Pressure-induced Phase Transition Sequence Research Articles

Size-Dependent High-Pressure Behavior of Pure and Eu3+-Doped Y2O3 Nanoparticles: Insights from Experimental and Theoretical Investigations.

We report a joint high-pressure experimental and theoretical study of the structural, vibrational, and photoluminescent properties of pure and Eu3+-doped cubic Y2O3 nanoparticles with two very different average particle sizes. We compare the results of synchrotron X-ray diffraction, Raman scattering, and photoluminescence measurements in nanoparticles with ab initio density-functional simulations in bulk material with the aim to understand the influence of the average particle size on the properties of pure and doped Y2O3 nanoparticles under compression. We observe that the high-pressure phase behavior of Y2O3 nanoparticles depends on the average particle size, but in a different way to that previously reported. Nanoparticles with an average particle size of ~37 nm show the same pressure-induced phase transition sequence on upstroke and downstroke as the bulk sample; however, nanoparticles with an average particle size of ~6 nm undergo an irreversible pressure-induced amorphization above 16 GPa that is completed above 24 GPa. On downstroke, 6 nm nanoparticles likely consist of an amorphous phase.

Pressure-induced successive phase transitions and Fano resonance engineering in lead-free piezoceramics KNbO3

We carried out detailed high-pressure Raman scattering studies on lead-free perovskite piezoceramics KNbO3 (KNO) with a smaller pressure interval so that the successive phase transition pressures are precisely determined (PO→T: 8.2 GPa; PT→C: 10.9 GPa). The pressure-quenched phase does not return to the ambient orthorhombic phase completely. In this work, the pressure-induced phase transition sequence is discussed in detail. Also, based on the pressure-dependent Raman spectra of KNO, we have discovered clear evidence of a pressure-induced Fano resonance. It reveals the role played by the Raman-active continuum and discrete energy states in the condensed matter system under high pressure and enables the high-pressure Fano resonances engineering in arbitrary geometries [q∈(−∞, 0)]. These findings highlight the opportunity for designing and tuning the wave transmission and optical switch properties of lead-free piezoceramics KNO via high pressure.

A cotunnite-type new high-pressure phase of Fe2S

Abstract We examined pressure-induced phase transitions in Fe2S based on high-pressure and high-temperature X-ray diffraction measurements in a laser-heated diamond-anvil cell. Fe2S is not stable at ambient pressure but is known to form above 21 GPa with the Fe2P-type (C22) structure. Our experiments demonstrate a novel phase transition in Fe2S from the C22 to C23 phase with the Co2P-type cotunnite structure above ~30 GPa. The experiments also reveal a transformation from the C23 to C37 (Co2Si-type) phase above ~130 GPa. While the C23 and C37 structures exhibit the same crystal-lographic symmetry (orthorhombic Pnma), the coordination number of sulfur increases from nine in C23 to ten in C37. Such a sequence of pressure-induced phase transitions in Fe2S, C22 → C23 → C37, are similar to those of Fe2P, while they are not known in oxides and halogens that often adopt the C23 cotunnite-type structure. The newly found cotunnite-type Fe2S phase could be present in solid iron cores of planets, including Mars.

Pressure-Induced Synthesis and Properties of an H2S-H2Se-H2 Molecular Alloy.

The chalcogens are known to react with one another to form interchalcogens, which exhibit a diverse range of bonding and conductive behavior due to the difference in electronegativity between the group members. Through a series of high-pressure diamond anvil experiments combined with density functional theory calculations, we report the synthesis of an S-Se hydride. At pressures above 4 GPa we observe the formation of a single solid composed of both H2Se and H2S molecular units. Further compression in a hydrogen medium leads to the formation of an alloyed compound (H2SxSe1-x)2H2, after which there is a sequence of pressure-induced phase transitions associated with the arrested rotation of molecules. At pressures above 50 GPa, there is a symmetrization of hydrogen bonds concomitantly with a closing band gap and increased reflectivity of the compound, indicative of a transition to a metallic state.

Fluorides of Silver Under Large Compression*.

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3 F4 and Ag2 F3 ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag2 F5 , containing both Ag2+ and Ag3+ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

Evidence for Orientational Order in Nanothreads Derived from Thiophene.

