| As an important tool to study condensed materials, high pressure can shorten the atomic spacing of solid materials, effective change materials density, change the structure morphology and electronic states. Every material average appeared five phase transition under 100GPa. High pressure can provide five times new substances than the current materials, therefore using high-pressure we can obtain new phenomena and new rules which not exist in the atmosphere. High-pressure science has produced important role in energy materials, information material, the deep earth resource utilization, environmental protection and marine resources development fields, high pressure is becoming an important impetus for the develope high-tech industry. In recent years, structure phase transitions of materials and mechanism behavior research has become a very important high-pressure scientific research fields under the condition of extreme. High pressure synchrotron X-ray diffraction is one of the most direct and effective methods to investigate the structural phase transitions and states. High pressure Raman spectroscopy is one of the most convenient and powerful tools to study High-pressure phase stability and crystal structure. This paper using angle dispersive X-ray diffraction, Raman scattering technology with diamond anvil cell and the first principle theoretical calculation to study physical properties of several compounds such as A3N2(Mg3N2,Ca3N2和Zn3N2) and LnB6(CaB6,SrB6,BaB6,SmB6,NdB6 and GdB6) under high pressure and room temperature.In recent years, there caused widespread interest in rare earth sesquioxide (example:Sc2O3, Gd2O3, and Sm2O3 etc.) for scientists because which have a very rich structure transformations and the new physical phenomenons under high-pressure, studing high pressure phase transitions of rare earth sesquioxide have became one of the hot issue of high pressure scientific research. GroupⅡmetal nitrides crystallizes in the cubic anti-bixbyite (anti-C-type, space group:Ia-3) structure under ambient conditions. Therefore, there have some vital extremely significance to research high-pressure structure behaviors of group II metal nitrides for understanding the behaviors of rare earth sesquioxide. Combining in situ high pressure X-ray diffraction, Raman spectroscopy and ab initio calculations, we will further understand the phase changes of group II metal nitrides under high pressure.1. We firstly report the outcome of a high pressure study on anti-C-type Mg3N2 up to 40.7 GPa at room temperature using in-situ angle-dispersive X-ray diffraction measurement (ADXD). A high pressure phase with the monoclinic C2/m (anti-B-type) structure was observed for the first time in Mg3N2 at pressures higher than 20.6 GPa. In addition, our theoretical results predicted that there is a second phase transition from the anti-B-type to the hexagonal P-3m1(anti-A-type) structure at about 67 GPa. The caculated results show that the band gap energy of Mg3N2 crystals will be increased while the increasing of pressure, it also show that Mg3N2 has excellent optical properties under high pressure. The phase sequence observed in Mg3N2 is in accordance with the systematic behavior of C→B→A phase transitions occurring in most sesquioxides. In the process of the phase transition from cubic anti-C type to anti-B type structure, along with the destruction or shift between bonds and its anion and cation got to arrangement, the phase transformation should be first order heavy configuration structure transformation. The coordination number of N atoms is from 6 to 7 for the phase transition. These data can provides important knowledge reserves for our follow-up study to the other group II alkaline-earth metal nitrides.2. Because Ca atoms and Mg atoms belong to the same group, Ca3N2 and Mg3N2 have the same crystal structure under ambient, so we guess Ca3N2 should have the same high pressure phase sequence with Mg3N2. However, a recent theory article put forward an unusual high pressure phase transitions sequence to Ca3N2:anti-C→γ-Ca3N2→anti-B→anti-A→λ-Ca3N2-This sequence different from Mg3N2, and were not reported in the sesquioxide object. Therefore, experimental detection of high pressure phase transitions in Ca3N2 is quite necessary.We have undertaken a high-pressure study on the anti-C type Ca3N2 at room temperature by using in situ synchrotron angle-dispersive X-ray diffraction (ADXD) and Raman spectroscopy techniques in a diamond anvil cell. Two high pressure phase transitions from anti-C to anti-B and then to anti-A type structure were observed for the first time in Ca3N2. The