Pressure-induced Isostructural Phase Transition Research Articles

Study on pressure-induced isostructural phase transition and electrical transport properties of BiOBr.

In this work, we prepared a BiOBr powder sample by the coprecipitation method for in situ high-pressure AC impedance spectroscopy tests, in situ high-pressure Raman measurements and in situ high-pressure X-ray diffraction experiments to explore its structural properties and electrical transport processes under compression. Two pressure-driven isostructural phase transitions, T-T' and T'-T'' (T - tetragonal, T' - tetragonal 1 and T'' - tetragonal 2), were discovered at around 10.0 and 15.0 GPa, respectively. The pressure-induced changes in the crystal structure and electrical transport of BiOBr can provide a reference for explaining the mechanism of the isostructural phase transition of other similar compounds after compression.

Coupling of organic cation and inorganic lattice in methylammonium lead halide perovskites: Insights into a pressure-induced isostructural phase transition

Organic-inorganic hybrid metal-halide perovskite (OIHP) materials provide a tunable platform for engineering their optoelectronic properties. Although several high-pressure studies have been conducted from the OIHP family of single crystals and films, the exact nature of the dynamic coupling of the $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}$ (MA) cation with the octahedral lattice framework and the mechanisms responsible for the structural phase transformation under pressure are not well captured. By combined photoluminescence (PL), synchrotron-based x-ray diffraction, and Raman-scattering studies as a function of pressure from methylammonium lead bromide ($\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$), we shed light on an isostructural phase transition due to the coupling of the MA cation and the $\mathrm{Pb}{\mathrm{Br}}_{6}$ lattice through hydrogen bonding. The sharp discontinuities at $\ensuremath{\sim}$ 1 GPa and $\ensuremath{\sim}$ 3 GPa in the PL peak positions correlate with the structural changes observed in high-pressure XRD and Raman-scattering studies. The electronic band edge as a function of pressure is calculated within density-functional theory. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman-scattering studies. The frequencies of the lattice modes and the C--H/N--H bending and stretching modes of the MA cation show anomalous changes and other nuances at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II ($Im\overline{3}$) to orthorhombic ($Pnma$) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this paper highlights the impact of molecular vibrations on the electronic properties of $\mathrm{M}\mathrm{A}\mathrm{Pb}{\mathrm{Br}}_{3}$ under pressure.

Pressure-induced isostructural phase transition in Ti3AlC2: experimental and theoretical investigation.

The structural stability of Ti3AlC2 under high pressure is important for understanding its mechanical properties. Here, we conducted a high hydrostatic pressure synchrotron X-ray diffraction experiment and no structural phase transition was observed. Like most other MAX phases, Ti3AlC2 showed an anisotropic compression behavior. Most importantly, an anomaly in c/a ratio was observed at 20.3 GPa, indicating that a pressure-induced isostructural phase transition occurred here. Analysis of the electronic band structure and Fermi surface revealed that three bands crossed the Fermi surface under compression, which suggested that this isostructural phase transition can be considered to be motivated by an electronic topological transition. The subsequent Hall-effect measurements reconfirmed this variation of the electronic band at the Fermi surface, which can be regarded as the electronic origin for the observed isostructural phase transition. These results enrich the basic property data of Ti3AlC2 and would benefit the further understanding of this promising material.

Isostructural phase transition-induced bulk modulus multiplication in dopant-stabilized ZrO2 solid solution**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0305900) and the National Natural Science Foundation of China (Grant Nos. 11774126, 11774174, 1674404, and 51772125).

The electrical transport properties and structures of Y2O3/ZrO2 solid solution have been studied under high pressure up to 23.2 GPa by means of in situ impedance spectroscopy and x-ray diffraction (XRD) measurements. In the impedance spectra, it can be found that the pressure-dependent resistance of Y2O3/ZrO2 presents two different change trends before and after 13.3 GPa, but the crystal symmetry still remains stable in the cubic structure revealed by the XRD measurement and Rietveld refinement. The pressure dependence of the lattice constant and unit cell volume shows that the Y2O3/ZrO2 solid solution undergoes an isostructural phase transition at 13.1 GPa, which is responsible for the abnormal change in resistance. By fitting the volume data with the Birch–Murnaghan equation of state, we found that the bulk modulus B0 of the Y2O3/ZrO2 solid solution increases by 131.9% from 125.2 GPa to 290.3 GPa due to the pressure-induced isostructural phase transition.