Nanothreads are one-dimensional sp3 hydrocarbons that pack within pseudohexagonal crystalline lattices. They are believed to lack long-range order along the thread axis and also lack interthread registry. Here we investigate the phase behavior of thiophene up to 35 GPa and establish a pressure-induced phase transition sequence that mirrors previous observations in low-temperature studies. Slow compression to 35 GPa results in the formation of a recoverable saturated product with a 2D monoclinic diffraction pattern along (0001) that agrees closely with atomistic simulations for single crystals of thiophene-derived nanothreads. Paradoxically, this lower-symmetry packing signals a higher degree of structural order since it must arise from constituents with a consistent azimuthal orientation about their shared axis. The simplicity of thiophene reaction pathways (with only four carbon atoms per ring) apparently yields the first nanothreads with orientational order, a striking outcome considering that a single point defect in a 1D system can disrupt long-range structural order.

High pressure structural stability of ThN: ab-initio study

The high pressure structural stability of thorium nitride (ThN) has been examined through first principle total energy calculations. The structural sequence predicted through present theoretical simulation is NaCl type structure (B1) to tetragonal phase (B10) to CsCl type (B2) phase with corresponding transition pressures ∼75 GPa and 120 GPa, respectively. The B1 → B10 transition with volume discontinuity of ∼10% is found to be of first order type, whereas, the B10 → B2 displays second order nature. The predicted high pressure structural sequence in ThN is different from those reported in earlier works [Gupta and Singh, Sol. State Sci., 2010 and Modak and Verma, Phys. Rev. B 2011]. The pressure induced phase transition sequence so obtained is further supported by examining the mechanical and phonon stability of the predicted structures in their structural stability regime. Also, the activation barrier for B1 to B10 transition determined from the calculated Bain path at the transition pressure turns out to be ∼0.75eV/f.u.(72.36 kJ/mol).

Experimental and Theoretical Studies on α-In2Se3 at High Pressure.

α(R)-In2Se3 has been experimentally and theoretically studied under compression at room temperature by means of X-ray diffraction and Raman scattering measurements as well as by ab initio total-energy and lattice-dynamics calculations. Our study has confirmed the α ( R3 m) → β' ( C2/ m) → β ( R3̅ m) sequence of pressure-induced phase transitions and has allowed us to understand the mechanism of the monoclinic C2/ m to rhombohedral R3̅ m phase transition. The monoclinic C2/ m phase enhances its symmetry gradually until a complete transformation to the rhombohedral R3̅ m structure is attained above 10-12 GPa. The second-order character of this transition is the reason for the discordance in previous measurements. The comparison of Raman measurements and lattice-dynamics calculations has allowed us to tentatively assign most of the Raman-active modes of the three phases. The comparison of experimental results and simulations has helped to distinguish between the different phases of In2Se3 and resolve current controversies.

Structural, magnetic and electronic properties of CrO2 at multimegabar pressures.

As the only half-metallic ferromagnetic material in 3d transition metal dioxides, CrO2 has attracted great scientific interest from materials science to physical chemistry. Here, an investigation into the structural, magnetic and electronic properties of CrO2 under high pressure has been conducted by first-principles calculations based on density functional theory. Static calculations have predicted that CrO2 undergoes structural transitions with the sequence of rutile-type → CaCl2-type → pyrite-type → Pnma → (Fe2P-type→) I4/mmm at high pressures. In addition, a transition from the ferromagnetic state to the non-magnetic state with the magnetic collapse of Cr is observed in CrO2 at the pyrite–Pnma transition. This transition also delocalizes the 3d electrons of Cr and leads to a metallic character of CrO2. The equation of state, elasticity and band gap for each energetically favorable phase of CrO2 are determined. Our results not only bridge the gap about the high-pressure behavior of CrO2 in previous studies but also extend our understanding of its properties up to multimegabar conditions. According to previous data and present results, we further discuss and summarize the high-pressure behavior of various AO2 compounds. This can contribute to investigating properties of other AO2 compounds or exploring novel materials at high pressures.

Volume and bond length dependences of the electronic structure of 6-fold and 8-fold coordinated Co2+ in pressure transformed CoF2

This work reports an optical absorption study on CoF2 under high-pressure conditions in the 0-80 GPa range. A recent structural study1 performed on the pressure-induced phase-transition sequence of CoF2 reveals that this compound undergoes a structural transformation associated with a change of Co2+ coordination from 6 (rutile type) to 8 (fluorite type) at 15 GPa, what implies a profound change in the Co2+ electronic ground state associated with the inversion of d orbitals from (t2g5eg2) to (eg4t2g3). This allows us to explore the Co2+ electronic structure, and hence 10Dq, as a function of the density and the Co-F bond length in both CoF6 (rutile phase) and CoF8 (fluorite phase) coordinations. The results are compared with those obtained in KCoF3 (perovskite) in a wide range of Co-F distances (from 2.04 to 1.83 Å).

