Angle-dispersive X-ray Diffraction Research Articles

Evolutions of structural and optical properties of lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub> under high pressure

In recent years, organic-inorganic hybrid perovskite materials have been widely used in solar cells, photodetectors, and light-emitting diodes due to their advantages such as high light absorption coefficient, good carrier mobility, and long carrier diffusion length. However, the high toxicity of lead and poor stability still restrict the application and promotion of such materials. The lead-free double perovskite material derived from the concept of “heterovalent substitution”, while maintaining the high symmetrical structure of perovskite, avoids using the toxic lead elements, which has the advantages of environmental friendly, stable structure, and suitable band gap. At present, the limited research on lead-free double perovskite materials still leaves a big room to researchers, and such a limited research seriously restricts the development and promotion of such materials. Therefore, the relationship between the structure and performance of lead-free double perovskite materials needs further exploring in order to provide theoretical basis for the practical application of such materials. Here in this work, the lead-free double perovskite material Cs<sub>2</sub>TeCl<sub>6</sub> is prepared by the solution method. The crystal structure and optical properties of the lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub> under high pressure are investigated by using diamond anvil cell combined with <i>in-situ</i> high-pressure angle-dispersive X-ray diffraction and ultraviolet-visible absorption technology. The results show that the crystal structure of Cs<sub>2</sub>TeCl<sub>6</sub> is not changed within the experimental pressure range of 0－50.0 GPa, and the structural symmetry of <i>Fm-</i>3<i>m</i> is still maintained, indicating the sample has good stability. The lattice constant and volume of Cs<sub>2</sub>TeCl<sub>6</sub> gradually decrease within the pressure range of 0－50.0 GPa. The volume and pressure of Cs<sub>2</sub>TeCl<sub>6</sub> are fitted using the third-order Birch-Mumaghan equation of state, the bulk elastic modulus is obtained to be <i>B</i><sub>0</sub> = (18.77 ± 2.88) GPa. The smaller bulk elastic modulus indicates that the lead-free double perovskite material Cs<sub>2</sub>TeCl<sub>6</sub> has higher compressibility. The optical band gap of Cs<sub>2</sub>TeCl<sub>6</sub> is 2.68(3) eV at 1 atm and its optical band gap gradually decreases with the increase of pressure, which is related to the shrinkage of octahedral [TeCl<sub>6</sub>]<sup>2–</sup> under high pressure. The calculation results show that the Cs<sub>2</sub>TeCl<sub>6</sub> possesses an indirect band gap, the valence band maximum is mainly composed of Cl 3p orbits, and the conduction band minimum is mainly composed of Te 5p and Cl 3p orbits. After the pressure is completely relieved, Cs<sub>2</sub>TeCl<sub>6</sub> returns to the initial state. The above conclusions further deepen the understanding of the crystal structure and optical properties of lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub>, and provide a theoretical basis for designing and optimizing the lead-free double perovskite materials.

Competing Interactions: Evolution of Inter and Intramolecular Hydrogen Bonds in Salicylic Acid at High Pressures.

Benzoic acid derivatives are important molecular systems in the pharmaceutical industry. Salicylic acid is distinct among the derivatives of benzoic acid due to the presence of an intramolecular hydrogen bond. With a view to study the evolution of inter and intramolecular hydrogen bonding at shorter length scales, in situ high pressure Raman spectroscopic measurements, angle dispersive X-ray diffraction experiments, and density functional theory (DFT) based first principle calculations have been carried out on crystalline salicylic acid. Subtle structural modifications are noted across ∼1 GPa leading to structural phase transition to a new crystalline phase above 7 GPa which is reversible. Substantial softening of the OH stretching Raman mode associated with intramolecular hydrogen bond is observed prior to the transition pressure. Possible molecular configurations associated with tautomerization are discussed with the aid of DFT calculations.

Abnormal physical behaviors of hafnium diboride under high pressure

Hafnium diboride (HfB2) is one of the most promising hard-brittle ceramic materials with unique physical properties. Here, we have synthesized the well-crystallized HfB2 by a high-pressure solid-state reaction and employ in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction to investigate the size-effect of HfB2. An abnormal physical behavior of HfB2 under high pressure is observed. The microsized HfB2 shows slight anisotropy along a and c axes; however, the nanosized HfB2 reveals a different compression behavior with pressure-induced shell structural transition from a disordered symmetric amorphous shell state to an asymmetric amorphous shell state. In addition, the results indicate that the pressure calibrations are sensitive to the grain size. The present findings offer insights into the physical behaviors of different sized HfB2, which may also provide valuable information for other transition metal borides under high pressure.