Pressure-induced isostructural phase transition in α-Ni(OH)2 nanowires**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA0305900), the National Natural Science Foundation of China (Grant Nos. 11874172, 11374120, 11634004, and 51320105007), and the Fund from Jilin University for Science and Technology Innovative Research Team (Grant No. 2017TD-01).

High pressure structural phase transition of monoclinic paraotwayite type α-Ni(OH)2 nanowires with a diameter of 15 nm–20 nm and a length of several micrometers were studied by synchrotron x-ray diffraction (XRD) and Raman spectra. It is found that the α-Ni(OH)2 nanowires experience an isostructural phase transition associated with the amorphization of the H-sublattice of hydroxide in the interlayer spaces of the two-dimensional crystal structure at 6.3 GPa–9.3 GPa. We suggest that the isostructural phase transition can be attributed to the amorphization of the H-sublattice. The bulk moduli for the low pressure phase and the high pressure phase are 41.2 (4.2) GPa and 94.4 (5.6) GPa, respectively. Both the pressure-induced isostructural phase transition and the amorphization of the H-sublattice in the α-Ni(OH)2 nanowires are reversible upon decompression. Our results show that the foreign anions intercalated between the α-Ni(OH)2 layers play important roles in their structural phase transition.

Pressure-induced isostructural phase transition and charge transfer in superconducting FeSe

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material.

Solid-State Associative Reactions and the Coordination Compression Mechanism.

Coordination polymers and metal-organic frameworks can be modified by high pressure, according to its effects on the radii of central and ligand atoms. The pressure reduces the ligands' radii, and the coordination number is usually increased. Such transformations of the coordination quite generally conform to the inverse rule of pressure and temperature effects, although the temperature-induced transformations are much less frequently observed. The two-dimensional coordination polymer Cd(APP)2NO3·NO3 [APP = 1,4-bis(3-aminopropyl)piperazine] undergoes a pressure-induced isostructural phase transition triggered by a topochemical reaction, yielding Cd(APP)2(NO3)2. The transition retains the symmetry of both phases, and their structures have been determined by X-ray diffraction for the single crystals compressed in a diamond-anvil cell. The reaction increases the Cd coordination, from 6-fold in phase I to 7-fold in phase II, where the new Cd-O bond involves an additional nitrate anion in the Cd coordination sphere.

Pressure-induced iso-structural phase transition and metallization in WSe2

We present in situ high-pressure synchrotron X-ray diffraction (XRD) and Raman spectroscopy study, and electrical transport measurement of single crystal WSe2 in diamond anvil cells with pressures up to 54.0–62.8 GPa. The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding, beginning at 28.5 GPa and not being completed up to around 60 GPa. The Raman data also reveals a dominant role of the in-plane strain over the out-of plane compression in helping achieve the transition. Consistently, the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2–61.7 GPa, where a mixed semiconducting and metallic feature is observed due to the coexisting low- and high-pressure structures.

Pressure-Driven Isostructural Phase Transition in InNbO4: In Situ Experimental and Theoretical Investigations.

The high-pressure behavior of technologically important visible-light photocatalytic semiconductor InNbO4, adopting a monoclinic wolframite-type structure at ambient conditions, was investigated using synchrotron-based X-ray diffraction, Raman spectroscopic measurements, and first-principles calculations. The experimental results indicate the occurrence of a pressure-induced isostructural phase transition in the studied compound beyond 10.8 GPa. The large volume collapse associated with the phase transition and the coexistence of two phases observed over a wide range of pressure shows the nature of transition to be first-order. There is an increase in the oxygen anion coordination number around In and Nb cations from six to eight at the phase transition. The ambient-pressure phase has been recovered on pressure release. The experimental pressure-volume data when fitted to a Birch-Murnaghan equation of states yields the value of ambient pressure bulk modulus as 179(2) and 231(4) GPa for the low- and high-pressure phases, respectively. The pressure dependence of the Raman mode frequencies and Grüneisen parameters was determined for both phases by experimental and theoretical methods. The same information is obtained for the infrared modes from first-principles calculations. Results from theoretical calculations corroborate the experimental findings. They also provide information on the compressibility of interatomic bonds, which is correlated with the macroscopic properties of InNbO4.

