Pressure-driven Phase Transition Research Articles

Design of Photoelectrochemical Materials Via Non-Native Nanostructures and Their Click Assembly into Photoreactors

We modulate photoelectrochemical properties by stabilization of ‘non-native’ structures which are nanostructures having discrete translational symmetry in sub-surface regions different from that present in sub-surface regions of large crystals. Enhanced photon cross-section and charge-carrier separation is addressed through fabrication of native@non-native heterostructures with stable photoelectrodes formed via click chemistry. Introduction The development of highly efficient corrosion-resistant photoelectrode materials and their processing technologies are critical in the context of efficient hydrogen production from solar energy through water photolysis. The central physicochemical parameters in meeting this are band-gap, band-edge alignments, band bending, surface electrocatalytic activity, stability in solution and most importantly, abundant availability at low cost. To date, no suitable material (nanostructure/film) has been demonstrated to meet all these requirements. While modulation of properties via variations in chemical composition has been explored extensively in the context of hydrogen generation using photo-electrochemical (PEC) systems, comprehensive modulation of properties via variation of nanostructure has not been pursued. We propose that modulation of desirable material properties can be achieved by stabilization of ‘non-native’ structures which are nanostructures having discrete translational symmetry in the sub-surface regions different from that present in the sub-surface regions of the thermodynamically most stable form of large crystals (1,2,3,4). These non-native structures have different physico-chemical properties (e.g. band-gap, band-edges) and catalytic activity in comparison to bulk ‘native’ structure due to different chemical coordination. Further, charge carrier separation and transport related problems can be addressed through formation of native@non-native isomaterial heterostructure. The problem of stabilization of lattice mismatched heterostructure is circumvented through ‘click’ chemistry. To illustrate the our methodology we have taken TiO2 as a model material and is also being extended to other transition metal oxides. Materials and Methods The ‘native’ rutile phase and ‘non-native’ anatase phase has been synthesized using sol-gel method. The heterostructure is made through ‘click’ chemistry of alkyne-azide linkage. Finally the hetero-structures are attached to the substrate using similar click reaction. Results and Discussion Anatase and rutile are two polymorphs of TiO2. Anatase is found to be stable in lower dimension nanostructure whie rutile is stable as a higher dimension nanostructure. According to our definition of ‘native’ and ‘non-native’ structure[1], we identify rutile phase as native structure while anatase as non-native structure. The order of photoelectrocatalytic(PEC) activity is found to be in the order of rutile@anatase>anatase>rutile. The advantage of iso-material heterostructures is that they have lesser interfacial defects due to better lattice match at the interface between phases. This helps in facilitating electron-hole separation. It can be noted that the interfacial charge carrier transport in TiO2 polymorphic heterojunctions can be explained in two main pathways: 1) due to lower CB minimum, rutile phase acts as a passive electron sink and captures electrons from anatase phase and 2) transport of electrons from the rutile to anatase occurs via lower energy electron trapping sites of anatase phase. The as synthesized heterostructures have high stability due to covalently bound interfaces which enhances long-term performance of the PEC water splitting. We have demonstrated this in a wide variety of metal-semiconductor heterostructures relevant to photoelectrochemical hydrogen production (5). We also point towards other transition metal oxide heterostuctures relevant to photoelectrical hydrogen generation that are being designed using the heuristics developed using non-native nanostructures. Significance (1) Non-native nanostructures are catalytically more active than native nanostructures in case of TiO2, (2) Isomaterial stable heterostructures can be formed using click chemistry which are found to have better activity than any individual polymorphs. We attribute this to better electron-hole separation. References “Hydroxylation induced stabilization of near-surface rocksalt nanostructure on wurtzite ZnO structure”, M. Pandey and R. G. S. Pala, J. Chem. Phys.,138, 224701 (2013) “Increased loading of Eu3+ in monazite LaVO4 nanocrystals via pressure driven phase transitions”, P. Gangwar, M. Pandey, S. Sivakumar, R. G. S. Pala, G. Parthasarathy, Cryst. Growth Des., 13 (3),2344-2349(2013). “Stabilization of non-native Rocksalt CdSe at atmospheric pressures by pseudomorphic growth”, M. Pandey and R. G. S. Pala, J. Phys. Chem. C, 117, 7643-7647 (2013). “Stabilization and growth of non-native nanocrystals at low and atmospheric pressures”, M. Pandey and R. Pala, J. Chem. Phys., 136, 044703 (2012)“Generic Process for Highly Stable Metallic Nanoparticle-Semiconductor Heterostructures via “Click” Chemistry for Electro/Photocatalytic Applications”, A. Upadhyay, D. Behara, G. Sharma, A. Bajpai, N. Sharac, R. Ragan, R. Pala and S. Sivakumar, ACS Appl. Mater. Interfaces, 5, 9554 (2013)