High-pressure structural investigations on InPO4

High-pressure Raman scattering and angle dispersive x-ray diffraction investigations have been carried out on Indium phosphate (InPO4). Our experimental studies show that ambient Cmcm phase of InPO4 remains stable up to ~10 GPa. On the further increase of pressure to ~12 GPa, diffraction patterns show the signature of structural phase transformation. Rietveld refinements of diffraction patterns suggest that at high-pressure both the zircon (Space Group I41/amd) and scheelite (Space Group I41/a) type tetragonal phases are present along with the ambient orthorhombic Cmcm phase. In both the zircon and scheelite phases, the indium atoms become eight coordinated with respect to the oxygen atoms while phosphorous continues to be four coordinated.

Pressure-induced structural transitions and metallization in hollow ZnMn2O4 microspheres

High-pressure in situ angle dispersive X-ray diffraction (ADXRD) measurements combined with first-principles calculations were performed on hollow ZnMn2O4 microspheres up to 43.5 GPa at room temperature, and two high-pressure phases emerged at 15.9 GPa. Phase III was assigned to the CaMn2O4 type structure (space group Pbcm). It is supposed that phase II probably has a MgAl2S4 inverted spinel type structure (space group Imma). Moreover, phase I, phase II and phase III were probably coexisted when pressure was released. The compressional behaviors of phase I (space group I41/amd) and phase III were all determined. Besides, based on electronic band structure calculations, it was uncovered that phase I probably experiences a pressure-induced electronic topological transition (ETT) to a semimetallic state upon pressure, and phase III is a semimetallic state. Our results show that pressure play a dramatic role in tuning ZnMn2O4’s crystalline structures, electrical behavior and spin-state.

Pressure-temperature phase diagram and thermoelastic behavior of manganese fluoride up to 13.1 GPa and 700 K

Studying manganese fluoride (MnF2) under high-pressure and high-temperature (HP-HT) conditions is of great significance for understanding its important magnetic and piezomagnetic properties. The structural evolution and thermoelastic behavior of MnF2 were studied at HP-HT conditions up to ∼13.1 GPa and ∼700 K based on in situ synchrotron angle dispersive powder x-ray diffraction (XRD) combined with diamond anvil cell (DAC). Four phases of MnF2 are successfully observed through the pressure-temperature (P-T) range of the experiment. The structural phase transitions are with the following sequence: rutile-type (P42/mnm) → α-PbO2-type (Pbcn) → Pm → ZrO2-type (Pbcm) → SrI2-type (Pbca) at HP-HT conditions, where the α-PbO2-type (Pbcn) and the Pm structures are observed upon decompression. A phase diagram of MnF2 is presented in this study. In addition, the bulk modulus for the SrI2-type structure (Pbca) of MnF2 is 84 (3) GPa when fixed K0′ = 4. Moreover, we also obtain (∂K/∂T)P = −0.012 (7) GPaK−1, and the volumetric thermal expansion coefficient α0 = 3.8 (7) × 10−5 K−1 at 300 K. Furthermore, the thermal expansion coefficients for the rutile-type structure (P42/mnm) of MnF2 by fitting the temperature-volume data to the Fei-type thermal equation of state (EoS) are also obtained. The volumetric thermal expansion coefficient of MnF2 is 0.87 × 10−5 K−1, and its axial thermal expansivities are αa0 = 0.13 × 10−5 K−1 and αc0 = 0.64 × 10−5 K−1 along the a-axis and c-axis, respectively.

Pressure-Induced Emission (PIE) and Phase Transition of a Two-dimensional Halide Double Perovskite (BA)4 AgBiBr8 (BA=CH3 (CH2 )3 NH3 + ).

Two-dimensional (2D) halide perovskites have attracted significant attention due to their compositional flexibility and electronic diversity. Understanding the structure-property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure-induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA)4 AgBiBr8 (BA=CH3 (CH2 )3 NH3 + ). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the band gap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction data indicates that the observed PIE can be attributed to the emission from self-trapped excitons. This coincides with [AgBr6 ]5- and [BiBr6 ]3- inter-octahedral tilting which cause a structural phase transition. High-pressure study on (BA)4 AgBiBr8 sheds light on the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.