phase sequence observed in Ca3N2 is in accordance with Mg3N2 and the systematic behavior of C→B→A phase transitions occurring in most sesquioxides. But the phase sequences observed in Ca3N2 don't agree with previous theory predicts, we give out several possible reasons for experimental and theoretical inconsistent.3. We first report the outcome of a high pressure study on anti-C-type Zn3N2 up to 43.2 GPa at room temperature using in situ angle-dispersive X-ray diffraction measurement (ADXD). A high pressure phase with the monoclinic C2/m (anti-B-type) structure was observed for the first time in Zn3N2 at 5.9 GPa.It is extremely importance to propose two new structures of Mg3N2, Ca3N2 and Zn3N2, it can provide foundation for further explore its physical properties and potential application.LnB6-type rich boron metal compounds with high melting point, high hardness, high strength, high chemical stability and the favourable electrical characteristics, these superior performance in various decided it has broad prospects for application in industrial fields. For example as additives for oxidation resistant and corrosion resistant material can improve oxidation resistance of carbon refractory and as an effective deoxidizer for the copper alloy, and used as nuclear protective materials and manufacturing thermal emission cathode. LnB6-type rich boron compounds have many special functions, such as metallurgy, absorbing materials and electromagnetic components etc, they show broad application prospect. So far the team at home and abroad research for group LnB6-type rich boron compounds confined to the interesting ambient conditions sample preparation and ground state property research aspects, however in situ high pressure research have few literatures in the research reports. In order to clear the structure phase stability of the group LnB6-type rich boron compounds in room temperature under high pressure, using in situ high pressure X-ray diffraction, and Raman spectroscopy in diamond anvil cell, we will further understand the phase changes of group LnB6-type rich boron compounds in room temperature under high pressure. Structure phase transitions and transformation mechanism research of LnB6-type rich boron compounds under extreme conditions, we are aimed at revealing their structure phase transitions rules, and exploring their potential applications under high pressure.1. We have undertaken a high-pressure study on the CaB6 at room temperature by using in situ synchrotron angle-dispersive X-ray diffraction (ADXD) and Raman spectroscopy techniques in a diamond anvil cell up to 40 and 28.1 GPa, respectively. High-pressure Raman experimental results show that the CaB6 occurring a pressure induced structural transformation at 12.5 GPa. Through the Raman experimental data analysis, we obtained change curve of the Raman shift with pressure, and calculated the pressure constant and Gruneisen parameter values of the different Raman vibration models of CaB6. Under the pressure, the atomic spacing of CaB6 decreased due to the compression effect, thus caused covalent bond enhancing, Raman peaks occurred blue shift. Through the Gruneisen parameter values, anharmonic effect of Alg symmetry vibration mode is the largest. It is precisely because the influences of different vibration mode have differences, which destroys the original symmetry of the crystal structure and led to the occurrence of phase transition. Based on the X-ray diffraction experimental data analysis, one first-order phase transformations were observed at 12.1 GPa accompanied by small volume collapses of 1%. The high pressure phase was identified as Orthorhombic (Pban) structures by Rietveld refinement. With the pressure derivatives fixed at 4, the bulk modulus of Cubic, and Orthorhombic structures were determined as 128.89 and 184.05 GPa, respectively, indicating the increased incompressibility of CaB6 under high pressure. The phase sequence observed in CaB6 is in accordance with the systematic behaviors of phase transition occurring in LaB6.2. Through the experimental data observed in SrB6,it occurred phase transition from cubic Pm-3m to Pban structure while 15.8 GPa. However, BaB6 didn't have phase transition under 31.1 GPa.3. We have undertaken a high-pressure study on the SmB6, NdB6, and GdB6at room temperature by using in situ synchrotron angle-dispersive X-ray diffraction (ADXD) in a diamond anvil cell. Conclusion is that their phase transition pressure is 6.5 GPa,8.3 GPa and 2.5 GPa, respectively. They have the same of the phase trantion sequence:cubic Pm-3m to orthogonal Pban. |
PDF Full Text Request