Direct probe of Fermi surface evolution across a pressure-induced quantum phase transition

The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition, although the evolution of the Fermi surface may be tied to the emergence of exotic phenomena. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin- and charge-density-wave systems near quantum phase transitions. In particular, we track the gradual evolution of the Fermi surface in elemental chromium and delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition.

Room-temperature vibrational properties of potassium gadolinium double tungstate under compression up to 32 GPa

KGd(WO4)2 has been studied by high-pressure Raman spectroscopy at room temperature up to 32.2GPa. Evidences of two pressure-driven phase transitions have been detected. The low-pressure monoclinic phase undergoes a phase transition at 7.2(±0.9)GPa. The second transition is found at 14.2(±1.6)GPa. Both transitions are reversible. No evidence of pressure-induced amorphization is found up to 32GPa. The pressure dependence of the Raman active modes of the low- and high-pressure phases is reported. A Raman mode is detected to exhibit a weak softening in the low-pressure phase. Three Raman modes are also observed to have a similar behavior in the second high-pressure phase. The reported results are discussed comparing with the studies available in related tungstates. Emphasis is placed on the physical implications of the results.

High-pressure structural and elastic properties of Tl2O3

The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is theoretically predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic α-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa.

Pressure-induced magnetic, structural, and electronic phase transitions in LaFeO3: A density functional theory (generalized gradient approximation) + U study

We have investigated the behavior of orthoferrite LaFeO3 at ambient conditions and under pressure using DFT (generalized gradient approximation (GGA)) + U approach. Ground state electronic (band gap) and magnetic properties are considerably improved due to the Hubbard correction. Moreover, the experimentally observed pressure-driven phase transition, namely, the simultaneous occurrence of spin crossover, isostructural volume collapse, and drastic reduction in electrical resistance (electronic phase transition) is nicely described by GGA + U calculations. In particular, despite a sharp drop in resistance, a small band gap still remains in the low spin state indicating an insulator to semiconductor phase transition, in good agreement with the experiments but in contrast to GGA, which predicts metallic behavior in low spin state. We discuss the origin of variation in electronic structure of LaFeO3 in low spin state as obtained from GGA to GGA + U methods. These results emphasize the importance of correlation effects in describing the pressure-driven phase transition in LaFeO3 and other rare-earth orthoferrites.

Local structure analysis of NaNbO3

NaNbO3 has an antiferroelectric structure at room temperature and finds important technological applications. It exhibits an unusual complex sequence of temperature-and pressure-driven structural phase transitions. NaNbO3 shows unambiguous evidence for the presence of the ferroelectric R3c phase coexisting with an antiferroelectric phase (Pbcm) over a wide range of temperatures. We have carried out atomic pair-distribution function (PDF) analysis on NaNbO3 to understand the phase transitions. High-energy X-ray PDF using powder samples were carried out at the SPring-8, which is provided to become rhombohedral structure if A-site atoms did order and to become orthorhombic structure if A-site atoms did disorder.

