High-pressure structural behavior and equation of state of NaZnF3

We report the results of density functional theory ab-initio calculations and monochromatic synchrotron X-ray diffraction study carried out for orthorhombic NaZnF₃ in the pressure range 0-40 GPa. Perovskite-to-postperovskite phase transition was anticipated by first-principles computations and then observed in high-pressure diamond anvil cell synchrotron diffraction experiment between 14 and 22 GPa. Above 25 GPa postperovskite structure (CaIrO₃ type, space group Cmcm) coexists with another phase, yet unidentified. On decompression, pure postperovskite-type structure was found to be stable down to 4 GPa; below this pressure sample contained both perovskite and postperovskite modifications. Fit of experimental P-V data to the third-order Birch-Murnaghan equation of state gave bulk moduli, K_P,0 64.98±2.67 and 69.88±3.69 GPa for perovskite and postperovskite modifications, respectively. Both phases demonstrated strong anisotropy of compressibility. For postperovskite NaZnF₃, the highest compression was observed along the direction perpendicular to the planes of ZnF₆ octahedra arrangement.     

Pressure‐Dependent Polymorphism and Band‐Gap Tuning of Methylammonium Lead Iodide Perovskite

We report the pressure‐induced crystallographic transitions and optical behavior of MAPbI₃ (MA=methylammonium) using in situ synchrotron X‐ray diffraction and laser‐excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin‐orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO₃‐type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase‐mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction.     

Raman scattering studies on n-heptane under high pressure

High-pressure Raman scattering studies at ambient temperature are performed on n-heptane. We observe a liquid-solid transition around 1.5 GPa from the changes in the Raman spectra. This has been reported in earlier works. With increasing pressure, we observe large changes in the Raman modes and the spectra show a distinct change around 7.5 GPa. This marks the solid-solid transition at 7.5 GPa observed in n-heptane for the first time. As predicted in theoretical work, we observe dampening of methyl rotation in n-heptane below 7.5 GPa. With increase in pressure above 7.5 GPa we observe a definitive conversion of gauche to trans conformation in the solid phase. Upon release of pressure we do not observe any hysteresis, which suggests that the solid-solid transition takes place with no volume change or is a second-order transition. In this paper we propose this transition to be an orientational order-disorder transition driven by the dampening of the rotation of the methyl group.     

Modulated structure and molecular dissociation of solid chlorine at high pressures

Among diatomic molecular halogen solids, high pressure structures of solid chlorine (Cl(2)) remain elusive and least studied. We here report first-principles structural search on solid Cl(2) at high pressures through our developed particle-swarm optimization algorithm. We successfully reproduced the known molecular Cmca phase (phase I) at low pressure and found that it remains stable up to a high pressure 142 GPa. At 150 GPa, our structural searches identified several energetically competitive, structurally similar, and modulated structures. Analysis of the structural results and their similarity with those in solid Br(2) and I(2), it was suggested that solid Cl(2) adopts an incommensurate modulated structure with a modulation wave close to 2∕7 in a narrow pressure range 142-157 GPa. Eventually, our simulations at >157 GPa were able to predict the molecular dissociation of solid Cl(2) into monatomic phases having body centered orthorhombic (bco) and face-centered cubic (fcc) structures, respectively. One unique monatomic structural feature of solid Cl(2) is the absence of intermediate body centered tetragonal (bct) structure during the bco → fcc transition, which however has been observed or theoretically predicted in solid Br(2) and I(2). Electron-phonon coupling calculations revealed that solid Cl(2) becomes superconductors within bco and fcc phases possessing a highest superconducting temperature of 13.03 K at 380 GPa. We further probed the molecular Cmca → incommensurate phase transition mechanism and found that the softening of the A(g) vibrational (rotational) Raman mode in the Cmca phase might be the driving force to initiate the transition.     

Synthesis, structural transformation, thermal stability, valence state, and magnetic and electronic properties of PbNiO3 with perovskite- and LiNbO3-type structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.     

高压下两种8-羟基喹啉络合物的发光行为和结构变化

测定了在高压条件下两种金属（钙和锌）的8 羟基喹啉络合物的晶体粉末样品的发光行为和原位X光衍射光谱.结果表明,压力对其发光性质产生极大的影响.随着压力的增加,8 羟基喹啉钙的发光强度在3 GPa以内时大大增加,随后发光强度快速下降,到7 GPa左右时几乎为零.而8 羟基喹啉锌的发光强度随压力的增加而逐渐降低,到7 GPa左右时约为常压的10％.高压下的原位X光衍射结果表明,8 羟基喹啉钙的晶体在3～4 GPa开始 发生非晶化相变,在7 GPa时该非晶化相变完成,样品的X光衍射完全消失.而8 羟基喹啉锌在压力的作用下（至16 GPa）没有发生明显的相变.     