Giant strain geared to transformable H-bonded network in compressed β-D-mannose.

Networks of OH···O bonds in sugars and H2O ices are hardly affected by temperature, but transform under pressure. Such a transition at 1.95 GPa in β-D-mannose induces strong strain, non-destructive for the single crystal and easily visible in its shape deformation. This transition occurs at the lowest pressure of all sugars investigated at high pressure so far. The giant strain propagates perpendicular to the clearly visible zero-strain planes. The transition reconstructs the 3-dimensional pattern of OH···O hydrogen bonds, changes the conformation of molecules and preserves the space-group symmetry, like the analogous transitions of α-D-glucose and D-sucrose. However, new repulsing O···O contacts have been generated at high pressure in D-mannose only.

Anomalous anisotropic compression behavior of superconducting CrAs under high pressure

CrAs was observed to possess the bulk superconductivity under high-pressure conditions. To understand the superconducting mechanism and explore the correlation between the structure and superconductivity, the high-pressure structural evolution of CrAs was investigated using the angle-dispersive X-ray diffraction (XRD) method. The structure of CrAs remains stable up to 1.8 GPa, whereas the lattice parameters exhibit anomalous compression behaviors. With increasing pressure, the lattice parameters a and c both demonstrate a nonmonotonic change, and the lattice parameter b undergoes a rapid contraction at ∼ 0.18-0.35 GPa, which suggests that a pressure-induced isostructural phase transition occurs in CrAs. Above the phase transition pressure, the axial compressibilities of CrAs present remarkable anisotropy. A schematic band model was used to address the anomalous compression behavior of CrAs. The present results shed light on the structural and related electronic responses to high pressure, which play a key role toward understanding the superconductivity of CrAs.

Pressure-induced isostructural phase transition in Bi2Sr2CaCu2O8+δ

The high-pressure structures of an underdoped cuprate superconductor Bi2Sr2CaCu2O8+δ have been studied by synchrotron X-ray diffraction at pressures up to 36.5 GPa. We find that this superconductor retains its orthogonal structure with the space group Amaa in the pressure range studied. Upon compression, both the a and b axes first shrink monotonically up to 17.4 GPa from their ambient pressure values and keep these behaviors with positive compressibilities up to 36.5 GPa after experiencing expansion with negative compressibilities in the pressure regime between 17.4 and 23.7 GPa. However, the c axis decreases continuously with increasing pressure with a slow change at about 23.7 GPa. The results indicate an isostructural phase transition starting at 17.4 GPa and a structural collapse at around 23.7 GPa.

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.

Correction to “Pressure-Induced Isostructural Metastable Phase Transition of Ammonium Nitrate”

Correction to “Pressure-Induced Isostructural Metastable Phase Transition of Ammonium Nitrate”

Compressibility anomaly in the superconducting material Nb3Al under high pressure

Abstract Nb 3 Al, which is widely used in high field magnets, was studied under a range of pressures up to 39.5 GPa using diamond anvil cell. The Nb 3 Al superconductor is structurally stable up to the highest pressure of the present investigation from previous reports. However, an anomaly of the compressibility beyond 19.2 GPa was detected in the pressure versus volume plot. The curve of volume versus pressure shows the existence of a plateau around 18.0 GPa as seen in several other highly correlated electrons systems, The observed pressure-induced isostructural phase transition was accomplished with a volume inclination without any symmetrical change (space group, Wyckoff position). The physical mechanism behind this isostructural phase transition is the interesting issue for further studies.

