Zhang Xinyue, Li Luo, Yu Yingxin, Zhang Qingchun, Sun Ningyu, Mao Zhu, Zhang Dongzhou
Deep Space Exploration Laboratory/School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China.
CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei 230026, China.
ACS Omega. 2024 May 22;9(22):23675-23687. doi: 10.1021/acsomega.4c01347. eCollection 2024 Jun 4.
The phase transition of AF difluorides strongly depends on pressure, temperature, and cationic radius. Here, we have investigated the phase transition of three difluorides, including MgF, CaF, and BaF, at simultaneously high pressures and temperatures using Raman spectroscopy and X-ray diffraction in externally heated diamond anvil cells up to 55 GPa at 300-700 K. Rutile-type difluoride MgF with a small cationic radius undergoes a transition to the CaCl-type phase at 9.9(1) GPa and 300 K, to the HP-PdF-type phase at 21.0(2) GPa, and to the cotunnite-type phase at 44.2(2) GPa. The phase transition pressure to the HP-PdF and cotunnite structure at 300 K for our single crystal was found to be higher than that in previous studies using polycrystalline samples. Elevating the temperature increases the transition pressure from rutile- to the CaCl-type phase but has a negative influence on the transition pressure when MgF transforms from the HP-PdF- to cotunnite-type phase. Meanwhile, the transition pressure from the CaCl- to HP-PdF-type phase for MgF was identified to be independent of the temperature. Raman peaks suspected to belong to the α-PbO-type phase were observed at 14.6-21.0(1) GPa and 400-700 K. At 300 K, difluorides CaF and BaF in the fluorite structure with larger cationic radii transform to the cotunnite-type phase at 9.6(3) and 3.0(3) GPa at 300 K, respectively, and BaF further undergoes a transition to the NiIn-type phase at 15.5(4) GPa. For both CaF and BaF, elevating the temperature leads to a lower transition pressure from fluorite- to the cotunnite-type phase but has little influence on the transition to the NiIn structure. Raman data provide valuable insights for mode Grüneisen parameters. We note that the mode Grüneisen parameters for both difluorides and dioxides vary linearly with the cation radius. Further calculations for the mode Grüneisen parameters at high pressures for MgF, CaF, and BaF yield a deeper understanding of the thermodynamic properties of the difluorides.
氟化亚金属的相变强烈依赖于压力、温度和阳离子半径。在此，我们利用拉曼光谱和X射线衍射，在外部加热的金刚石对顶砧池中，于300 - 700 K温度及高达55 GPa压力下，研究了三种氟化亚金属（包括MgF、CaF和BaF）的相变。阳离子半径较小的金红石型氟化亚金属MgF在9.9(1) GPa和300 K时转变为CaCl型相，在21.0(2) GPa时转变为HP - PdF型相，在44.2(2) GPa时转变为方铅矿型相。我们发现，对于我们的单晶，在300 K时转变为HP - PdF相和方铅矿结构的相变压力高于以往使用多晶样品的研究结果。升高温度会增加从金红石型到CaCl型相的转变压力，但当MgF从HP - PdF型转变为方铅矿型相时，对转变压力有负面影响。同时，MgF从CaCl型到HP - PdF型相的转变压力被确定与温度无关。在14.6 - 21.0(1) GPa和400 - 700 K时观察到疑似属于α - PbO型相的拉曼峰。在300 K时，阳离子半径较大的萤石结构中的氟化亚金属CaF和BaF分别在9.6(3) GPa和3.0(3) GPa时转变为方铅矿型相，并且BaF在15.5(4) GPa时进一步转变为NiIn型相。对于CaF和BaF两者，升高温度都会导致从萤石型到方铅矿型相的转变压力降低，但对向NiIn结构的转变影响很小。拉曼数据为模式格林艾森参数提供了有价值的见解。我们注意到，氟化亚金属和二氧化物的模式格林艾森参数都随阳离子半径线性变化。对MgF、CaF和BaF在高压下的模式格林艾森参数进行进一步计算，能更深入地了解氟化亚金属的热力学性质。