Reinterpretation of the bond-valence model with bond-order formalism: An improved bond-valence-based interatomic potential for PbTiO3

We present a modified bond-valence model of PbTiO${}_{3}$ based on the principles of bond-valence and bond-valence vector conservation. The relationship between the bond-valence model and the bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, bond-valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved bond-valence model can be applied to a broad range of inorganic materials.

High-pressure powder X-ray diffraction study of Cu5Si and pressure-driven isostructural phase transition

The structural behaviour of Cu5Si under high-pressure (HP) has been studied by angular dispersive X-ray diffraction up to 49.9 GPa. The experimental results suggest that a pressure-induced isostructural phase transition occurs around 13.5 GPa as revealed by a discontinuity in volume as a function of pressure. The lattice parameter decreases with the pressure increasing for both low-pressure (LP) and HP phases of Cu5Si. However, a plot of the lattice parameter vs. pressure shows the existence of a plateau between 11.7 and 15.3 GPa. The bulk moduli, derived from the fitting of Birch–Murnaghan equation of state, are 150(2) GPa and 210(3) GPa for the LP phase and the HP phase of Cu5Si, respectively. A change in the electronic state of the copper is assumed to govern the observed structural phase transition.

Pressure-driven Phase Transition in CaFeAsF at 40 and 300 K

We carried out systematic investigation of high pressure crystal structure and structural phase transition upto 46 GPa at 40 K and 25 GPa at 300 K in CaFeAsF using powder synchrotron x-ray diffraction experiments. Rietveld analysis of the diffraction data at 40 K reveals a structural phase transition from an orthorhombic to a monoclinic phase at Pc = 13.7 GPa, while increasing pressure. The transition to a lower symmetry phase (orthorhombic to monoclinic) in this 1111 compound under pressure is in contrast with the transition to a high symmetry phase (orthorhombic to tetragonal) as observed in the '122' type compounds. On heating from 40 K at high pressure, CaFeAsF undergoes a monoclinic to tetragonal phase transition around 25 GPa and 200 K. Further, it does not show any post-tetragonal phase transition, as observed in '122' compounds (CaFe2As2, BaFe2As2), and remains in the tetragonal phase upto 25 GPa at 300 K. The dPc/dT slope is found to be positive for CaFeAsF and CaFe2As2, unlike a negative gradient was found in case of BaFe2As2.

High Pressure Phase Transitions in Barium Tungstate

Alkaline-earth tungstates exhibit an interesting phase diagram with respect to pressure. BaWO4 is found to undergo a pressure-driven phase transition at 7.1 GPa from tetragonal scheelite structure to monoclinic fergusonite structure. It is observed to become amorphous beyond 45 GPa. In this work, we report molecular dynamics simulation studies on transformations of the scheelite phase in BaWO4 with increasing pressure at ambient temperature. Our calculated equation of state is in very good agreement with reported experimental data and first principles calculations. In our calculations there is no apparent volume discontinuity at the scheelite to fergusonite transformation in BaWO4 around 7.5 GPa. The tungstate transforms to an amorphous phase beyond 35 GPa with discernable volume collapse. The pair correlation functions show that there are subtle changes in the arrangement of the BaO8 polyhedra with increasing pressure. However, the WO4 tetrahedra remain unperturbed with increasing pressure until amorphization occurs.

Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using ab initio calculations. The calculations reveal that α-FeSe makes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe are also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from α-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe–Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from α-FeSe to the other phases of FeSe. The experimentally observed decrease in Tc of superconducting α-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest an increase of the Tc of α-FeSe if such phase transitions are frustrated by suitable methods.

