The author of the paper: Ma Yanming's research group's oxidation state reflects the chemical state of elements in compounds and their electronic gains and losses in the chemical reaction process, so it is closely related to the physical and chemical properties of substances
Compounds containing new oxidation states of elements usually have novel structures and electronic properties
Supercoordination is an important prerequisite for the formation of new oxidation states of elements
Therefore, the preparation of super coordination compounds has always been a frontier scientific issue in the field of condensed matter physics and chemistry
Among the halogens, iodine has a large atomic radius, weak electronegativity, and is easy to polarize
These properties make iodine different from other P-region elements in the formed compounds
In addition, the over coordinated iodine compounds can be used as environmentally friendly catalysts, and their catalytic ability is closely related to the oxidation state of iodine
At present, among the neutral iodine compounds, the highest coordination number of iodine is 7, and the representative compound is if 7
It is necessary to obtain electrons from the outside in the form of anions (if 8 -)
As a basic thermodynamic parameter, pressure can not only overcome the energy barrier in chemical reaction, but also make the electrons not involved in the reaction have chemical activity at atmospheric pressure
Recently, based on the first principle structure prediction method, Professor Ma Yanming's research group in the school of physics of Jilin University found that if 8, a stable supervalent cubic structure under high pressure, is an unprecedented electron deficient I-F bond molecule
Relevant research results were published in Chem
SCI
(DOI: 10.1039 / c8sc04635b)
The corresponding authors are Professor Ma Yanming of Jilin University, Professor Yang Guochun of Northeast Normal University and Assistant Professor Martin Rahm of chalms University of technology in Sweden
The research group of Professor Ma Yanming of Jilin University has made pioneering progress in the exploration of supervalent iodine molecules
Based on the Calypso structure prediction software independently developed by the research group, the author studied the high-pressure chemical reaction of iodine (I 2) and fluorine (F 2) in the environment of 0 K, and constructed the binary high-pressure phase diagram of I-F (Fig
1)
The results not only reproduce if x (x = 3,5,7) compounds at atmospheric pressure, but also find a series of if x (x = 8,10,11,12) compounds with novel ratio
It is worth noting that the highest coordination number of I is 8
In addition, the calculation results of phonon spectrum show that these structures are dynamic stable
Figure 1
High pressure phase diagram of I-F compounds relative to I 2 and F 2 monomers
(a) Energy convex hull diagram at 100, 200, 300 GPa
The hollow point represents the thermodynamic instability of compounds, while the solid point represents the stability; (b) the pressure stability range of I-F compounds
(source: chem
SCI.) at high pressure, if 3 was polymerized and a p 21 / M phase containing "Z-shaped" iodine chain was obtained
The distance between I-I and I-I in the iodine chain is 2.7 Å, which is close to the distance between I-I in the atmospheric pressure I 2 molecule, which means that I-I in the iodine chain is connected with strong covalent bond, which is conducive to obtain ultra-high conductivity in this direction
What's more exciting is the appearance of neutral r-3-if 8 molecule (Fig
2I) at 300 GPA, breaking the record of the highest coordination number of molecular compounds of iodine element of 7
At the same time, if 8 is a quasi cubic structure, which is different from the VSEPR theory that if 7 and if 8 - follow
Therefore, it is of great significance to explore the theoretical basis for the stable existence of if 8 molecules for the study of high coordination compounds
Fig
2
Stable lattice structure of I-F system
（ a ） Pnma -IF 3 ,0 GPa； （ b ） P 2 1 /m -IF 3 , 100 GPa； （ c ） C 2/ c -IF 5 ,0 GPa； （ d ） P- 1-IF 5 , 100GPa； （ e ） Pmmn -IF 5 , 200GPa； （ f ） Aea 2-IF 7 , 0 GPa； （ g ） P 2 1 /m -IF 7 , 100 GPa； （ h ） P- 1-IF 10 , 200 GPa； （ i ） R -3-IF 8 ,300 GPa； （ j ） P- 1-IF 11 , 300GPa； （ k ） P- 1-IF 12 , 300GPa 。 (source: chem
SCI.) in order to explore the stability mechanism of if 8, the authors studied the pressure-induced changes of atomic orbital energy levels
At atmospheric pressure, the energy level of I 5D orbital is 8.2ev higher than that of 5P, which usually does not affect the chemical properties of I in I-based compounds, but the 5D energy level of I changes obviously at high pressure
In order to study the effect of I 5D orbitals on the stability of if 8 molecules with cubic structure, we simulate the molecular orbitals (MO) combined with I and F 8 cages with cubic structure to describe the interaction between I 5S, 5p and 5D orbitals and f 2p orbital energy levels (Fig
3)
The results show that with the increase of pressure, the overlapping degree of I and f atomic orbitals increases continuously, the energy of T 2G and e g molecular orbitals of I-F bond in if 8 decreases continuously because of being occupied by electrons, and the cubic if 8 molecule is stabilized together with the decreasing a 1g and t 1U orbitals
Therefore, the decrease of I 5D orbital energy levels related to t 2G and e g plays an important role in the stability of if 8
Fig
3
Molecular orbital diagram: I 5S, 5p, 5D and f 2p interaction (source: chem
SCI.) in order to further explore the electronic properties of if 8 molecule, the author calculated the band, PDOS and Cohp of if 8 molecule (Fig
4)
The Cohp integrals of the nearest (F1 and F2) and next nearest (F3) I-F bonds below Fermi level are - 6.4ev, - 6.2ev and - 0.7eV, respectively, indicating that if 8 is a molecular crystal
In Fig
4b, PDOS near Fermi level shows that i5s, 5p, 5D orbitals are hybridized with F2P orbitals
The negative Cohp below Fermi level in Figure 4C clearly shows that the interaction between i5d-f2p can effectively stabilize the molecular structure of if 8
However, the Cohp above Fermi level shows that there is an anti bond state between i5s-f2p and f2p-f2p, which is completely consistent with the Mo analysis of Fig
3 if 8
In Fig
4d-e, the interaction between five I 5D orbitals and f 2p orbitals shows that more electrons may be occupied in the direction of I d Z 2, which may be the main reason why if 8 molecules change from cubic structure to quasi cubic structure
The metallicity of if 8 shown in Figure 4 (band and DOS Fermi levels are not zero) is mainly derived from the incompletely filled f 2p orbital
Figure 4
Electronic structure of r-3-if 8 at 300 GPa
（a） Energy band; (b) partial density of states; (c) COHP of I-F2 bond, positive value represents bonding state, negative value represents anti bonding state; (d) partial density of states of I 5D orbit; (E) COHP of I 5d-F 2p
(source: chem
SCI.) to sum up, the phase stability and structure of I-F compounds under high pressure were studied by first principles group structure search calculation
The pressure is propitious to the stability of if x (x = 3,5,7,8,10,11,12)
Some I-F compounds are predicted to undergo pressure-induced phase transition from molecular phase to condensed phase, along with the transition from semiconductor to metal
The author mainly analyzed if 8 of R-3 phase, which showed a unique cubic molecular structure, which is rare in the main group of element compounds
If 8 molecular crystal has electron deficient structure, which makes it have metal properties
Because I 5S, 5p and 5D orbitals are hybridized under high pressure, I is in the state of valence expansion, so if 8 is considered as supervalent molecule
The main reason for the stable existence of if 8 is that the I 5D orbital energy level drops to the bonding state
This achievement was published on chem
SCI
(DOI: 10.1039 / c8sc04635b)
The authors are Dongbao Luo +, Jian LV +, Feng Peng, yancao Wang, Guochun Yang *, Martin Rahm *, and Yang Ma *.
