Structure Stability And Phase Transition In Metal Oxides A₂O₃(C-RES) And BO₃ Under High Pressure

As well known, to study high-pressure physics behavior is main including the state equation of the materials, various phase transition of the physics at high pressure and abundant of physical and chemical characters of substances at high pressure, etc. High pressure Raman spectroscopy is one of the most convenient and powerful tools to study hydrogen bonds and to investigate the conformations and dynamics of molecules in the condensed phase. High pressure synchrotron X-ray diffraction is one of the most direct and effective methods to investigate the structural phase transitions and states. A complementary insight into high-pressure behavior of materals is obtained by carrying out the ab initio calculations. Combining in situ high pressure X-ray diffraction, Raman spectroscopy and ab initio calculations, we will further understand the changes of materials under high pressure.In recent years, III metal sesquioxides have recently attracted particular attention because of their unique physical and chemical properties, and potential applications in nuclear engineering, light-triggered semiconductor devices, antireflection coatings, solar cells, field-effect transistors, barrier layer in tunnel junctions, sensing material in gas sensors, liquid crystal displays, and organic light emitting diodes, etc. In addition, studies on the polymorphs of rare-earth sesquioxides have become a hot topic in science, because it is crucial to understand the characters and formation of different phases of these metal sesquioxides systems. At ambience conditions, the molar volume of metal sesquioxides decreases in the transition C-RES→B-RES→A-RES with increasing cation coordination number. Hence the transition sequence of metal sesquioxides is expected to give similar sequential changes under high pressure. However, there are many reports which indicated the transition sequence of C-RES→B-RES→A-RES not to be simply applicable to all metal sesquioxides. Until now, the high-pressure behaviors of In₂O₃,Sc₂O₃,Tb₂O₃,Dy₂O₃,Ho₂O₃,Er₂O₃,Tm₂O₃ and Lu₂O₃ have received relatively less attention. In order to understand the structural behavior of In₂O₃,Sc₂O₃,Tb₂O₃,Dy₂O₃,Ho₂O₃,Er₂O₃,Tm₂O₃ and Lu₂O₃ under pressure and to verify the theoretical prediction and high temperature-high pressure data reported in the literatures, we expect to undertake a comprehensive investigation of In₂O₃,Sc₂O₃,Tb₂O₃,Dy₂O₃,Ho₂O₃,Er₂O₃,Tm₂O₃ and Lu₂O₃ at room temperature in DAC performing in situ synchrotron radiation x-ray diffraction. What is more, our works will be helpful for the understanding of physical properties of In₂O₃,Sc₂O₃,Tb₂O₃,Dy₂O₃,Ho₂O₃,Er₂O₃,Tm₂O₃ and Lu₂O₃ and give a basis for researching phase transition sequence of metal sesquioxides in the future.In recent years, significant interest has been focused on molybdenum oxide (MOO3) and related hydrated (MOO3·xH2O) for specific applications, such as battery electrodes, large–area display devices, gas sensor, etc. molybdenum trioxides can exist in five forms. It is of great importance to study the pressure-induced phase transformations among transition metal oxides polymorphs in order to establish the stable and metastable phase relations between different crystalline modifications, explore new phases of materials, and evaluate their production under different synthesis conditions. Raman spectroscopy is one of the most convenient and powerful tools to study hydrogen bonds and to investigate the conformations and dynamics of molecules in the condensed phase. In molybdenum trioxide hydrates, the cohesion between the layers is maintained by O–H…O hydrogen bonds. However, until now, there is no high pressure study on molybdenum trioxide hydrates x-ray diffraction. To establish the stable and metastable phase relations between different crystalline modifications and to obtain further information on weak O–H…O interactions in molybdenum trioxide hydrates, we study the MOO3·xH2O using their Raman spectra at various pressures.1. We first experimentally confirm static high pressure-induced structural phase transition of In₂O₃ by taking synchrotron x-ray diffraction (XRD) and Raman spectral measurements. A pressure-induced C-RES to hexagonal corundum-type phase transition starts at 12.8-15.3 GPa, which is very swift compared with the pressure-induced transition in shock-induced method. But the value significantly disagreed with earlier theoretical results. A bulk modulus B0=212.85±8.32 GPa at a fixed B0′=4.62 for corundum-In₂O₃ was estimated from the patterns obtained at high pressure. This value is slightly higher than theoretical data results. However, the broaden peaks