Theoretical study of pressure-driven phase transitions in HgSe and HgTe

We present the results of a density-functional-theory study of the stability of high-pressure phases of HgSe and HgTe up to the so-called Cmcmphase, which is the highest-pressure phase that has been fully characterized in experiments in these compounds. We compare the results obtained using different exchange-correlation functionals for the energetics, stability, and structure of the high-pressure phases, finding that the qualitative picture is very similar and essentially in agreement with the experimental observations. We study the $\text{cinnabar}\ensuremath{\rightarrow}\text{NaCl}$ and the $\text{NaCl}\ensuremath{\rightarrow}\text{Cmcm}$ transitions in detail and provide the structural evolution under pressure of the cinnabar and Cmcm phases. We find that the mechanism for the $\text{NaCl}\ensuremath{\rightarrow}\text{Cmcm}$ transition is somewhat different from other related compounds exhibiting this transition, with cell deformation playing an important role in the stabilization of the Cmcm state in HgSe and HgTe. Our results support the view of a $\text{NaCl}\ensuremath{\rightarrow}\text{Cmcm}$ transition that is distinctly first order, in good agreement with the experimental observations and contrary to other materials where this transition is of second order.

Pressure-driven phase transitions in TiOCl and the family(Ca, Sr, Ba)Fe2As2

Motivated by recent experimental measurements on pressure-driven phasetransitions in Mott insulators as well as the new iron pnictide superconductors, weshow that first principles Car–Parrinello molecular dynamics calculations area powerful method to describe the microscopic origin of such transitions. Wepresent results for (i) the pressure-induced insulator to metal phase transitionin the prototypical Mott insulator TiOCl as well as (ii) the pressure-inducedstructural and magnetic phase transitions in the family of correlated metalsAFe2As2 (A = Ca, Sr, Ba). Comparison of our predictions with existing experimental results yields very goodagreement.

Analytical stress tensor and pressure calculations with the CRYSTAL code

The calculation of the stress tensor and related properties and its implementation in the CRYSTAL code are described. The stress tensor is obtained from the earlier implemented analytical gradients with respect to the cell parameters. Subsequently, the pressure and enthalpy are computed, and a test concerning the pressure-driven phase transition in KI is used as an illustration. Finally, the possibility of applying external pressure is implemented. The constant-pressure optimization offers an alternative optimization method in addition to the already implemented optimization at constant volume.

AMPLIMODES: symmetry-mode analysis on the Bilbao Crystallographic Server

AMPLIMODESis a computer program available on the Bilbao Crystallographic Server that can perform a symmetry-mode analysis of any distorted structure of displacive type. The analysis consists in decomposing the symmetry-breaking distortion present in the distorted structure into contributions from different symmetry-adapted modes. Given the high- and the low-symmetry structures,AMPLIMODESdetermines the atomic displacements that relate them, defines a basis of symmetry-adapted modes, and calculates the amplitudes and polarization vectors of the distortion modes of different symmetry frozen in the structure. The program uses a mode parameterization that is as close as possible to the crystallographic conventions, expressing all quantities for the asymmetric unit of the low-symmetry structure. Distorted structures are often related to their higher-symmetry counterparts by temperature- and/or pressure-driven phase transitions, ferroic phase transitions being a particular example. The automatic symmetry-mode analysis performed byAMPLIMODEScan be very useful for establishing the driving mechanisms of such structural phase transitions or the fundamental instabilities at the origin of the distorted phases.

Experimental and theoretical investigation ofThGeO4at high pressure

We report here the combined results of angle-dispersive x-ray diffraction experiments performed on ThGeO4 up to 40 GPa and total-energy density-functional theory calculations. Zircon-type ThGeO4 is found to undergo a pressure-driven phase transition at 11 GPa to the tetragonal scheelite structure. A second phase transition to a monoclinic M-fergusonite type is found beyond 26 GPa. The same transition has been observed in samples that crystallize in the scheelite phase at ambient pressure. No additional phase transition or evidence of decomposition of ThGeO4 has been detected up to 40 GPa. The unit-cell parameters of the monoclinic high-pressure phase are a = 4.98(2) A, b = 11.08(4) A, c = 4.87(2) A, and beta = 90.1(1), Z = 4 at 28.8 GPa. The scheelite-fergusonite transition is reversible and the zircon-scheelite transition non-reversible. From the experiments and the calculations, the room temperature equation of state for the different phases is also obtained. The anisotropic compressibility of the studied crystal is discussed in terms of the differential compressibility of the Th-O and Ge-O bonds.