Monoclinic-tetragonal-monoclinic phase transitions in Eu0.1Bi0.9VO4 under pressure

The promising technological material Eu0.1Bi0.9VO4, has been studied for the first time at room-temperature under high-pressure, up to 24.9 GPa, by means of in situ angle dispersive powder x-ray diffraction (XRD). The compound undergoes two phase transitions at 1.9 and 16.1 GPa. The first transition is from the monoclinic fergusonite-type structure (space group I2/a) to a tetragonal scheelite-type structure (space group I41/a), being a ferroelastic-paraelastic transformation similar to that previously reported for isomorphic pristine BiVO4. The second phase transition is first-order in nature. The scheelite-type and the second high-pressure phase coexist in a wide pressure range. A monoclinic structure (space group P21/n) is proposed for the second high-pressure phase. Both transitions are reversible upon decompression. Details of the different crystal structures are reported. All the three observed structures are composed of network of VO4 tetrahedra and BiO8 (or EuO8 due to the substitution of Bi by Eu) dodecahedra. The room-temperature P–V equation of state and axial anisotropic compressibilities of the fergusonite and scheelite polymorphs are also given. In particular, the isothermal compressibility tensor for the monoclinic fergusonite phase has been calculated.

Operando Energy Dispersive X-Ray Diffraction (EDXRD) of Bulk Cathode Material for Multivalent Secondary Batteries

Lithium-ion batteries have been at the forefront of secondary battery technology for the last decade and a half, being the preferred means for powering almost all small electronics. The characteristics of lithium-ion batteries (high energy density, high power) are extremely well suited to these applications. However, for many emerging applications such as solar and wind power backup, other characteristics are desired, including high safety, low toxicity, and low cost. It is for these reasons that new alternatives to lithium-ion must be explored. Energy dispersive X-ray diffraction (EDXRD) from a high energy source allows the evolution of materials to be tracked from deep within large specimens, due to (a) the use of highly penetrating X-rays and (b) the ability to define a well-controlled diffraction gauge volume in space.1,2 This non-destructive characterization tool lends itself to the study of batteries, whose electrodes are generally composites of compressed particles, with electrolyte filling the pore space, and a sealed containment surrounding the cell designed to be as compact as possible. Using EDXRD, is it possible to study the structure of the compressed particles that make up these electrodes. Normal angle dispersive X-ray diffraction (ADXRD) is not capable of probing past the sealed containers of batteries making it mandatory to take out the active material of the battery to be analyzed. This means it is impossible to view the materials in disequilibrium states and risks exposing the sample to atmospheric gas or other contaminants. In contrast, EDXRD uses X-rays of high enough energy to pervade into the bulk material during charge/discharge cycles making it possible to observe dynamic crystallographic changes of the active material. EDXRD opens new doors into the ability to correlate voltage and current data with structure to better understand how a material behaves during chemical reactions. Specifically, it offers the opportunity to gain further insight into mechanism of intercalation of working cations into anionic frameworks such as the Mo6S8 chevrel phase. Uniquely, this phase has been shown to intercalate divalent cations such as Zn2+ and even trivalent ions such as Al3+. This makes it an important material of study. However, the discharge products of these batteries are exceedingly susceptible to oxidation, making ex situ XRD incredibly difficult. It is known that two second order phase transitions are expected during intercalation of a guest ion A into the host chevrel: Mo6S8 → AMo6S8 → A2Mo6S8.3 Figure 1 shows operando XRD data of Zn2+ inserting into the chevrel phase. The peaks initially around 2θ = 30.5°, 2θ = 34° and 2θ = 41.7° on the red curve (bottom curve) show two distinct shifts or steps. This supporting a two-step phase transition mechanism . Further experiments can shed new light onto how the effects of different working ion and doping of the chevrel phase wit selenium in place of sulfur can modify the charge/discharge characteristics of secondary batteries using the chevrel phase as a cathode material. Figure 1: Operando XRD data of the chevrel phase during Zn2+ intercalation. References M. Croft, V. Shukla, E. K. Akdogan, N. Jisrawi, Z. Zhong, R. Sadangi, A. Ignatov, L. Balarinni, K. Horvath and T. Tsakalakos, J Appl Phys, 105 (2009).M. Croft, V. Shukla, N. M. Jisrawi, Z. Zhong, R. K. Sadangi, R. L. Holtz, P. S. Pao, K. Horvath, K. Sadananda, A. Ignatov, J. Skaritka and T. Tsakalakos, Int J Fatigue, 31, 1669 (2009).E. Levi, E. Lancry, A. Mitelman, D. Aurbach, G. Ceder, D. Morgan and O. Isnard, Chem Mater, 18, 5492 (2006). Figure 1

Study of the compression behavior and elastic properties of HfB2 ceramics using experimental method and first-principles calculations

