Crystal structure of low-pressure and high-pressure (HP) structures of Fe(IO3)3. Both described by space group P63. FeO6 octahedra are shown in yellow and the coordination polyhedra of I atoms in purple. There is a coordination change at the phase transition. The I polyhedral units are shown separately to illustrate it. The zeolitic channels along the c-axis can be identified in the figure.
Cerdanyola del Vallès, 28th July 2020 The high-pressure behaviour of iron iodate – Fe(IO3)3 – have been studied up to 35 GPa by means of synchrotron powder X-ray diffraction and infrared microspectroscopy measurements, which have been combined with density-functional theory calculations. This is the first time an iodate is studied by high-pressure Fourier Transform Infrared (FTIR) microspectroscopy in particular in the Far-IR spectral range. It was found an evidence of the existence of a pressure driven phase transition in this iodate. Fe(IO3)3 shows a pressure-induced structural phase transition at 15-22 GPa.
This is the first time these techniques are used combined to study the high-pressure behavior of an iodate. This is also a first time an isostructural first-order transition has been found in this family of compounds.
This high-pressure phase can be described by the same space group (P63) than the low-pressure phase but with a substantial different c/a ratio. The pressure dependence of infrared and Raman phonon frequencies and unit-cell parameters have been obtained. A mode assignment is proposed for phonons based upon ab-initio calculations. The pressure dependence of unit-cell parameters for the two phases has been determined.
The discovered phase transition involves a large volume collapse and a change in the coordination polyhedron of iodine, being a first-order transition. It also produces substantial changes in the infrared and Raman vibrational spectra. The phase transition is related to the change of the activity of iodine lone pair electrons, which under compression prefer to link to oxygen atoms.
A successful combination of two synchrotron techniques
Synchrotron powder X-ray diffraction was employed to obtain the structure of the high-pressure phase at the Shanghai Synchrotron Radiation Facility (SSRF, BL15U1 beamline) using an x-ray wavelength of 0.6199 Å. FTIR micro-spectroscopy measurements were performed at MIRAS beamline of the ALBA Synchrotron. Calculations of the total energy were performed within the framework of the density functional theory and the projector-augmented wave (PAW) method as implemented in the Vienna ab initio simulations package (VASP).
The interest of this study that is the first experiment performed at MIRAS in extreme conditions (high pressure) using a bolometer detector optimized for operation in a range covering the Far-infrared spectral region (660-100 cm-1), illustrating the synergy of infrared microspectroscopy with other available techniques at ALBA, in particular at MSPD beamline, as a complementary analytical method. It has to be highlighted that infrared measurements at high pressure provide a non-intrusive method to get further structural insights on phase transitions and also to characterize the electronic properties of materials. These results will pave the way for further joint experiments using the two beamlines as complementary techniques in this field of research.
Metal iodates and high-pressure research
The family of metal iodates has been extensively studied at ambient pressure because of their dielectric, magnetic or nonlinear optical properties. Many of them have been also studied for the reason that they are superionic conductors. Such properties make them excellent barocaloric materials, very promising for the development of eco-friendly solid-state cooling technologies.
On the other hand, numerous iodates are fascinating because they have IO3 units with lone-pair orbitals, which give materials particular characteristics. High-pressure research is known to be an efficient tool to determine the characteristics of materials. In general, high-pressure reduces the interatomic distances in materials in a controlled manner, which results in significant changes of the physical and chemical characteristics. The discoveries in high-pressure research include multiple structural phase transitions which trigger interesting phenomena like metallization or superconductivity. However, the high-pressure behavior of metal iodates can be extremely complex and should be further studied for the better understanding of the high-pressure behavior of this family of compounds.
In particular, the interest on Fe(IO3)3 in this work comes from the fact described above, and also from the circumstance that its crystal structure contains zeolitic-like channels, which could lead to an interesting high-pressure behavior.
The study has been designed and coordinated by Daniel Errandonea from the University of Valencia in collaboration with Catalin Popescu from MSPD beamline and Ibraheem Yousef from MIRAS beamline of ALBA. The studied is part of the Ph.D. thesis of Akung Liang at University of Valencia. It involves the collaboration of A. Muñoz and P. Rodriguez-Hernandez from Universidad de la Laguna, who run computing simulations, S. Rahman and H. Saqib from Center for High Pressure Science and Technology Advanced Research (Shangai) and an industrial collaborator, G. Nénert, from Malvern Panalytical, who prepared the samples and contribute with X-rays diffraction.
Reference: Akun Liang, Saqib Rahman, Hajra Saqib, Placida Rodriguez-Hernandez, Alfonso Muñoz, Gwilherm Nénert, Ibraheem Yousef, Catalin Popescu, Daniel Errandonea. First-Order Isostructural Phase Transition Induced by High Pressure in Fe(IO3)3 Journal of Physical Chemistry C (2020). 124, 16, 8669–8679. DOI: 10.1021/acs.jpcc.0c02080.