Benzene under high pressure: a story of molecular crystals transforming to saturated networks, with a possible intermediate metallic phase

In a theoretical study, benzene is compressed up to 300 GPa. The transformations found between molecular phases generally match the experimental findings in the moderate pressure regime (<20 GPa): phase I (Pbca) is found to be stable up to 4 GPa, while phase II (P4(3)2(1)2) is preferred in a narrow pressure range of 4-7 GPa. Phase III (P2(1)/c) is at lowest enthalpy at higher pressures. Above 50 GPa, phase V (P2(1) at 0 GPa; P2(1)/c at high pressure) comes into play, slightly more stable than phase III in the range of 50-80 GP, but unstable to rearrangement to a saturated, four-coordinate (at C), one-dimensional polymer. Actually, throughout the entire pressure range, crystals of graphane possess lower enthalpy than molecular benzene structures; a simple thermochemical argument is given for why this is so. In several of the benzene phases there nevertheless are substantial barriers to rearranging the molecules to a saturated polymer, especially at low temperatures. Even at room temperature these barriers should allow one to study the effect of pressure on the metastable molecular phases. Molecular phase III (P2(1)/c) is one such; it remains metastable to higher pressures up to ～200 GPa, at which point it too rearranges spontaneously to a saturated, tetracoordinate CH polymer. At 300 K the isomerization transition occurs at a lower pressure. Nevertheless, there may be a narrow region of pressure, between P = 180 and 200 GPa, where one could find a metallic, molecular benzene state. We explore several lower dimensional models for such a metallic benzene. We also probe the possible first steps in a localized, nucleated benzene polymerization by studying the dimerization of benzene molecules. Several new (C(6)H(6))(2) dimers are predicted.     

The chemistry of cyanuric acid (H3C3N3O3) under high pressure and high temperature

We have studied cyanuric acid (H(3)C(3)N(3)O(3)) at static pressures up to 8.1 GPa and simultaneous temperatures up to 750 K, using primarily infrared absorption spectroscopy and visual observation. The corresponding phase diagram compares favorably with theoretical predictions of metastable organic materials. Two reactions were observed and characterized; both are irreversible. Below 2 GPa, melting is accompanied by a decomposition reaction, and upon cooling, cyanuric acid is not recovered. Above 2 GPa, heating results in a solid product recoverable at ambient conditions. Corresponding infrared spectra suggest that pressure leads to the formation of heterocycles of increasing complexity and biological potential, with the composition determined by the pressure of formation. Cyanuric acid is of interest at these conditions because it and its monomer, isocyanic acid, are "prebiotic" compounds found in stellar dust clouds, meteorites, and other remnants of the early Earth.     

Pressure-induced phase transitions in PbTiO3: a query for the polarization rotation theory

Our first-principles computations show that the ground state of PbTiO3 under hydrostatic pressure transforms discontinuously from P4mm to R3c at 9 GPa. Spontaneous polarization decreases with increasing pressure so that the R3c phase transforms to the centrosymmetric Rc phase at around 27 GPa. The first-order phase transition between the tetragonal and rhombohedral phases is exceptional since there is no evidence for a bridging phase. The essential feature of the R3c and Rc phases is that they allow the oxygen octahedron to increase its volume VB at the expense of the cuboctahedral volume VA around a Pb ion. This is further supported by the fact that neither the R3m nor Cm phase, which keep the VA/VB ratio constant, is a ground state within the pressure range between 0 and 40 GPa. Thus, tetragonal strain is dominant up to 9 GPa, whereas at higher pressures, efficient compression through oxygen octahedra tilting plays the central role for PbTiO3. Previously predicted pressure induced colossal enhancement of piezoelectricity in PbTiO3 corresponds to unstable Cm and R3m phases. This suggests that the phase instability, in contrast to the polarization rotation, is responsible for the large piezoelectric properties observed in systems like Pb(Zr,Ti)O3 in the vicinity of the morphotropic phase boundary.     

Quasi-hydrostatic X-ray powder diffraction study of the low- and high-pressure phases of CaWO4 up to 28 GPa

We have studied CaWO₄ under compression using Ne as pressure-transmitting medium at room temperature by means of synchrotron X-ray powder diffraction. We have found that CaWO₄ beyond 8.8 GPa transforms from its low-pressure tetragonal structure (scheelite) into a monoclinic structure (fergusonite). The high-pressure phase remains stable up to 28 GPa and the low-pressure phase is totally recovered after full decompression. The pressure dependence of the unit-cell parameters, as well as the pressure–volume equation of state, has been determined for both phases. Compared with previous studies, we found in our quasi-hydrostatic experiments a different behavior for the unit-cell parameters of the fergusonite phase and a different transition pressure. These facts suggest that deviatoric stresses influence on the high-pressure structural behavior of CaWO₄ as previously found in related compounds. The reported experiments also provide information on the pressure dependence of interatomic bond distances, shedding light on the transition mechanisms.     

