Turnbull Robin, Sánchez Martín Josu, Liang Akun, Díaz-Anichtchenko Daniel, Popescu Catalin, Sandeep Rao K, Achary S Nagabhusan, Muñoz Alfonso, Panchal Vinod, Errandonea Daniel
Departamento de Física Aplicada-ICMUV, Universidad de Valencia, Dr Moliner 50, Burjassot, 46100 Valencia, Spain.
Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, EH9 3FD Edinburgh, UK.
Dalton Trans. 2024 Aug 27;53(34):14278-14288. doi: 10.1039/d4dt01886a.
The monoclinic polymorph of CoPO (space group 2/), isomorphic to farringtonite (MgPO) type orthophosphates, was studied up to 21 GPa using synchrotron powder X-ray diffraction and density-functional theory simulations to investigate the influence of pressure in the crystal structure. This study revealed a pressure induced structural phase transition for monoclinic cobalt phosphate, CoPO, and the details of crystal structure of the new high-pressure polymorph were delineated. The evolution of XRD pattern with pressure indicate that the onset of a phase transition occurs around 2.90(5) GPa, and the low- and high- pressure phases coexist up to 10.3(1) GPa. The high-pressure phase also has a monoclinic lattice (space group 2/), and a discontinuous change of unit-cell volume of about 6.5% occurs at the transition. A reorganization of atomic positions with a change in the cobalt coordination sphere occurs in the phase transition. Notably, the high-pressure polymorph has a defect-olivine-type structure like chopinite-type orthophosphates. Using a third-order Birch-Murnaghan equation of state, the bulk moduli of the low pressure (LP) phase (75(2) GPa) and high-pressure (HP) phase (92(2) GPa) were determined. For the low-pressure polymorph, the principal axes of compression and their compressibility were also determined. Density-functional theory calculations also provided the frequencies of Raman- and infrared-active modes which can be used for mode assignment in future experiments.
采用同步辐射粉末X射线衍射和密度泛函理论模拟，对与镁橄榄石（MgPO）型正磷酸盐同构的CoPO单斜晶型多晶型物在高达21 GPa的压力下进行了研究，以考察压力对晶体结构的影响。该研究揭示了单斜磷酸钴CoPO的压力诱导结构相变，并描绘了新型高压多晶型物的晶体结构细节。XRD图谱随压力的演变表明，相变起始于约2.90(5) GPa，低压相和高压相在高达10.3(1) GPa时共存。高压相也具有单斜晶格（空间群2/），在转变时单位晶胞体积发生约6.5%的不连续变化。在相变过程中，钴配位球发生变化，原子位置重新排列。值得注意的是，高压多晶型物具有类似氯磷灰石型正磷酸盐的缺陷橄榄石型结构。使用三阶Birch-Murnaghan状态方程，确定了低压（LP）相（75(2) GPa）和高压（HP）相（92(2) GPa）的体模量。对于低压多晶型物，还确定了压缩主轴及其压缩性。密度泛函理论计算还提供了拉曼活性和红外活性模式的频率，可用于未来实验中的模式归属。
