pressure is a sought-after trend in crystallography, pharmaceutics, solid state chemistry, and materials sciences. The di-p-tolyl disulfide (CH₃−C₆H₄−S−)₂ system is a bright example of high-pressure polymorphism. It contains “conventional” solid–solid transition and a “hidden” form which may be obtained only from solution at elevated pressure. In this work, we apply force field and periodic DFT computational techniques to evaluate the thermodynamic stability of three di-p-tolyl disulfide polymorphs as a function of pressure. Theoretical pressures and driving forces for polymorphic transitions are defined, showing that the compressibility of the γ phase is the key point for higher stability at elevated pressures. Transition state energies are also estimated for α → β and α → γ transitions from thermodynamic characteristics of crystal structures, not exceeding 5 kJ/mol. The β → γ transition does not occur experimentally in the 0.0–2.8 GPa pressure range because transition state energy is greater than 18 kJ/mol. Relations between free Gibbs energy (in assumption of enthalpy) of phases α, β, and γ, as a function of pressure, are suggested to supplement and refine experimental data. A brief discussion of the computational techniques used for high-pressure phase transitions is provided. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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12 pages, 3554 KiB

Open AccessEditor’s ChoiceArticle

An Investigation of the Pressure-Induced Structural Phase Transition of Nanocrystalline α-CuMoO₄

by Vinod Panchal, Catalin Popescu and Daniel Errandonea

Crystals 2022, 12(3), 365; https://doi.org/10.3390/cryst12030365 - 9 Mar 2022

Cited by 2 | Viewed by 2861

Abstract

The structural behavior of nanocrystalline α-CuMoO₄ was studied at ambient temperature up to 2 GPa using in situ synchrotron X-ray powder diffraction. We found that nanocrystalline α-CuMoO₄ undergoes a structural phase transition into γ-CuMoO₄ at 0.5 GPa. The structural sequence [...] Read more.

The structural behavior of nanocrystalline α-CuMoO₄ was studied at ambient temperature up to 2 GPa using in situ synchrotron X-ray powder diffraction. We found that nanocrystalline α-CuMoO₄ undergoes a structural phase transition into γ-CuMoO₄ at 0.5 GPa. The structural sequence is analogous to the behavior of its bulk counterpart, but the transition pressure is doubled. A coexistence of both phases was observed till 1.2 GPa. The phase transition gives rise to a change in the copper coordination from square-pyramidal to octahedral coordination. The transition involves a volume reduction of 13% indicating a first-order nature of the phase transition. This transformation was observed to be irreversible in nature. The pressure dependence of the unit-cell parameters was obtained and is discussed, and the compressibility analyzed. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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Review

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19 pages, 4623 KiB

Open AccessEditor’s ChoiceReview

Recent Progress in Phase Stability and Elastic Anomalies of Group VB Transition Metals

by Yixian Wang, Hao Wu, Yingying Liu, Hao Wang, Xiangrong Chen and Huayun Geng

Crystals 2022, 12(12), 1762; https://doi.org/10.3390/cryst12121762 - 5 Dec 2022

Cited by 4 | Viewed by 2381

Abstract

Recently discovered phase transition and elastic anomaly of compression-induced softening and heating-induced hardening (CISHIH) in group VB transition metals at high-pressure and high-temperature (HPHT) conditions are unique and interesting among typical metals. This article reviews recent progress in the understanding of the structural [...] Read more.

Recently discovered phase transition and elastic anomaly of compression-induced softening and heating-induced hardening (CISHIH) in group VB transition metals at high-pressure and high-temperature (HPHT) conditions are unique and interesting among typical metals. This article reviews recent progress in the understanding of the structural and elastic properties of these important metals under HPHT conditions. Previous investigations unveiled the close connection of the remarkable structural stability and elastic anomalies to the Fermi surface nesting (FSN), Jahn–Teller effect, and electronic topological transition (ETT) in vanadium, niobium, and tantalum. We elaborate that two competing scenarios are emerging from these advancements. The first one focuses on phase transition and phase diagram, in which a soft-mode driven structural transformation of BCC→RH1→RH2→BCC under compression and an RH→BCC reverse transition under heating in vanadium were established by experiments and theories. Similar phase transitions in niobium and tantalum were also proposed. The concomitant elastic anomalies were considered to be due to the phase transition. However, we also showed that there exist some experimental and theoretical facts that are incompatible with this scenario. A second scenario is required to accomplish a physically consistent interpretation. In this alternative scenario, the electronic structure and associated elastic anomaly are fundamental, whereas phase transition is just an outcome of the mechanical instability. We note that this second scenario is promising to reconcile all known discrepancies but caution that the phase transition in group VB metals is elusive and is still an open question. A general consensus on the relationship between the possible phase transitions and the mechanical elasticity (especially the resultant CISHIH dual anomaly, which has a much wider impact), is still unreached. Full article

(This article belongs to the Special Issue Pressure-Induced Phase Transformations (Volume II))

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