The ground structure and the pressure-induced structural and electronic phase transitions for for rare-earth trihydride GdH3

The ground structure and the pressure-induced structural and electronic phase transitions for rare-earth trihydride GdH3 are investigated extensively using first-principles calculations where f electrons are considered. The P3¯c1, P63cm andP63 structure models are taken as the possible ground state structure. With a higher plane-wave basis-800 eV cutoff energy, the optimized structural parameters under CASTEP code are very close to the experimental data; furthermore, it is found that both in the ferromagnetic state and the antiferromagenic state, P63cm structure has the lowest energy, followed, in this order, by theP63 and P3¯c1 structures; but under VASP code, only liking YH3, P63 structure has the lowest energy, followed, in this order, by theP63cm and P3¯c1 structures. The standard DFT band structure calculations also show that the band structure of GdH3 is similar to that of YH3, and P63 structure has the largest band gap among the structures, GGA+U calculations only can improve the band gap of GdH3 very limitedly. So the main factors that affect the opening of the band structure of GdH3 should still be algorithms. On the other hand, under compression, GdH3 follows the pressure-induced phase transition sequence AFM P3¯c1→AFMC2/m→FM C2/m→FMfcc→FMhcp→FMCmcm. In conjunction with the phase transition sequence, the pure DFT-GGA calculations of electronic structures demonstrate that the critical pressure that makes the band gap of GdH3 close is close to the transition pressure from an intermediate phase (C2/m phase) to cubic phase. In contrast to the case of YH3, it seems that the presentation of 4f electrons doesn't make rare-earth trihydride arise any abnormality in the electronic properties and the pressure-induced phase transitions with CASTEP code.

Electronic Topological Transition in Ag2Te at High-pressure.

Recently, Ag2Te was experimentally confirmed to be a 3D topological insulator (TI) at ambient pressure. However, the high-pressure behaviors and properties of Ag2Te were rarely reported. Here, a pressure-induced electronic topological transition (ETT) is firstly found in Ag2Te at 1.8 GPa. Before ETT, the positive pressure coefficient of bulk band-gap, which is firstly found in TIs family, is found by both first-principle calculations and in situ high-pressure resistivity measurements. The electrical resistivity obtained at room temperature shows a maximum at 1.8 GPa, which is nearly 3.3 times to that at ambient pressure. This result indicates that the best bulk insulating character and topological nature in Ag2Te can be obtained at this pressure. Furthermore, the high-pressure structural behavior of Ag2Te has been investigated by in situ high-pressure synchrotron powder X-ray diffraction technique up to 33.0 GPa. The accurate pressure-induced phase transition sequence is firstly determined as P21/c → Cmca → Pnma. It is worth noting that the reported isostructural P21/c phase is not existed, and the reported structure of Cmca phase is corrected by CALYPSO methodology. The second high-pressure structure, a long puzzle to previous reports, is determined as Pnma phase. A pressure-induced metallization in Ag2Te is confirmed by the results of temperature-dependent resistivity measurements.

High pressure phase transitions for CdSe

The structure and pressure-induced phase transitions for CdSe are investigated using first-principles calculations. The pressure-induced phase transition sequence WZ/ZB → Rs → Cmcm → CsCl for CdSe is drawn reasonably for the fist time, the corresponding transition pressures are 3.8, 29 and 107 GPa, respectively and the intermediate states between the Cmcm structure and the CsCl structure should exist.

Pressure-induced phase-transition sequence in CoF2: An experimental and first-principles study on the crystal, vibrational, and electronic properties

We report a complete structural study of CoF${}_{2}$ under pressure. Its crystal structure and vibrational and electronic properties have been studied both theoretically and experimentally using first-principles density functional theory (DFT) methods, x-ray diffraction, x-ray absorption at Co $K$-edge experiments, Raman spectroscopy, and optical absorption in the 0--80 GPa range. We have determined the structural phase-transition sequence in CoF${}_{2}$ and corresponding transition pressures. The results are similar to other transition-metal difluorides such as FeF${}_{2}$ but different to ZnF${}_{2}$ and MgF${}_{2}$, despite that the Co${}^{2+}$ size (ionic radius) is similar to Zn${}^{2+}$ and Mg${}^{2+}$. We found that the complete phase-transition sequence is tetragonal rutile ($P{4}_{2}/mnm$) $\ensuremath{\rightarrow}$ CaCl${}_{2}$ type (orthorhombic $Pnnm$) $\ensuremath{\rightarrow}$ distorted PdF${}_{2}$ (orthorhombic $Pbca$)+PdF${}_{2}$ (cubic $Pa\overline{3}$) in coexistence $\ensuremath{\rightarrow}$ fluorite (cubic $Fm\overline{3}m$) $\ensuremath{\rightarrow}$ cotunnite (orthorhombic $Pnma$). It was observed that the structural phase transition to the fluorite at 15 GPa involves a drastic change of coordination from sixfold octahedral to eightfold cubic with important modifications in the vibrational and electronic properties. We show that the stabilization of this high-pressure cubic phase is possible under nonhydrostatic conditions since ideal hydrostaticity would stabilize the distorted-fluorite structure (tetragonal $I4/mmm$) instead. Although the first rutile $\ensuremath{\rightarrow}$ CaCl${}_{2}$-type second-order phase transition is subtle by Raman spectroscopy, it was possible to define it through the broadening of the ${E}_{g}$ Raman mode which is split in the CaCl${}_{2}$-type phase. First-principles DFT calculations are in fair agreement with the experimental Raman mode frequencies, thus providing an accurate description for all vibrational modes and elastic properties of CoF${}_{2}$ as a function of pressure.

