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Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

2022; Elsevier BV; Volume: 3; Issue: 2 Linguagem: Inglês

10.1016/j.xinn.2022.100226

2666-6758

Xin Zhong, John S. Tse, Russell J. Hemley, Hanyu Liu,

Nuclear Materials and Properties

Main textThe search for room-temperature superconductivity has been one of the great challenges in condensed matter physics ever since the first observation of superconductivity with a critical temperature (Tc) of 4 K in mercury in 1911. In recent years, we have been witness to the successful observation of superconductivity in highly compressed SH3 (Tc = 203 K at 155 GPa) and classes of clathrate hydrides (e.g., CaH6, YH6, YH9, LaH10), with LaH10 holding a record high Tc of 250–260 K at ∼180 GPa among these binary hydrides (Figure 1).1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Each of these experimental studies was guided by theoretical predictions. New experimental work on the Ca-H system has finally confirmed the theoretical calculation that launched this effort, the 2012 prediction of a high-Tc superconductivity in CaH6 above 200 K.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar, 2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar, 3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google ScholarFirst-principles structure-search methods, which require only the chemical composition as input without prior information about the arrangement of atoms, can identify the thermodynamically stable and metastable structures of materials at any pressure and 0 K. Armed with these state-of-the-art methods, several major materials discoveries under pressure have now been documented, one of which is the observation of an anti-Wilson transition:4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar the prototypical free-electron metal of sodium transforming to a transparent semiconductor at megabar pressures, in violation of the nearly 1-century old concept that the metallic state is the highest pressure form of materials. Subsequent structure-search simulations led to major discoveries of pressure-stabilized hydrides with Tc approaching room temperature, realizing materials-by-design for this new class of materials.In addition to the advanced structure-search methods, a key role in leading to the discovery of these high-Tc superconducting materials is that the phonon-mediated Bardeen-Cooper-Schrieffer theory describes the high-Tc superconducting properties of these hydrides remarkably well. According to this theory, high critical temperatures may be realized by a large density state at the Fermi level, a high average vibrational frequency, and strong electron-phonon coupling. In this regard, it has long been believed that atomic metallic hydrogen, which was first pointed out by Ashcroft in 1968, would be a very-high-Tc superconductor, as it is predicted to exhibit the above electronic and vibrational properties.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar However, the pressures required to produce atomic metallic hydrogen are estimated to become extremely high (∼500 GPa), which creates a challenge for experimental characterization.An alternative approach was later proposed by Ashcroft in 2004, in which hydrogen-rich materials may exhibit "chemical precompression" that could reduce the onset pressure of metallic hydrogen-like superconductivity by forming dense hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar The structures of all high-Tc hydrides studied to date can be viewed as various clathrate-like structures. From a Zintl-Klemm description, the metal atoms donate their valence electrons and weaken the H2 molecules in these clathrate-like structures. Thus, the stability of these clathrate hydride structures can be considered to be a balance between the number of electrons available by charge transfer from the metal ions and the size of those metal ions. Electrons are shared to form connected networks of multi-center H … H … H bonds that resemble the bonding environment of atomic metallic hydrogen, and thus electronic properties such as high density of states at Fermi level, together with high characteristic vibrational frequencies and strong electron-phonon coupling.The high superconductivity observed in compressed earth/actinide hydrides has ushered in a new era of superconductivity research, while this distinct type of clathrate metal hydrides was first proposed for CaH6 using the crystal structure analysis by particle swarm optimization (CALYPSO) crystal structure prediction method in 2012.3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google Scholar Recent laser-heated diamond anvil cell experiments have confirmed this seminal prediction, with a Tc of 215 K at ∼170 GPa.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar X-ray diffraction measurements are consistent with the predicted clathrate structure of CaH6.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar The results demonstrate that high Tc above 200 K is indeed possible in clathrate hydrides formed from the main group, not only lanthanide/actinide, elements. These findings continue to demonstrate the role of structure search methods in this exciting new field.In 2015, a variety of Tc up to 203 K at ∼155 GPa in sulfur hydrides was measured for the first time and exhibited high-Tc above 200 K, thus sparking a flurry of interest in pressure-stabilized hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar In this experiment, the initial sample of SH2 has long been believed to dissociate into elemental hydrogen and sulfur at sufficiently high pressures. However, a theoretical study Led by Li and Ma in 2014 predicted two stable high-pressure structures, which were theoretically estimated to have the highest Tc of 80 K at megabar pressure.