High-pressure studies of cyclohexane to 40 GPa

We present data from two room temperature synchrotron X-ray powder diffraction studies of cyclohexane up to approximately 40 and approximately 20 GPa. In the first experiment, pressure cycling was employed wherein pressure was varied up to approximately 16 GPa, reduced to 3.5 GPa, and then raised again to 40 GPa. Initially, the sample was found to be in the monoclinic phase (P12(1)/n1) at approximately 8.4 GPa. Beyond this pressure, the sample adopted triclinic unit cell symmetry (P1) which remained so even when the pressure was reduced to 3.5 GPa, indicating significant hysteresis and metastability. In the second experiment, pressure was more slowly varied, and the monoclinic unit cell structure (P12(1)/n1) was observed at lower pressures up to approximately 7 GPa, above which a phase transformation into the P1 triclinic unit cell symmetry occurred. Thus, the pressure onset of the triclinic phase may be dependent upon the pressurizing conditions. High-pressure Raman data that further emphasize a phase transition (probably into phase VI) around 10 GPa are also presented. We also have further evidence for a phase VII, which is probably triclinic.     

Structural, electronic, and optical properties of crystalline iodoform under high pressure: a first-principles study

The high pressure phases, electronic structure, and optical properties of iodoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density-functional theory. A new high pressure polar monoclinic structure with space group Cc, denoted as β phase, has been observed after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from α to β phase occurs at 40.1 GPa. Electronic structure calculated results showed that the insulator-metal transition in α phase due to band overlap is found at about 32 GPa. In addition, the calculated absorption spectra of iodoform are consistent with the experimental results.     

Pressure-induced cubic to monoclinic phase transformation in erbium sesquioxide Er(2)O(3)

Cubic Er(2)O(3) was compressed in a symmetric diamond anvil cell at room temperature and studied in situ using energy-dispersive X-ray diffraction. A transition to a monoclinic phase began at 9.9 GPa and was complete at 16.3 GPa and was accompanied by a approximately 9% volume decrease. The monoclinic phase was stable up to at least 30 GPa and could be quenched to ambient conditions. The normalized lattice parameter compression data for both phases were fit to linear equations, and the volume compression data were fit to third-order Birch-Murnaghan equations of state. The zero-pressure isothermal bulk moduli (B(0)) and the first-pressure derivatives (B(0)') for the cubic and monoclinic phases were 200(6) GPa and 8.4 and also 202(2) GPa and 1.0, respectively. Ab initio density functional theory calculations were performed to determine optimized lattice parameters and atom positions for the cubic, monoclinic, and hexagonal phases of Er(2)O(3). The calculated X-ray spectra and predicted transition pressure are in good qualitative agreement with the experimental results.     

Application of principal component analysis and Raman spectroscopy in the analysis of polycrystalline BaTiO3 at high pressure

The principal component analysis (PCA) was applied to Raman spectra of polycrystalline BaTiO(3) under pressure from atmospheric pressure to approximately 6.72 GPa. For the system utilized, PCA was able to distinguish spectral features and to determine the phase transition pressure: tetragonal to cubic at approximately 2.0 GPa. The present study demonstrates the potentialities of the application of PCA to the investigation on phase transitions at high pressure by Raman spectroscopy.     

Compression behaviors of binary skutterudite CoP3 in noble gases up to 40 GPa at room temperature

The binary skutterudite CoP(3) has a large void at the body-centered site of each cubic unit cell and is, therefore, called a nonfilled skutterudite. We investigated its room-temperature compression behavior up to 40.4 GPa in helium and argon using a diamond-anvil cell. High-pressure in situ X-ray diffraction and Raman scattering measurements found no phase transition and a stable cubic structure up to the maximum pressure in both media. A fitting of the present pressure-volume data to the third-order Birch-Murnaghan equation of state yields a zero-pressure bulk modulus K(0) of 147(3) GPa [pressure derivative K(0)' of 4.4(2)] and 171(5) GPa [where K(0)' = 4.2(4)] in helium and argon, respectively. The Gru?neisen parameter was determined to be 1.4 from the Raman scattering measurements. Thus, CoP(3) is stiffer than other binary skutterudites and could therefore be used as a host cage to accommodate large atoms under high pressure without structural collapse.     