Chemically induced renormalization phenomena in Pb-based relaxor ferroelectrics under high pressure

The pressure-induced phase transition sequence in PbSc0.5Ta0.5O3 (PST) and PbSc0.5Nb0.5O3 (PSN) heavily doped with homo- and heterovalent cations on the A- or B-site of the perovskite-type structure (ABO3) was analysed by in situ synchrotron x-ray diffraction and Raman spectroscopy up to pressures of 25 GPa. We focused on the structural phenomena occurring above the first pressure-induced phase transition at pc1 from a relaxor state to a non-polar rhombohedral phase with antiphase tilting of the BO6 octahedra. The samples studied were PST doped with Nb5+ and Sn4+ on the B-site, PST doped with Ba2+ and La3+ on the A-site and PSN doped with Sr2+ and La3+ on the A-site. All of them exhibit a second pressure-induced phase transition at pc2, similar to pure PST and PSN. The second transition involves the development of either order of antiparallel Pb2+ displacements and complementary a+b−b− octahedral tilts, or a−b−b− (0 ≤ a < b) tilting alone. As in pure PST and PSN, the second phase transition is preceded by the occurrence of unequal octahedral tilts on the local scale. The substitution of Nb5+ for Ta5+ as well as the coupled substitution of Sn4+ for Sc3+ + Ta5+ on the octahedral B sites increases the second critical pressure. The doping by Nb5+ also reduces the length of coherence of antipolar Pb2+ order developed at pc2. The isovalent substitution of the larger Ba2+ for Pb2+ on the A-site suppresses the antipolar Pb2+ order due to the induced local elastic stresses and thus significantly increases pc2. The substitution of smaller cations for Pb2+ on the A-site generally favours the development of long-range order of antiparallel Pb2+ displacements because of the chemically enhanced a−a−a− octahedral tilts. However, this ordering is less when the dopant is aliovalent, due to the charge imbalance on the A-site. For all of the relaxors studied here, the dynamic compressibility estimated from the pressure derivative of the wavenumber of the soft mode associated with the first phase transition is larger in the pressure interval between pc1 and pc2 than above pc2. The dynamic compressibility of the phase above pc2 decreases if the antipolar Pb2+ order is disturbed.

First-principles study on thermodynamic properties and phase transitions inTiS2

Structural and vibrational properties ofTiS2 withthe CdI2 structure have been studied to high pressures from density functional calculations with thelocal density approximation (LDA). The calculated axial compressibility of theCdI2-type phase agrees well with experimental data and is typical of layered transition-metaldichalcogenides. The obtained phonon dispersions show a good correspondence withavailable experiments. A phonon anomaly is revealed at 0 GPa, but is much reduced at20 GPa. The thermodynamic properties of this phase were also calculated at highpressures and high temperatures using the quasi-harmonic approximation. OurLDA study on the pressure-induced phase transition sequence predicts that theCdI2-typeTiS2, the phasestable at ambient conditions, should transform to the cotunnite phase at 15.1 GPa, then to a tetragonalphase (I4/mmm) at 45.0 GPa. The tetragonal phase remains stable to at least 500 GPa. The existence of thetetragonal phase at high pressures is consistent with our previous findings inNiS2 (Yu and Ross 2010 J. Phys.: Condens. Matter 22 235401). The cotunnite phase, althoughonly stable in a narrow pressure range between 15.1 and 45.0 GPa, displays the formation ofa compact S network between 100 and 200 GPa, which is evidenced by a kink in thevariation of unit cell lengths with pressure. The electron density analysis in cotunniteshows that valence electrons are delocalized from Ti atoms and concentrated near the Snetwork.

