Structural phase transformations of Mg3N2 at high pressure : experimental and theoretical studies

The structural behavior of Mg₃N₂ has been investigated up to 40.7 GPa at room temperature by means of angle-dispersive X-ray diffraction. A reversible, first-order structural phase transition from the ambient cubic phase (Ia3̅) to a high-pressure monoclinic phase (C2/m) is found to start at ~ 20.6 GPa and complete at ~ 32.5 GPa for the first time. The equation of state determined from our experiments yields bulk moduli of 110.7(2) and 171.5(1) GPa for the cubic and monoclinic phases, respectively, indicating higher incompressibility of the high-pressure phase of Mg₃N₂. First-principles calculations reproduced the phase stability and transition pressure determined in our experiment. In addition, a second phase transition from the monoclinic phase to a hexagonal phase (P3̅m₁) was predicted around 67 GPa for Mg₃N₂. The electronic band structures of three phases of Mg₃N₂ are also calculated and discussed.

Pressure-induced phase transition in hydrogen-bonded supramolecular adduct formed by cyanuric acid and melamine

Single-crystal samples of the 1:1 adduct between cyanuric acid and melamine (CA·M), an outstanding case of noncovalent synthesis, have been studied by Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell up to pressures of 15 GPa. The abrupt changes in Raman spectra around 4.4 GPa have provided convincing evidence for pressure-induced structural phase transition. This phase transition was confirmed by angle dispersive X-ray diffraction (ADXRD) experiments to be a space group change from C2/m to its subgroup P2₁/m. On release of pressure, the observed transition was irreversible, and the new high-pressure phase was fully preserved at ambient conditions. We propose that this phase transition was due to supramolecular rearrangements brought about by changes in the hydrogen bonding networks.

High-pressure phase transition in cyclo-octane

Structural behaviour of cyclo-octane under high pressure is studied by using a synchrotron x-ray source in a diamond anvil cell (DAC) up to 40.2 GPa at room temperature. The cyclo-octane firstly solidifies to the triclinic phase at 0.87GPa. With the increasing pressure, the phase of cyclo-octane changes to the tetragonal phase at about 6.0 GPa and then transforms to amorphous phase above 18.2 GPa, which is kept till to 40.2 GPa. All the phase transitions of cyclo-octane are irreversible.

Nature of hardness evolution in nanocrystalline NiTi shape memory alloys during solid-state phase transition

Due to a distinct nature of thermomechanical smart materials' reaction to applied loads, a revolutionary approach is needed to measure the hardness and to understand its size effect for pseudoelastic NiTi shape memory alloys (SMAs) during the solid-state phase transition. Spherical hardness is increased with depths during the phase transition in NiTi SMAs. This behaviour is contrary to the decrease in the hardness of NiTi SMAs with depths using sharp tips and the depth-insensitive hardness of traditional metallic alloys using spherical tips. In contrast with the common dislocation theory for the hardness measurement, the nature of NiTi SMAs' hardness is explained by the balance between the interface and the bulk energy of phase transformed SMAs. Contrary to the energy balance in the indentation zone using sharp tips, the interface energy was numerically shown to be less dominant than the bulk energy of the phase transition zone using spherical tips.

Comparative studies of structural transition between AlN nanocrystals and nanowires

The structural transition of AIN nanocrystals and nanowires were investigated simultaneously under pressures up to 37.2 GPa by in situ angle dispersive high-pressure x-ray diffraction using synchrotron radiation source and a single diamond anvil cell. The size of hexagonal AIN nanocrystals and the diameter of nanowires are 45 nm on average. A pressure-induced wurtzite to rocksalt phase transition starts at 21.5 GPa and completes at 27.8 GPa for the nanocrystals and nanowires, respectively. The high-pressure behaviors of AlN nanocrystals the same as the AIN nanowires might arise from the similar size and diameter in nanocrystals and nanowires. Hexagonal AIN nanocrystals (45 nm) display an apparent volumetric contraction as compared to the AlN nanocrystals (10 nm) which might induce the difference of transition pressure.

Pressure-induced structural transitions in MOO3·xH2O (x = 1/2, 2) molybdenum trioxide hydrates : a raman study

The high-pressure behaviors of M_OO₃·1/2H₂O and M_OO₃·2H₂O have been investigated by Raman spectroscopy in a diamond anvil cell up to 31.3 and 30.3 GPa, respectively. In the pressure range up to around 30 GPa, both M_OO₃·1/2H₂O and M_OO₃·2H₂O undergo two reversible structural phase transitions. We observed a subtle structural transition due to O−H···O hydrogen bond in M_OO₃·1/2H₂O at 3.3 GPa. We found a soft mode phase transition in M_OO₃·2H₂O at 6.6 GPa. At higher pressures, a frequency discontinuity shift and appearance of new peaks occurred in both M_OO₃·1/2H₂O and M_OO₃·2H₂O, indicating that the second phase transition is a first-order transition. The frequency redshift of the O−H stretching bands of M_OO₃·1/2H₂O and M_OO₃·2H₂O are believed to be related to the enhancement of the O−H···O weak hydrogen bonds under high pressures.