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Stimulated by this excellent prediction, researchers synthesized two distinct high-temperature superconductors. One sample was prepared at low temperature and measured to exhibit Tc from 30 to 150 K, in accordance with the predicted SH2 superconductor. The other sample was annealed at room temperature and measured to have unexpected high-Tc from 180 to 203 K, which was understood as the decomposition of SH2 into high-Tc SH3 and S, where SH3 is a known stoichiometry of (SH2)2H2 that has been synthesized at a pressure of 7 GPa and was predicted to be a 200 K superconductor at megabar pressures.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar This SH3, as a matter of fact, was not molecular, and was also viewed as having a clathrate structure with S encapsulated in the cages of the hydrogen framework.4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar These excellent findings demonstrate that the theoretical work plays a critical role in the experiments into high-Tc superconductors.There is now a focus on broader chemical compositions, starting with ternary systems. A ternary phase consisting of carbon, sulfur, and hydrogen was reported to have a Tc of 287 K at 267 GPa, but the composition and detailed structure at the highest critical temperature remain to be determined.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar There is growing evidence that the introduction of even small amounts of a third element can enhance the superconductivity of binary hydrides through hole or electron doping as shown for the C-S-H and the La-N-H systems. A pressing task is also whether one can design a material with relatively high Tc that is also stable at much lower pressures and even recoverable as a superconductor in ambient conditions. As shown in previous work, one of the key factors appears to spread favorable electron-phonon coupling over much of the vibrational spectrum in terms of these high-Tc hydrides.5Tanaka K. Tse J.S. Liu H. Electron-phonon coupling mechanisms for hydrogen-rich metals at high pressure.Phys. Rev. B. 2017; 96: 100502(R)Google Scholar Beyond hydrides, other likely candidates are compounds consisting of other light elements such as Li, Be, B, and C, which ensure an essential ingredient that the system has a relatively high characteristic frequency. The recently predicted LaBH8 and LaBeH8 show intriguingly dynamic stability below 50 GPa, despite being thermodynamically stable at or above approximately 100 GPa.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar These findings point to the potential of ternary hydrides as room temperature and "hot" superconductors under pressure as well as very-high-Tc materials that are stable at low pressures. It is reasonable to expect that new classes of high-Tc structures will be predicted and synthesized in the near future. To this end, recent advances in crystal structure prediction methods, data-driven machine learning techniques, and molecular dynamic simulations will undoubtedly accelerate the discovery of new and practical high-Tc superconductors, as well as foster a deep understanding of physical phenomena, such as the possible coexistence of proton quantum dynamics together with hydrogen-induced superconductivity in very-high-Tc hydrides. Main textThe search for room-temperature superconductivity has been one of the great challenges in condensed matter physics ever since the first observation of superconductivity with a critical temperature (Tc) of 4 K in mercury in 1911. In recent years, we have been witness to the successful observation of superconductivity in highly compressed SH3 (Tc = 203 K at 155 GPa) and classes of clathrate hydrides (e.g., CaH6, YH6, YH9, LaH10), with LaH10 holding a record high Tc of 250–260 K at ∼180 GPa among these binary hydrides (Figure 1).1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Each of these experimental studies was guided by theoretical predictions. New experimental work on the Ca-H system has finally confirmed the theoretical calculation that launched this effort, the 2012 prediction of a high-Tc superconductivity in CaH6 above 200 K.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar, 2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar, 3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google ScholarFirst-principles structure-search methods, which require only the chemical composition as input without prior information about the arrangement of atoms, can identify the thermodynamically stable and metastable structures of materials at any pressure and 0 K. Armed with these state-of-the-art methods, several major materials discoveries under pressure have now been documented, one of which is the observation of an anti-Wilson transition:4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar the prototypical free-electron metal of sodium transforming to a transparent semiconductor at megabar pressures, in violation of the nearly 1-century old concept that the metallic state is the highest pressure form of materials. Subsequent structure-search simulations led to major discoveries of pressure-stabilized hydrides with Tc approaching room temperature, realizing materials-by-design for this new class of materials.In addition to the advanced structure-search methods, a key role in leading to the discovery of these high-Tc superconducting materials is that the phonon-mediated Bardeen-Cooper-Schrieffer theory describes the high-Tc superconducting properties of these hydrides remarkably well. According to this theory, high critical temperatures may be realized by a large density state at the Fermi level, a high average vibrational frequency, and strong electron-phonon coupling. In this regard, it has long been believed that atomic metallic hydrogen, which was first pointed out by Ashcroft in 1968, would be a very-high-Tc superconductor, as it is predicted to exhibit the above electronic and vibrational properties.