“Chemical templates” stabilize metal superhydrogens and guide new material design

On November 9, 2022, Dr. Yuanhui Sun and Professor Maosheng Miao of California State University, Northridge, USA, published a research paper entitled “Chemical templates that assemble the metal superhydrides” in the journal Chem.

This report reports that a new bonding theory has successfully explained the stability of metal superhydrogenates and their changes to different metals, and found that there are hydrogen units with three-dimensional aromatics in a large number of structures, which provides a guiding scheme for further exploring the structure of new binary and multimetallic superhydrogens.

The first author of the paper is Sun Yuanhui; The corresponding author is Miao Maosheng.

Metal hydride superconductivity is one of the hot research directions in the field of superconductivity and high pressure. With the help of high-pressure technology, theoretical and experimental studies of binary metal hydrides have exhausted almost all metal elements, and many types of metal superhydrogen structures have been discovered. However, it is generally accepted that the chemical bonding between metals and hydrogen is not the chemical driving force of the hydrogen covalent network in stable structures. Traditional chemical bonds cannot explain the changing laws of superhydrogenic stability. For example, why are metals at the boundaries of the S-D zone of the periodic table more likely to form metallic superhydrogens and why do superhydrocarbons formed from different metals have the same structural prototype? Finding this mechanism to effectively understand these structures is of great significance for the stability and evolution of different metal superhydrides and them under different pressures, and for further exploring new binary and multimetallic superhydride materials.

Recently, Dr. Yuanhui Sun and Professor Maosheng Miao of California State University, Northridge proposed a new bonding theory to successfully explain the stability of metal superhydrogens and their changes to different metals. They split many of the reported metal superhydrogen structures into metal sublattice and hydrogen lattice (covalent network), and found that significant electrons already exist at the gap positions of these metal sublattices through electronic structure calculations. The position occupied by these electrons coincides exactly with the position of the hydrogen atom (or hydrogen unit) in the hydrogen lattice that is placed. Metal orbital wave function analysis shows that the electrons of the metal sublattice occupy the fixed orbital (quasiatomic orbital) at the void and have a similar position distribution in real space to the unoccupied state of the hydrogen lattice. Therefore, when the two sublattices of metal and hydrogen merge to form a compound, the electrons occupying the quasiatomic orbital of the metal sublattice naturally occupy the hydrogen lattice orbital and are shared by the two to a certain extent, thereby reducing the energy of the system. The authors name this chemical drive the “template effect.” Based on high-throughput first-principles calculations, the authors confirm that the chemical template effect is most significant for metals at the boundaries of the S-D region of the periodic table, and that the metals in the currently reported stable metal superhydrogenates also happen to come from this region. Since the 70s of the last century, many chemists have proposed the idea of understanding compounds in terms of anion insertion into metal matrices, and in this way explain the structural stability of many compounds. This work develops this semi-empirical view into a novel chemical bond theory based on quasiatomic fixed orbitals, and successfully explains the stability and structural laws of metal superhydrogens.

Based on this finding, the authors try to design new superhydrocompound structures by using metal lattices with strong “template effect” as the initial structure, and find that the template effect can greatly improve the efficiency of searching for new superhydrogenates. Using this method, they discovered and reported a Pbam SrH16 binary phase and a P63/mmc Sc3MgH24 ternary phase structure, with theoretical calculations predicting superconducting transition temperatures of 138 K and 40 K, respectively.

It is also worth mentioning that the aromatic hydrogen unit in the hydrogen lattice itself also promotes the stability of the structure. The aromatic functional groups in the traditional organic structure are composed of carbon bone structures. Its aromatism comes from the electron delocalization formed by the p-orbital conjugation of carbon atoms perpendicular to the direction of the C-C bond. To ensure the delocalization of the conjugate, the carbon atom needs to be in a plane and satisfy the 4n+2 rule. The latter ensures that the bonded state is filled by electrons and the anti-bonded state is completely empty. Unlike the p-orbitals of carbon atoms, the 1s electron bonding of hydrogen atoms does not have a specific directionality. The corresponding electronic delocalization does not need to be confined to a planar structure. Correspondingly, the lowest energy state of the 3D hydrogen unit does not meet the 4n+2 rule, but depends on the specific structure. The key is to achieve the lowest energy occupation mode of all bonded occupancy and anti-bonding non-occupancy. Through the analysis of different hydrogen units, the authors found that a large number of aromatic hydrogen units exist in the superhydrogen structure, including three-dimensional aromatic hydrogen units represented by H8 cubic units and H6 crown units.

The relevant calculations of this paper are completed by the material design software JAMIP developed by the team of Professor Zhang Lijun of the School of Materials Science and Engineering of Jilin University and the crystal structure prediction software CALYPSO developed by the team of Professor Ma Yanming of the School of Physics of Jilin University. Dr. Yuanhui Sun, the first author of this paper, was involved in the development of JAMIP.

Figure 1: Structure, stability, and bonding properties of metal superhydrogenates. (Image source: Chem)

Three typical metal superhydrogen structures: Im3m CaH6, Fm3m LaH10, and P63/mmc CeH9. The authors point out that electron transfer, atomic orbital bonding, etc. are not the main reasons for stabilizing the structure of metal superhydrogens.