Polymerization of nitrogen in sodium azide

The high-pressure behavior of nitrogen in NaN(3) was studied to 160 GPa at 120-3300 K using Raman spectroscopy, electrical conductivity, laser heating, and shear deformation methods. Nitrogen in sodium azide is in a molecularlike form; azide ions N(3-) are straight chains of three atoms linked with covalent bonds and weakly interact with each other. By application of high pressures we strongly increased interaction between ions. We found that at pressures above 19 GPa a new phase appeared, indicating a strong coupling between the azide ions. Another transformation occurs at about 50 GPa, accompanied by the appearance of new Raman peaks and a darkening of the sample. With increasing pressure, the sample becomes completely opaque above 120 GPa, and the azide molecular vibron disappears, evidencing completion of the transformation to a nonmolecular nitrogen state with amorphouslike structure which crystallizes after laser heating up to 3300 K. Laser heating and the application of shear stress accelerates the transformation and causes the transformations to occur at lower pressures. These changes can be interpreted in terms of a transformation of the azide ions to larger nitrogen clusters and then polymeric nitrogen net. The polymeric forms can be preserved on decompression in the diamond anvil cell but transform back to the starting azide and other new phases under ambient conditions.     

Theoretical Study on the High-Temperature P6 and P6' Phases of Si3N4: A Tool to Aid in Ceramics Development

Atomistic modeling based on the density functional theory combined with the quasi-harmonic approximation is used to investigate the lattice parameters and elastic moduli of the P6 and P6' phases of Si3N4. β-Si3N4 is set as a benchmark system since accurate experiments are available. The calculated lattice constants and elastic constants of β-Si3N4 are in good agreement with the experimental data. The crystal anisotropy, mechanical stability, and brittle behavior of P6- and P6'-Si3N4 are also discussed in the pressure range of 30-55 GPa. The results show that these two polymorphs are metallic compounds. The brittleness and elastic anisotropy increase with applied pressure increasing. Besides, the phase boundariesof the β→P6'→δ transitions are also analysed. The β phase is predicted to undergo a phase transition to the P6' phase at 40.0 GPa and 300 K. Upon further compression, the P6'→δ transition can be observed at 53.2 GPa. The thermal and pressure effects on the heat capacity, cell volume and bulk modulus are also determined. Some interesting features are found at high temperatures.     

不同结晶度的乙二醇及其水溶液玻璃化转变与焓松弛

为了考察晶体成分对无定形成分玻璃化转变和结构松弛行为的影响,利用差示扫描量热法(DSC),结合低温显微技术,研究了乙二醇(EG)及其50％水溶液在不同结晶度时的玻璃化转变和焓松弛行为.采用等温结晶方法控制骤冷的部分结晶玻璃体中的晶体份额.DSC结果表明,对于部分结晶的EG,只有单一的玻璃化转变过程,而对于50％EG,当结晶度不同时,不同程度地表现出两次玻璃化转变（无定形相Ⅰ和无定形相Ⅱ）.相Ⅰ的玻璃化转变温度和完全无定形态的含水EG的玻璃化转变温度相一致;相Ⅱ的玻璃化转变温度要比此温度约高6 ℃.低温显微观察结果印证了DSC实验结果.DSC等温退火的实验和KWW(Kohlrausch-Williams-Watts)衰变函数分析结果表明,EG无定形和50％EG中的两种无定形有不同的焓松弛行为.     

Melting and dissociation of ammonia at high pressure and high temperature

Raman spectroscopy and synchrotron x-ray diffraction measurements of ammonia (NH(3)) in laser-heated diamond anvil cells, at pressures up to 60 GPa and temperatures up to 2500 K, reveal that the melting line exhibits a maximum near 37 GPa and intermolecular proton fluctuations substantially increase in the fluid with pressure. We find that NH(3) is chemically unstable at high pressures, partially dissociating into N(2) and H(2). Ab initio calculations performed in this work show that this process is thermodynamically driven. The chemical reactivity dramatically increases at high temperature (in the fluid phase at T > 1700 K) almost independent of pressure. Quenched from these high temperature conditions, NH(3) exhibits structural differences from known solid phases. We argue that chemical reactivity of NH(3) competes with the theoretically predicted dynamic dissociation and ionization.     

High pressure behavior of α-NaVO3: A Raman scattering study

The reported pressure-induced amorphization in α-NaVO₃ has been re-investigated using Raman spectroscopy. Discontinuous changes are noted in the Raman spectrum above 5.6 GPa implying large structural changes across the transition. The decrease in frequency of the V-O stretching mode across the transition suggests that the vanadium atom may be in octahedral coordination in the high pressure phase. Excessive broadening of the internal modes is observed above 6 GPa. New peaks characteristic of a crystalline phase gain in intensity at higher pressures in the bending modes region; however, the transformation is not complete even at 13 GPa. Co-existence of phases is noted over a significant pressure range above the onset of transition. Pressure released spectrum is found to be a mixture of crystalline α-phase, traces of crystalline β-phase and highly disordered phase consisting of V-O units in five- and six-fold coordination.