Prediction of high-pressure polymorphism inNiS2 at megabar pressures

A sequence of pressure-induced phase transitions of vaesite,NiS2, with pyrite structure, has been established from static LDA calculations. A dozenAX2 candidate structures have been studied at high pressures including cotunnite (α-PbCl2), which is commonlyobserved in other AX2 compounds at high pressures. At 150 GPa, vaesite transforms to a tetragonal phase (P42/n) rather than cotunnite. This tetragonal structure is characterized by layers of Ni atoms ineight-fold coordination with S atoms rather than the nine-fold coordination observed in cotunnite.With further compression to about 7.5 Mbar, the tetragonal phase transforms into a hexagonalAlB2-typestructure (P6/mmm) which is characterized by planar hexagonal layers of S intercalated by Ni atomswhere each Ni atom is 12-fold coordinated by S atoms. Calculated band structuresand valence electron density maps show S–S and Ni–S bonded interactions forNiS2 under these extremely compressed conditions. The tetragonal phase may have geophysicalimplications if present in the Earth’s core.

High-pressure study of the Sr 2CoWO 6 ordered double perovskite tungstate oxide

Using synchrotron radiation and Raman spectroscopy and a diamond anvil cell we measured the pressure dependence of the lattice parameters and Raman modes of polycrystalline Sr 2CoWO 6. Angle-dispersive X-ray diffraction patterns were indexed and showed that at 2.2 GPa, the material transforms from the I4/ m tetragonal structure to the P2 1/ n monoclinic structure. For pressure values between 2.2 and 12.7 GPa only monoclinic symmetries were found. We had difficulties with the convergence of the profile matching of the diffractogram taken at 12.7 GPa, indicating that the second phase-transition took place. To get more informations about these structural changes, an in-situ Raman spectroscopic study was conducted to explore the pressure-induced phase-transition sequence of Sr 2CoWO 6 to pressures of 40.8 GPa at room-temperature. Group theory yields nine Raman-active modes for Sr 2CoWO 6 ( I4/ m), all the predicted bands are observed at ambient conditions (phase I). The experimental results indicate that structural changes are observed at 2.15 and 11.15 GPa, which we attribute to phase transitions; thus, giving rise to two new phases, named as phase II and phase III, respectively. In the 9.33–14.84 GPa interval a coexistence of phases II and III is observed.

High-pressure studies of NaCo(H 2PO 3) 3·H 2O phosphite by Raman spectroscopy

High-pressure investigations of NaCo(H 2PO 3) 3·H 2O dihydrogenphosphite, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 69A g + 69B 1g + 69B 2g + 69B 3g Raman active modes for NaCo(H 2PO 3) 3·H 2O, of which only 36 bands are observed at ambient conditions. The pressure-induced phase transition sequence of NaCo(H 2PO 3) 3·H 2O dihydrogenphosphite to pressures up to 20 GPa was explored. Raman spectra showed that the investigated compound, NaCo(H 2PO 3) 3·H 2O, compressed smoothly up to 4.2 GPa and starts amorphizing at 6.8 GPa. The amorphous phase persists down to 1 atm upon release of pressure. The amorphization was due to a chemical decomposition and also might be due to pressure-induced distortions of the lattice upon compression associated with the Na + and Co 2+ cation displacements and strongly coupled with rigid rotations of the (PO 3) units. No P–H vibrations were seen after pressure release which means that P–H bonds were broken. The frequencies seen in the Raman spectra for the amorphous phase are characteristic of (PO 4) 3− or P 2 O 7 4 − tetrahedral short-range structure.

High-pressure studies of SrNi 3(P 2O 7) 2 pyrophosphate by Raman spectroscopy and X-ray diffraction

High-pressure investigations of SrNi 3(P 2O 7) 2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy and X-ray synchrotron radiation source. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30A g+30B g for SrNi 3(P 2O 7) 2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi 3(P 2O 7) 2 pyrophosphate to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, SrNi 3(P 2O 7) 2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The new high-pressure phase persists metastably down to 1 atm. upon release of pressure. We also measured the pressure dependence of the lattice parameters of the polycrystalline SrNi 3(P 2O 7) 2 sample up to a pressure of 48 GPa. The compressibility of SrNi 3(P 2O 7) 2 along the b-axis is larger than that along the a-axis. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to a tetragonal structure started. The bulk modulus of SrNi 3(P 2O 7) 2 ( P2 1/ c) is 124±2 GPa with a pressure derivative of 4.37±0.12.