In-situ high pressure synchrotron angle-dispersive x-ray diffraction (AXRD) studies have been performed for HfB2, an ultra high temperature refractory alloy with AlB2 structure, by using a diamond anvil cell (DAC) at room temperature. The mechanical constants (bulk modulus K, shear modulus G, Young's modulus E, Poisson's ratio ν, density ρ, and Debye temperatures ΘD) of bulk HfB2 are measured by ultrasound experiments at room temperature. Furthermore, the compression behavior and elastic properties of HfB2 under pressure are investigated using density functional theory. The experimental results are in good agreement with the theoretical calculations. The pressure dependence of mechanical constants and elastic anisotropy of HfB2 are discussed. The experimental data and theoretical calculations results show that HfB2 has a distinct anisotropic compression behavior along the a- and c-axes, and the anisotropy increases as the pressure is increasing. These markedly elastic properties are attributes to the strongly directional bonding between Hf and B atoms.

Effect of high pressure on the typical 2D hydrogen-bonded crystal azodicarbonamide

We have performed in situ Raman scattering and synchrotron angle-dispersive X-ray diffraction (ADXRD) investigations to explore high-pressure behaviors of azodicarbonamide (C2N4O2H4, ADC) to 21.6 and 23.8 GPa, respectively. ADC exhibits the representative two-dimensional (2D) hydrogen-bonded networks, and is the most hugely used foaming agent in industry both under ambient and high pressures. Careful identification of external modes and indexation of Bragg diffraction peaks under different pressures demonstrate ADC crystal remains the P21/c symmetry in this study. The bulk modulus (B0) and its pressure derivative (B0’) are determined to be 13.2(7) GPa and 8.2(2) by fitting the isothermal third Vinet equation of state (EOS). The conformation change of ADC molecule has been observed at 12.7 GPa. And this is evidenced by the splitting of skeleton atoms vibrations and the discontinuous evolutions with respect to hydrogen bond donor (NH) vibrations. First principle calculation reveals the observed confirmation change arises from the discontinuous variation of the torsion angel between H2NC=O and CN=NC skeleton groups. Hirshfeld surface analysis indicates that the extensive hydrogen bonds within 2D networks dominate the intermolecular close contacts even under high pressures. The intermolecular interaction energy calculation also implies that the energy between neighboring molecules in the 2D layer dominates the networks stability, whereas the energy between neighboring molecules in the second nearest adjacent layer is the primary factor for crystal instability. The cooperativity of the molecular flexibility and directional hydrogen bonds is responsible for structural stability under high pressures.

Measurement of residual stresses in titanium alloys using synchrotron radiation

The residual stress distribution due to laser peening without coating (LPwC) was measured by using the synchrotron radiation X-rays from BL-11 at INDUS-2, RRCAT, Indore, India. This was accomplished by using the effective and quick method of angle-dispersive X-ray diffraction (AD-XRD). LPwC was performed on the titanium TC6 and Ti-6Al-4V alloys with a laser wavelength of 1064 and 532 nm and samples were peened with three different laser power densities of 3, 6 and 9 GW cm−2 and subjected to multiple impacts (1/3/5). Followed by that, the residual stress measurements using synchrotron radiation beam energy (SRBE) of 20 and 30 keV were carried out. The results show that LPwC induces higher and deep residual stress depending upon the titanium alloy. The change of residual stress with laser power densities and impact times are calculated and discussed. Although a compressive residual stress (CRS) was observed in majority of the cases, the more detrimental tensile residual stress (TRS) was observed in few cases only, especially with the TC6 alloy. TC6 showed TRS with peening radiations of 532 nm (power density of 3 GW cm−2) and 1064 nm (power density of 9 GW cm−2). Besides, TRS was observed in multiple peened samples (5 times) at both radiations. In each of the instances the residual stress was measured at SRBE of 20 keV. It is suggested that for beneficial CRS, peening may be employed in conditions other than these.