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar However, the pressures required to produce atomic metallic hydrogen are estimated to become extremely high (∼500 GPa), which creates a challenge for experimental characterization.An alternative approach was later proposed by Ashcroft in 2004, in which hydrogen-rich materials may exhibit "chemical precompression" that could reduce the onset pressure of metallic hydrogen-like superconductivity by forming dense hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar The structures of all high-Tc hydrides studied to date can be viewed as various clathrate-like structures. From a Zintl-Klemm description, the metal atoms donate their valence electrons and weaken the H2 molecules in these clathrate-like structures. Thus, the stability of these clathrate hydride structures can be considered to be a balance between the number of electrons available by charge transfer from the metal ions and the size of those metal ions. Electrons are shared to form connected networks of multi-center H … H … H bonds that resemble the bonding environment of atomic metallic hydrogen, and thus electronic properties such as high density of states at Fermi level, together with high characteristic vibrational frequencies and strong electron-phonon coupling.The high superconductivity observed in compressed earth/actinide hydrides has ushered in a new era of superconductivity research, while this distinct type of clathrate metal hydrides was first proposed for CaH6 using the crystal structure analysis by particle swarm optimization (CALYPSO) crystal structure prediction method in 2012.3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google Scholar Recent laser-heated diamond anvil cell experiments have confirmed this seminal prediction, with a Tc of 215 K at ∼170 GPa.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar X-ray diffraction measurements are consistent with the predicted clathrate structure of CaH6.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar The results demonstrate that high Tc above 200 K is indeed possible in clathrate hydrides formed from the main group, not only lanthanide/actinide, elements. These findings continue to demonstrate the role of structure search methods in this exciting new field.In 2015, a variety of Tc up to 203 K at ∼155 GPa in sulfur hydrides was measured for the first time and exhibited high-Tc above 200 K, thus sparking a flurry of interest in pressure-stabilized hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar In this experiment, the initial sample of SH2 has long been believed to dissociate into elemental hydrogen and sulfur at sufficiently high pressures. However, a theoretical study Led by Li and Ma in 2014 predicted two stable high-pressure structures, which were theoretically estimated to have the highest Tc of 80 K at megabar pressure.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Stimulated by this excellent prediction, researchers synthesized two distinct high-temperature superconductors. One sample was prepared at low temperature and measured to exhibit Tc from 30 to 150 K, in accordance with the predicted SH2 superconductor. The other sample was annealed at room temperature and measured to have unexpected high-Tc from 180 to 203 K, which was understood as the decomposition of SH2 into high-Tc SH3 and S, where SH3 is a known stoichiometry of (SH2)2H2 that has been synthesized at a pressure of 7 GPa and was predicted to be a 200 K superconductor at megabar pressures.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar This SH3, as a matter of fact, was not molecular, and was also viewed as having a clathrate structure with S encapsulated in the cages of the hydrogen framework.4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar These excellent findings demonstrate that the theoretical work plays a critical role in the experiments into high-Tc superconductors.There is now a focus on broader chemical compositions, starting with ternary systems. A ternary phase consisting of carbon, sulfur, and hydrogen was reported to have a Tc of 287 K at 267 GPa, but the composition and detailed structure at the highest critical temperature remain to be determined.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar There is growing evidence that the introduction of even small amounts of a third element can enhance the superconductivity of binary hydrides through hole or electron doping as shown for the C-S-H and the La-N-H systems. A pressing task is also whether one can design a material with relatively high Tc that is also stable at much lower pressures and even recoverable as a superconductor in ambient conditions. As shown in previous work, one of the key factors appears to spread favorable electron-phonon coupling over much of the vibrational spectrum in terms of these high-Tc hydrides.5Tanaka K. Tse J.S. Liu H. Electron-phonon coupling mechanisms for hydrogen-rich metals at high pressure.Phys. Rev. B. 2017; 96: 100502(R)Google Scholar Beyond hydrides, other likely candidates are compounds consisting of other light elements such as Li, Be, B, and C, which ensure an essential ingredient that the system has a relatively high characteristic frequency. The recently predicted LaBH8 and LaBeH8 show intriguingly dynamic stability below 50 GPa, despite being thermodynamically stable at or above approximately 100 GPa.