Raman Investigation of BaWO4-II Phase under Hydrostatic Pressures up to 14.8 GPa

The BaWO4-II phase is synthesized at 5.0 GPa and 610°C with a cubic-anvil apparatus and identified by XRD. Raman scattering measurement is carried out to investigate the phase behaviour of a pure BaWO4-II phase (space group P21/n, Z = 8) under hydrostatic pressures up to 14.8 GPa at ambient temperature. In each spectrum recorded for this phase, 27 Raman modes are observed, and all bands shift toward higher wavenumber with a pressure dependence ranging from 3.8 to 0.2 cm−1/GPa. No pressure-driven phase transition occurs in the entire pressure range in this study. Our results indicate that the previously reported high pressure phase of BaWO4 at pressure above about 10 GPa and room temperature (Errandonea et al. Phys. Rev. B 73(2006)224103) is not the BaWO4-II phase.

Inelastic neutron scattering and lattice dynamics of NaNbO3 and Sr0.70Ca0.30TiO3

NaNbO 3 and (Sr,Ca)TiO 3 exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice dynamical models have been developed for SrTiO 3 and CaTiO 3 which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO 3 .

Molecular dynamics simulation of the six- to four-coordinate pressure-driven transition inMXnanocrystals: Mechanistic consequences ofΣ3grain boundaries in the high-pressure starting structure

Constant-pressure molecular dynamics has been used to simulate the six- to four-coordinate downstroke pressure-driven phase transition in B1 nanocrystals. The nanocrystals considered have previously been formed in upstroke $\text{B}3\ensuremath{\rightarrow}\text{B}1$ simulations, giving them an amorphous surface region and interior ${\ensuremath{\Sigma}}_{3}$ grain boundaries. Nucleation occurs in the interior of the crystal, with multiple nucleation events observed along grain boundaries, in contrast to previous decompression simulations of single-domain nanocrystals with crystallographically well-defined surfaces. Competing mechanisms give rise to B3, B4, and d-BCT domains in the productstructures. Four distinct mechanisms are observed, including two $\text{B}4\ensuremath{\leftrightarrow}\text{B}1$ mechanisms. The $\text{B}4\stackrel{\text{I}}{\ensuremath{\leftrightarrow}}\text{B}1$ mechanism is the same as seen in previous simulations of single-domain nanocrystalline and bulk systems, while the observed $\text{B}3\ensuremath{\leftrightarrow}\text{B}1$ and $\text{B}4\stackrel{\text{II}}{\ensuremath{\leftrightarrow}}\text{B}1$ display a ${[111]}_{\text{B}3}\ensuremath{\leftrightarrow}{[100]}_{\text{B}1}$ correspondence, in agreement with experimental observations of repeatedly transformed CdSe nanocrystals; both of which are different to those seen in previous upstroke simulations. The interaction between these competing mechanisms determines the domain structure of the product nanocrystals.

High-pressure structural integrity and structural transformations of glass-derived nanocomposites: A review

A review of our recent and ongoing extensive high-pressure synchrotron X-ray diffraction and high-pressure optical spectroscopy studies of nanocrystalline composites is presented. These heterophased, nano-architectured composites consist of amorphous matrices with dispersed nanocrystals or quantum dots. We show how besides compositional variations, additional tuning of these glass-derived nanocomposites can be done by exploiting elevated pressure. We examine stability and pressure-driven phase transitions occurring in nanocrystals as well as structural changes occurring in the glass matrix. Finally, we discuss the influence of the glass matrix of a composite on the structural transformations occurring in the embedded nanocrystals.