Figure 2: Occupancy of electrons in metal lattice at quasiatomic orbitals. (Image source: Chem)

Electron local distribution plot of FCC La lattice in the structure of metal superhydrogenate LaH10, showing the presence of extreme distributions at its octahedral center (EO) and tetrahedral center (ET) positions. A deeper analysis of the orbital wave function found that the distribution of these extremes comes from the occupied state energy level of the electron, which is composed of the quasiatomic orbitals of the metal lattice. This is present in many stable metal superhydrogen structures.

Figure 3: Evolution of quasiatomic occupancy in a metal lattice. (Image source: Chem)

As the lattice parameters of the metal lattice decrease (external pressure rises), the electrons in the gap shift from a low-coordination position to a high-coordination position, and this transfer corresponds to a different lattice parameter for different metals. By comparing different metals of the same period in the periodic table, it is found that the electron occupancy at the lattice gap of the early metal is significantly stronger than that of the late metal. Interestingly, the authors found that when early metals such as Ca, Y, and La formed superhydrocompound structures, the corresponding metal lattices had large values at EO and ET positions; Late-stage metals, such as Al, do not have strong electron occupancy at these locations. The shadow part in Fig. 3A and 3B corresponds to the lattice variation range of the Fm3m CaH10 and Im3m CaH6 structures at 100-300 GPa pressure, and the shadow part in Fig. 3E and 3F corresponds to the lattice change range of the Fm3m AlH10 and Im3m AlH6 structures at 100-300 GPa pressure.

Figure 4: Constituent units of a hydrogen lattice in a superhydrogen structure. (Image source: Chem)

Through analysis, it is found that the hydrogen lattice is composed of many different types of hydrogen units. Some of the hydrogen units are aromatic, such as the H6 hexagonal, H8 cubic and H6 coronal units in the figure. Due to the presence of these aromatic hydrogen units (Figure 4D), the hydrogen lattice energy is significantly reduced. Among them, the hydrogen unit represented by H8 cubic and H6 crown unit is three-dimensional aromatic. Unlike the aromatism formed under the condition of conjugate π carbon atoms in the structure of organic matter, the aromatism of hydrogen units does not need to be confined to the plane, and its electron counting is different from the 4n+2 rule.

Figure 5: Metal template assists in the formation of hydrogen lattice. (Image source: Chem)

The metal lattice in metallic superhydrogens has electrons occupied at the gap position, and when the hydrogen lattice is placed back into the metal lattice, the hydrogen atom sits just at the extreme value of the electron occupation distribution. The analysis found that the electron occupied state energy level of the metal lattice and the unoccupied state energy level of the hydrogen lattice have the same position distribution in real space, so when the two form compounds, the electrons of the metal lattice at the hydrogen lattice position can be smoothly transferred to the hydrogen lattice, thereby reducing the energy of the system. Based on high-throughput first-principles calculations, the authors confirm that the chemical driving force is most pronounced for metals at the boundaries of the S-D region of the periodic table, and that the metals in the currently reported stable metal superhydrogen structure also happen to come from this region. The authors name this chemical drive the “template effect.”

Figure 6: Understanding and prediction of the structural stability of monometallic superhydrocarbons by the “template effect”. (Image source: Chem)

The “template effect” theory has a good ability to explain the preferred superhydrogen structure formed by different metals. When the quasiatoms at the gap positions of the metal lattice are strong, it is biased to form such a superhydrogen structure. Based on this conclusion, the authors used the metal lattice with a strong “template effect” as the starting structure, combined with the crystal structure prediction method, to find the possible stable metal superhydrogen structure by adding hydrogen atoms of different components. The result is a Pbam SrH16 structure that begins to stabilize above 186 GPa. In addition, based on this theory, potential stable structures can be quickly and accurately found that cannot be explored under conventional structure search settings.

Figure 7: Prediction of bimetallic superhydrogens by the “template effect”. (Image source: Chem)

By substituting metallic elements for the reported metal superhydrogen structure, a variety of bimetallic superhydrogen structures can be constructed. The “template effect” theory also has a positive guiding ability for its design. By comparing the changes of electron occupancy state in the lattice before and after element substitution with the corresponding change of formation energy, the authors found that the formation energy of the bimetallic superhydrogen structure with enhanced electron occupancy after replacement is mostly negative, which indicates that the increase of the “template effect” is conducive to stabilizing the bimetallic superhydrogen structure. Similarly, the authors also designed a P63/mmc Sc3MgH24 ternary phase structure with a formation energy of -0.54 meV/atom relative to the stable ScH6, MgH4, and H2 at 200 GPa, which is also undetectable under conventional structure search settings.

This study proposes a universal principle for understanding the structural stability of metal superhydrogens. The metal lattice with strong “template effect” can effectively reduce the formation energy of the hydrogen covalent network, thereby stabilizing the overall structure. With the help of this theory, using a metal lattice with a strong “template effect” as the precursor structure can effectively improve the efficiency of searching for superconducting hydride structures, especially the ternary and multivariate superhydrogen compound systems that are difficult to explore in first-principles calculations. The authors also point out that chemical template effects are widespread in many solid compounds. (Source: Web of Science)

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