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar These findings point to the potential of ternary hydrides as room temperature and "hot" superconductors under pressure as well as very-high-Tc materials that are stable at low pressures. It is reasonable to expect that new classes of high-Tc structures will be predicted and synthesized in the near future. To this end, recent advances in crystal structure prediction methods, data-driven machine learning techniques, and molecular dynamic simulations will undoubtedly accelerate the discovery of new and practical high-Tc superconductors, as well as foster a deep understanding of physical phenomena, such as the possible coexistence of proton quantum dynamics together with hydrogen-induced superconductivity in very-high-Tc hydrides. The search for room-temperature superconductivity has been one of the great challenges in condensed matter physics ever since the first observation of superconductivity with a critical temperature (Tc) of 4 K in mercury in 1911. In recent years, we have been witness to the successful observation of superconductivity in highly compressed SH3 (Tc = 203 K at 155 GPa) and classes of clathrate hydrides (e.g., CaH6, YH6, YH9, LaH10), with LaH10 holding a record high Tc of 250–260 K at ∼180 GPa among these binary hydrides (Figure 1).1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Each of these experimental studies was guided by theoretical predictions. New experimental work on the Ca-H system has finally confirmed the theoretical calculation that launched this effort, the 2012 prediction of a high-Tc superconductivity in CaH6 above 200 K.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar, 2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar, 3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google Scholar First-principles structure-search methods, which require only the chemical composition as input without prior information about the arrangement of atoms, can identify the thermodynamically stable and metastable structures of materials at any pressure and 0 K. Armed with these state-of-the-art methods, several major materials discoveries under pressure have now been documented, one of which is the observation of an anti-Wilson transition:4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar the prototypical free-electron metal of sodium transforming to a transparent semiconductor at megabar pressures, in violation of the nearly 1-century old concept that the metallic state is the highest pressure form of materials. Subsequent structure-search simulations led to major discoveries of pressure-stabilized hydrides with Tc approaching room temperature, realizing materials-by-design for this new class of materials. In addition to the advanced structure-search methods, a key role in leading to the discovery of these high-Tc superconducting materials is that the phonon-mediated Bardeen-Cooper-Schrieffer theory describes the high-Tc superconducting properties of these hydrides remarkably well. According to this theory, high critical temperatures may be realized by a large density state at the Fermi level, a high average vibrational frequency, and strong electron-phonon coupling. In this regard, it has long been believed that atomic metallic hydrogen, which was first pointed out by Ashcroft in 1968, would be a very-high-Tc superconductor, as it is predicted to exhibit the above electronic and vibrational properties.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar However, the pressures required to produce atomic metallic hydrogen are estimated to become extremely high (∼500 GPa), which creates a challenge for experimental characterization. An alternative approach was later proposed by Ashcroft in 2004, in which hydrogen-rich materials may exhibit "chemical precompression" that could reduce the onset pressure of metallic hydrogen-like superconductivity by forming dense hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar The structures of all high-Tc hydrides studied to date can be viewed as various clathrate-like structures. From a Zintl-Klemm description, the metal atoms donate their valence electrons and weaken the H2 molecules in these clathrate-like structures. Thus, the stability of these clathrate hydride structures can be considered to be a balance between the number of electrons available by charge transfer from the metal ions and the size of those metal ions. Electrons are shared to form connected networks of multi-center H … H … H bonds that resemble the bonding environment of atomic metallic hydrogen, and thus electronic properties such as high density of states at Fermi level, together with high characteristic vibrational frequencies and strong electron-phonon coupling. The high superconductivity observed in compressed earth/actinide hydrides has ushered in a new era of superconductivity research, while this distinct type of clathrate metal hydrides was first proposed for CaH6 using the crystal structure analysis by particle swarm optimization (CALYPSO) crystal structure prediction method in 2012.3Wang H. Tse J.S. Tanaka K. et al.Superconductive sodalite-like clathrate calcium hydride at high pressures.Proc. Natl. Acad. Sci. U S A. 2012; 109: 6463Google Scholar Recent laser-heated diamond anvil cell experiments have confirmed this seminal prediction, with a Tc of 215 K at ∼170 GPa.