Structural behavior of a stuffed derivative of α-quartz, Mg0.5AlSiO4, at high temperature: an in situ synchrotron XRD study

High-temperature structural behavior of a stuffed derivative of α-quartz, Mg0.5AlSiO4, has been investigated using in situ synchrotron-based angle-dispersive powder X-ray diffraction (XRD) from 299 to 1273 K. Rietveld analysis of the XRD data indicates that the framework of Mg0.5AlSiO4 remains isostructural with α-quartz throughout the temperature range tested. As in α-quartz, unit-cell parameters a and c and cell volume V of Mg0.5AlSiO4 increase with increasing temperature, primarily due to progressive tilting of [(Al,Si)O4] tetrahedra along the a axes. However, the rates of increase in the cell parameters and the rate of decrease in the tetrahedral tilt angle (δ) are much smaller for Mg0.5AlSiO4 than for α-quartz. This behavior can be attributed to the occupancy of Mg2+ over the octahedral channel sites in the α-quartz-type framework, effectively hindering the [(Al,Si)O4] tetrahedral tilting. As a result, the α- to β-quartz phase transformation, which exists in silica at 846 K, does not occur in Mg0.5AlSiO4 up to 1273 K, and probably beyond, to its melting point.

Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin Bromide Perovskites.

Low-dimensional halide perovskites easily suffer from the structural distortion related to significant quantum confinement effects. Organic tin bromide perovskite C4N2H14SnBr4 is a unique one-dimensional (1D) structure in which the edge sharing octahedral tin bromide chains [SnBr42-]∞ are embraced by the organic cations C4N2H142+ to form the bulk assembly of core-shell quantum wires. Some unusual phenomena under high pressure are accordingly expected. Here, an intriguing pressure-induced emission (PIE) in C4N2H14SnBr4 was successfully achieved by means of a diamond anvil cell. The observed PIE is greatly associated with the large distortion of [SnBr6]4- octahedral motifs resulting from a structural phase transition, which can be corroborated by in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction spectra. The distorted [SnBr6]4- octahedra would accordingly facilitate the radiative recombination of self-trapped excitons (STEs) by lifting the activation energy of detrapping of self-trapped states. First-principles calculations indicate that the enhanced transition dipole moment and the increased binding energy of STEs are highly responsible for the remarkable PIE. This work will improve the potential applications in the fields of pressure sensors, trademark security, and information storage.

The experimental compression behavior of platinum hydride to 128 GPa

To date, the exact structure of PtH, the key to resolve the controversial hexagonal close-packed (hcp) phase of SiH4, is still absent above 100 GPa. Therefore, PtH has been synthesized and its compression behavior is determined through the angle dispersive X-ray diffraction method. We find that hcp PtH keeps stable up to at least 128 GPa and its experimental lattice parameters are highly consistent with those of the hcp one in the SiH4 experiment. The present results provide reliable experimental information of PtH under high pressure, and directly clarify the debate between hcp PtH and “SiH4”.

High-pressure study of a nanostructured SnSe1−xSx (x = 0.5) solid solution by in-situ X-ray diffraction and ab-initio calculations

A nanostructured solid solution of SnSe1−xSx with x = 0.5 synthesized by mechanical alloying and its structural stability was studied by calorimetric measurements under high pressure conditions. The structural behavior under pressure was investigated using angle-dispersive X-ray diffraction in a synchrotron source and first-principles density functional theory (DFT) calculations. The variations of the cell structure phases were described by a third-order Birch–Murnaghan equation of state.

High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO4 were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO4 transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO4 are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

Pressure induced phase transformations in diisopropylammonium bromide

Effect of high pressure on ferroelectric diisopropylammonium bromide (DIPAB) is investigated using Raman spectroscopy and angle-dispersive x-ray diffraction (XRD) measurements using a diamond anvil cell up to ∼10 GPa. Raman spectroscopic studies indicate that the monoclinic (m-II) ferroelectric phase of DIPAB is unstable under slight compression, leading to changes in molecular conformation. We have observed new peaks appearing in the lattice mode region and CH3 stretching region giving indication of conformational change. XRD investigations reveal that the m-II phase (space group P21) is partially transformed into a centrosymmetric, non-polar triclinic (P-1) structure at 0.7 GPa. Ferroelectricity is expected to weaken under compression above 0.7 GPa and ferroelectricity may disappear at a very high pressure when the m-II phase fully converted into triclinic P-1 phase. On releasing the pressure the ambient structure is fully recovered. The role of hydrogen bonds in the phase transformation is also discussed.

Microstrain-assisted polymorphic phase transitions in (Eu1−x La x )2O3

Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.

High pressure studies on α-cristobalite form of Al0.5Ga0.5PO4: hydrostatic versus non-hydrostatic conditions

ABSTRACTHigh pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al0.5Ga0.5PO4. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C2221 phase transforms to orthorhombic Cmcm phase at 4.9 GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19 GPa. The values of bulk modulus for C2221 and Cmcm phases are 19(2) and 126(4) GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C2221 phase to Cmcm phase has not observed up to 17.4 GPa. Instead, a new monoclinic phase P21 is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.