Spectroscopic study of the effects of pressure media on high-pressure phase transitions in natrolite

Structural phase transitions in natrolite have been investigated as a function of pressure and different hydrostatic media using micro-Raman scattering and synchrotron infrared (IR) spectroscopy. Natrolite undergoes two reversible phase transitions at 0.86 and 1.53 GPa under pure water pressure medium. These phase transitions are characterized by the changes in the vibrational frequencies of four- and eight-membered rings related to the variations in the bridging T−O−T angles and the geometry of the elliptical eight-ring channels under pressure. Concomitant to the changes in the framework vibrational modes, the number of the O−H stretching vibrational modes of natrolite changes as a result of the rearrangements of the hydrogen bonds in the channels caused by a successive increase in the hydration level under hydrostatic pressure. Similar phase transitions were also observed at relatively higher pressures (1.13 and 1.59 GPa) under alcohol−water pressure medium. Furthermore, no phase transition was found up to 2.52 GPa if a lower volume ratio of the alcohol−water to natrolite was employed. This indicates that the water content in the pressure media plays a crucial role in triggering the pressure-induced phase transitions in natrolite. In addition, the average of the mode Grüneisen parameters is calculated to be about 0.6, while the thermodynamic Grüneisen parameter is found to be 1.33. This might be attributed to the contrast in the rigidity between the TO4 tetrahedral primary building units and other flexible secondary building units in the natrolite framework upon compression and subsequent water insertion.

High-pressure structural transitions of Sc2O3 by X-ray diffraction, raman spectra, and ab initio calculations

The high-pressure behavior of scandium oxide (Sc₂O₃) has been investigated by angle-dispersive synchrotron powder X-ray diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 46.2 and 42 GPa, respectively. An irreversible structural transformation of Sc₂O₃ from the cubic phase to a monoclinic high-pressure phase was observed at 36 GPa. Subsequent ab initio calculations for Sc₂O₃ predicted the phase transition from the cubic to monoclinic phase but at a much lower pressure. The same calculations predicted a second phase transition at 77 GPa from the monoclinic to hexagonal phase.

A study of phase behavior and conductivity of mixtures of the organic ionic plastic crystal N-methyl-N-methyl-pyrrolidinium dicyanamide with sodium dicyanamide

We report on the thermal, structural and conductivity properties of the organic ionic plastic crystal (OIPC) N-methyl-N-methyl-pyrrolidinium dicyanamide [C1mpyr][N(CN)2] mixed with the sodium salt Na[N(CN)2]. The DSC thermal traces indicate that an isothermal transition, which may be a eutectic melting, occurs at ~ 89 °C, below which all compositions are entirely in the solid phase. At 20 mol% Na[N(CN)2], this transition is the final melt for this mixture, and a new liquidus peak grows beyond 20 mol% Na[N(CN)2]. The III- > II solid-solid phase transition continues to be evident at ~- 2 °C. The microstructure for all the mixtures indicated a phase separated morphology where precipitates can be clearly observed. Most likely, these precipitates consist of a Na-rich second phase. This was also suggested from the vibrational spectroscopy and the 23Na NMR spectra. The lower concentrations of Na[N(CN)2] present complex 23Na MAS spectra, suggesting more than one sodium ion environment is present in these mixtures consistent with complex phase behavior. Unlike other OIPCs where the ionic conductivity usually increases upon doping or mixing in a second component, the conductivity of these mixtures remains relatively constant and above 10- 4 S cm- 1 at ∼ 80 °C, even in the solid state. Such high conductivities suggest these materials may be promising to be used for all solid-state electrochemical devices.

LiI-doped N,N-Dimethyl-Pyrrolidinium iodide, an archetypal rotator-phase ionic conductor

N,N-Dimethyl-pyrrolidinium iodide, and the effect of doping with LiI, has been investigated using DSC, NMR, and impedance spectroscopy. It was found that the addition of a small amount of LiI enhances the ionic conductivity by up to 3 orders of magnitude for this ionic solid. Furthermore, a slight decrease in phase transition onset temperatures, as well as the appearance of a superimposed narrow line in the ¹H NMR spectra with dopant, suggest that the LiI facilitates the mobility of the matrix material, possibly by the introduction of vacancies within the lattice. ⁷Li NMR line width measurements reveal a narrow Li line width, decreasing in width and increasing in intensity with temperature, indicating mobile Li ions.

High-pressure spectroscopic study of hydrous and anhydrous Cs-exchanged natrolites

Structural phase transitions in hydrous Cs-exchanged natrolite (Cs-NAT-hyd) and anhydrous Cs-exchanged natrolite (Cs-NAT-anh) have been investigated as a function of pressure and temperature using micro-Raman scattering and synchrotron infrared (IR) spectroscopy with pure water as the penetrating pressure medium. The spectroscopic results indicate that Cs-NAT-hyd undergoes a reversible phase transition around 4.72 GPa accompanied by the discontinuous frequency shifts of the breathing vibrational modes of the four-ring and helical eight-ring units of the natrolite framework. On the other hand, we observe that Cs-NAT-anh becomes rehydrated at 0.76 GPa after heating to 100 °C and then transforms into two distinctive phases at 2.24 and 3.41 GPa after temperature treatments at 165 and 180 °C, respectively. Both of these high-pressure phases are characterized by the absence of the helical eight-ring breathing modes, which suggests the collapse of the natrolite channel and formation of dense high-pressure polymorphs. Together with the fact that these high-pressure phases are recoverable to ambient conditions, our results imply a novel means for radionuclide storage utilizing pressure and a porous material.