of Raman spectra in high pressure phase indicate the phase is structurally disordered. These results should be helpful for understanding the physical properties of In₂O₃ and the future investigation of potential applications. In addition, high pressure phase of IIIA sesquioxides forms prior to the transition that increases the coordination number.2. We first have experimentally confirmed static high pressure induced structural phase transition of scandium oxide using in situ synchrotron radiation XRD and Raman spectra measurements up to 46.2 and 42 GPa, respectively. The pressure-induced phase transition from the cubic phase to a high pressure monoclinic phase occurred at 36 GPa, consistent with previously reported phase transition sequence by high temperature and high pressure method. The structural transitions suggested by variations in Raman parameters also support our XRD measurements. The pressure volume data of the new phase of Sc₂O₃ was analyzed using the Birch–Murnaghan equation of state. The zero pressure bulk modulus is B0=180(8) GPa and its pressure derivative is B0′= 4 for B-RES. Our complementary density-functional-theory calculations based on GGA confirm the experimental results and show a pressure-induced phase transition from C-RES to B-RESstructure. The calculations also suggest the occurrence of a second pressure induced phase transition from the B-RES to A-RES structure. These results should be helpful in evaluating their production under different synthesis conditions, and understanding the physical properties of Sc₂O₃.We first experimentally confirm static high pressure-induced structural phase transition of Tb₂O₃,Dy₂O₃,Ho₂O₃,Er₂O₃ and Tm₂O₃ by taking Raman spectral measurementsy. An irreversible structural transformation from the C-RES to A-RES high-pressure phase was verified at 9.3,13.4,13.2,19 and 20.5 GPa. We first experimentally confirm static high pressure-induced structural phase transition of Lu₂O₃ by taking Raman spectra up to 23.6 GPa. We did not found any phase transition. After release of pressure, the high-pressure phase transformed to a monoclinic structure.3. We first give a basis for researching phase transition sequence of C-RES sesquioxides under room temperature and high pressure. As a general trend, the volume of C-RES sesquioxides difference increases monotonically with increasing radius of cation. All Raman spectra are characterized by the presence of a very strong band in the range 330–420 cm?1, depending on the radius of cation of each oxide.The Raman strongest band of C-RES sesquioxides decreases monotonically with increasing radius of cation.4. We first performed in situ high-pressure, Raman spectrum and synchrotron XRD studies of molybdenum trioxides up to 30 GPa and 43 GPa, respectively. The goal was to compare the pressure behavior of the MOO3 with that of the previous work, in which they claimed that pressure alone would not bring about the transition fromα-MoO3 to MOO3-II. However, we observed the MOO3 has three different phases upon compression to 43 GPa. A phase transition from orthorhombicα-MoO3 into monoclinic modification of MOO3-II structure was observed around 12 GPa, indicating pressure alone did bring about the phase change in disagreement with the previous claims. In addition, a P21/c space group is proposed for the monoclinic cell for a second new phase, MOO3-III, around 25 GPa. The structural transitions suggested by variations of Raman parameters, which is also supported from recent XRD measurements. The pressure-volume data of the two new molybdenum trioxides phases were analyzed using the Birch-Murnaghan equation of state. The zero pressure bulk modulus was B0=48.25(1) GPa and its pressure derivative B0′=7 for theα-MoO3. The parameters for the two high-pressure polymorphs are B0=143.41(3) GPa, B0′=12 and B0=261.93(3) GPa, B0′=3.5.5. We first performed in situ high-pressure Raman spectrum studies of MOO3·12 H2O,α-MOO3·H2O,β-MOO3·H2O and MOO3·2H2O. In this work, we obtained a useful correlation of the O–H stretching frequencies with that of the O–H…O hydrogen bond parameters, which is associated with O–H unit's hydrogen atom of water molecule and the oxygen atom of MoO6 or MoO5(OH2) interactions in molybdenum trioxide hydrates. Our results show that MOO3·12 H2O,β-MOO3·H2O and MOO3·2H2O undergo reversibility structural phase transitions under high pressures. The frequencies of O–H stretching band display redshift with the pressure increasing. This phenomenon should be related to the same mechanism of O–H…O weak hydrogen bonds. The transition ofβ-MOO3·H2O and MOO3·2H2O is close to the soft phase transiton as suggested by the softening of lattice and stretching vibration ofνOMo2 modes around 700 cm-1.