Weiwei Xie
Research
Exploring Novel Quantum Materials from Chemistry Perspectives
Our research group will mainly be on non-molecular inorganic materials involving almost the whole periodic table and diverse theories and methods to design, predict and synthesize new materials.
We will work to answer three general questions:
- What are the chemical structures and physical properties of new metal-rich compounds?
- Why do they crystallize in their structure types and exhibit such physical properties?
- How does the structure influence physical properties?
To address these problems, powder and single crystal X-ray diffraction and neutron powder diffraction, if necessary, will be utilized to determine the crystal structure and phase information, in addition to magnetic and electrical conductivity measurements to be analyzed by a Quantum Design Physical Property Measurement System. With respect to theory, first-principle electronic structure calculations based on TB-LMTO-ASA, WIEN2k, and VASP packages will be employed to investigate patterns of atomic arrangements, structural phase transitions, magnetic ordering, superconductivity, and other properties.
Critical Charge-Transfer Pairs and Electron Counting Rules for Superconductivity and Magnetism
“Critical pairs” of atoms in the periodic table can be postulated where the balance between covalent and ionic bonding leads to just the right kind of charge transfer between the atoms so that the bond valence responds to perturbations from the other forces present to lead to instabilities in a compound’s electronic system that are delicately balanced with other factors such as electron-lattice coupling, magnetism or superconductivity. Well-known examples of such critical charge-transfer pairs in the periodic table are Cu-O and Fe-As, which lead to high-temperature superconducting properties. In contrast to the quantitative k space or Fermi surface view in physics, the concept of critical charge transfer pairs is clearly a qualitative, real space view of what can give rise to interesting physical properties. The design of new W5Si3-type superconductors T5Sb3-xRux (T = Hf, Zr) lead us to propose that Ru-Sb may be a third critical charge-transfer pair of elements for superconductivity in the periodic table along with Cu-O and Fe-As.
Moreover, based on our empirical theory, a fragmental formalism for making superconductors, we found that the superconductor LaRu4Sb12 could be viewed as stuffing RuSb6 octahedra at (¼, ¼, ¼) site in W-type “La”. Similarly, another superconductor Ca3Rh4Sn13 can be considered as putting RhSn6 trigonal prisms at (¼, ¼, ¼) site in Nb3Ge-type “Ca3Sn”.
Valence electron counting rules are favored by chemists because they can be useful to rationalize and predict the chemical behaviors of substances, for example, their structures and electrical properties. The Zintl–Klemm concept is such a valence electron counting scheme that has achieved wide usage among molecular and solid-state disciplines, similar to the octet (Lewis–Langmuir) rule applied to problems in organic, main-group inorganic, and biochemistry and the 18-electron counting rule for organometallic complexes.
Classical Zintl phases are considered to be valence precise semiconductors with electropositive cations (typically, Alkali and Alkali-earth or Rare-earth elements) donating their electrons to electronegative anions, which use electrons to form bonds in order to satisfy their valence. At the border between classical Zintl phases and normal metallic phases, for compounds called polar intermetallics, the semiconducting band gap diminishes and metallic conductivity can result. These compounds can, for example, be made good thermoelectric materials. Moreover, a new series of superconductors, like ReGa5, have been predicted and synthesized on the broader of Zintl phases with pseudo semiconducting band gap.
Spin-Orbit Coupling (SOC) Effects on Magnetism in 4d/5d Transition Metal Halides
The importance of spin-orbit coupling (SOC) to generate the electronic ground state in 4d/5d-based compounds has emerged and many novel routes to host unconventional physical states have been revealed, for example, quantum spin liquids, Weyl semimetals, and axion insulators. The major experimental and theoretical efforts in quantum spin-liquid state study have been solely undertaken to search for novel spin-orbit coupling systems in the various d5 system with S=1/2, for example, a-RuCl3. Few references have been reported concerning other situations, for example, S=3/2, because octahedral d3 configurations are expected to be orbitally quenched S= 3/2 states – in which case SOC enters only as a 3rd order perturbation. A common magnetic phenomenon related to SOC is spin-canting, which has been widely observed and investigated in many different systems. Spin-canting means spins are tilted a small angle about their axis rather than being accurately parallel. Generally, spin canting can be considered as a strong hint of large SOC in the different systems. We focus on the study of SOC on octahedral d3 configurations with spin-canting.
High-Pressure Single Crystal X-ray Diffraction on Solid State Materials
Pressure provides a useful tool to precisely tune the interatomic distances in quantum materials, which is critical to understanding the organizing principles that govern electron dynamics with strong quantum fluctuations. We focus on the study of crystal structure and physical properties under high pressure.