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar X-ray diffraction measurements are consistent with the predicted clathrate structure of CaH6.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar The results demonstrate that high Tc above 200 K is indeed possible in clathrate hydrides formed from the main group, not only lanthanide/actinide, elements. These findings continue to demonstrate the role of structure search methods in this exciting new field. In 2015, a variety of Tc up to 203 K at ∼155 GPa in sulfur hydrides was measured for the first time and exhibited high-Tc above 200 K, thus sparking a flurry of interest in pressure-stabilized hydrides.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar In this experiment, the initial sample of SH2 has long been believed to dissociate into elemental hydrogen and sulfur at sufficiently high pressures. However, a theoretical study Led by Li and Ma in 2014 predicted two stable high-pressure structures, which were theoretically estimated to have the highest Tc of 80 K at megabar pressure.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar Stimulated by this excellent prediction, researchers synthesized two distinct high-temperature superconductors. One sample was prepared at low temperature and measured to exhibit Tc from 30 to 150 K, in accordance with the predicted SH2 superconductor. The other sample was annealed at room temperature and measured to have unexpected high-Tc from 180 to 203 K, which was understood as the decomposition of SH2 into high-Tc SH3 and S, where SH3 is a known stoichiometry of (SH2)2H2 that has been synthesized at a pressure of 7 GPa and was predicted to be a 200 K superconductor at megabar pressures.1Gao G. Wang L. Li M. et al.Superconducting binary hydrides: theoretical predictions and experimental progresses.Mater. Today Phys. 2021; 21: 100546Google Scholar,2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar This SH3, as a matter of fact, was not molecular, and was also viewed as having a clathrate structure with S encapsulated in the cages of the hydrogen framework.4Ma Y. Eremets M. Oganov A.R. et al.Transparent dense sodium.Nature. 2009; 458: 182Google Scholar These excellent findings demonstrate that the theoretical work plays a critical role in the experiments into high-Tc superconductors. There is now a focus on broader chemical compositions, starting with ternary systems. A ternary phase consisting of carbon, sulfur, and hydrogen was reported to have a Tc of 287 K at 267 GPa, but the composition and detailed structure at the highest critical temperature remain to be determined.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar There is growing evidence that the introduction of even small amounts of a third element can enhance the superconductivity of binary hydrides through hole or electron doping as shown for the C-S-H and the La-N-H systems. A pressing task is also whether one can design a material with relatively high Tc that is also stable at much lower pressures and even recoverable as a superconductor in ambient conditions. As shown in previous work, one of the key factors appears to spread favorable electron-phonon coupling over much of the vibrational spectrum in terms of these high-Tc hydrides.5Tanaka K. Tse J.S. Liu H. Electron-phonon coupling mechanisms for hydrogen-rich metals at high pressure.Phys. Rev. B. 2017; 96: 100502(R)Google Scholar Beyond hydrides, other likely candidates are compounds consisting of other light elements such as Li, Be, B, and C, which ensure an essential ingredient that the system has a relatively high characteristic frequency. The recently predicted LaBH8 and LaBeH8 show intriguingly dynamic stability below 50 GPa, despite being thermodynamically stable at or above approximately 100 GPa.2Hilleke K.P. Zurek E. Tuning chemical precompression: theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures.J. Appl. Phys. 2022; 131: 070901Google Scholar These findings point to the potential of ternary hydrides as room temperature and "hot" superconductors under pressure as well as very-high-Tc materials that are stable at low pressures. It is reasonable to expect that new classes of high-Tc structures will be predicted and synthesized in the near future. To this end, recent advances in crystal structure prediction methods, data-driven machine learning techniques, and molecular dynamic simulations will undoubtedly accelerate the discovery of new and practical high-Tc superconductors, as well as foster a deep understanding of physical phenomena, such as the possible coexistence of proton quantum dynamics together with hydrogen-induced superconductivity in very-high-Tc hydrides. We thank Prof. Yanming Ma for valuable discussions. This work was supported by the National Key Research and Development Program of China (grant no. 2021YFA1400203 ), the National Natural Science Foundation of China (grant nos. 52090024 , 12074138 , and 11874176 ), the Fundamental Research Funds for the Central Universities (Jilin University [JLU]), the Program for JLU Science and Technology Innovative Research Team , and the Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDB33000000 ). R.J.H. acknowledges support from the U.S. National Science Foundation (grant no. DMR-210488 ). J.S.T. is supported by an NSERC Discovery Grant. The authors declare no competing interests.