Synthesis and Structural Study of Sr2CuO3 + δ Superconductor under High Pressure

A single-phase Sr2CuO3 + δ superconductor is synthesized under high temperature and high pressure, in which oxygen atoms only partially occupy the apical sites next to the CuO2 planes and act as hole-dopants. The superconducting transition temperature with Tcmax = 75 K is achieved in the material. Structure analysis from x-ray powder diffraction data show that this material crystallizes into a K2NiF4 structure with tetragonal unit cell of a = 3. 795(3) Å and c = 12. 507(1) Å. Energy-dispersive synchrotron x-ray-diffraction studies at ambient are performed on powder samples of Sr2CuO3 + δ in a diamond-anvil cell at pressure up to 35 GPa. Anisotropic compressibility is found. Pressure-induced isostructural phase transition might exist as revealed by the discontinuous change of crystal cell volume V with pressure.

Pressure-induced isostructural phase transition of metal-doped silicon clathrates

We propose an atomistic model for the pressure-induced isostructural phase transition of metal-doped silicon clathrates, Ba8Si46 and K8Si46, that has been observed at 14 GPa and 23 GPa, respectively. The model explains successfully the equation of state, transition pressure, change of Raman spectra and dependence on the doped cations as well as the effects of substituting Si(6c) atoms with noble metals.

Organic-inorganic hybrid semiconductorZnSe(C2H8N2)1∕2under hydrostatic pressure

Organic-inorganic hybrid semiconductor $\mathrm{Zn}\mathrm{Se}{({\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})}_{1∕2}$ under hydrostatic pressure is studied using a first-principles pseudopotential method with mixed-basis set, aimed at understanding its structural, mechanical, and electronic properties. The bulk modulus of the hybrid is determined to be $35.3\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which is considerably smaller than that of bulk ZnSe and, in fact, smaller than most known tetrahedral bulk semiconductors. We find that, when the hybrid is exposed to pressure, the chemical bonds of the inorganic constituent, not those of the organic constituent, are significantly compressed. This is important and allows the band-edge electronic properties of hybrid chalcogenides to be effectively tuned by applying pressure, since these properties can be substantially modified only by changing the inorganic $\mathrm{Zn}\text{\ensuremath{-}}\mathrm{Se}$ bonds. Our calculations further demonstrate that the pressure dependence of the band gap in hybrid semiconductor shows an unusual nonlinearity, with a bowing coefficient that is more than one order of magnitude larger than in bulk ZnSe. Moreover, our results reveal that the in-plane electron mobility mass is notably small $({m}_{e}^{*}\ensuremath{\sim}0.27)$ and remains unchanged over a wide range of pressure. The hybrids will thus continue to maintain a fast electron mobility when they are under mechanical loading. However, and interestingly, the in-plane hole mass along the $x$ and $y$ direction is predicted to undergo at $2.14\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ a dramatic change from 0.35 to 2.2 and from 2.0 to 0.24, respectively. A pressure-induced isostructural phase transition is found to be responsible for this behavior of hole mass.

Pressure-induced structural phase transition in NdAl x Ga 2-x

High-pressure neutron-diffraction experiments on the pseudo-ternary compound NdAlxGa2-x revealed a pressure-induced isostructural phase transition for x=1.00 and 1.08 above about 1.5 GPa at room temperature. Within its hexagonal AlB2 structure the compound changes its lattice ratio c/a discontinuously from 0.97 to 0.84 in analogy to chemical pressure for x>1.12. Pressures up to 5.2 GPa did not result in a pure high-pressure phase but rather in a mixture of both phases. The intermetallic MN2 phases of the AlB2 structure are known to appear in two distinct groups with different c/a ratios. The observed isostructural transition is discussed within the framework of near-neighbor diagrams.

Pressure-Induced Isostructural Phase Transition in Al-Rich NiAl Alloys

We predict the existence of an isostructural first-order phase transition in high-pressure Al-rich NiAl alloys, between vacancy-rich and antisite-rich phases. A critical point terminates the region of phase coexistence. The driving force for this phase transition is the positive defect mixing enthalpy originating from elastic interaction between vacancies and antisites on the Ni sublattice. We highlight the similarity between the critical behavior of Al-rich alloys and that of binary liquid mixtures. Analogous isostructural phase transitions are expected in other compounds with constitutional defects.