Rare earth chemistry – in memory of Professor Xu Guangxian on the centenary of his birth

Xu Guangxian, also known as Kwang-hsien Hsu (1920–2015), was born in Shaoxing, Zhejiang Province, China. He was a professor at Peking University, a famous chemist and educator, and was elected an academician of the Chinese Academy of Sciences in 1980.

Prof. Xu Guangxian graduated from the Department of Chemistry, Shanghai Jiao Tong University in 1944 with a Bachelor of Science degree. In 1949 and 1951, he received a Master of Science degree and a doctorate degree in physical chemistry from Columbia University in the United States. In May 1951, he was hired as an associate professor in the Department of Chemistry of Peking University and was promoted to full professor in 1961. He served as Director of the Laboratory of Radiochemistry, Deputy Director of the Department of Atomic Energy of Peking University, Director of the Rare Earth Chemistry Research Center (1986–1999) and Director of the State Key Laboratory of Rare Earth Materials Chemistry and Application. He served as Vice President of the Chinese Rare Earth Society (1980), President of the 22nd Council of the Chinese Chemical Society (1986), First Director of the Department of Chemistry of the National Natural Science Foundation of China (1986–1994) and President of the Federation of Asian Chemical Societies (1991). He served as Editor-in-Chief of Science in China Series B: Chemistry, Journal of Peking University (Natural Science Edition) and Journal of Chinese Rare Earths, and as an Associate Editor for Chemical Journal of Chinese Universities, Journal of Inorganic Chemistry, Science in China, and Science Bulletin.

Prof. Xu Guangxian had a wide range of research interests. He was adept at combining theoretical studies and quantum chemical calculations with experimental research, and combining basic research with industrial practice. He made important contributions in rare earth chemistry, especially in separation and coordination, and in theoretical and computational quantum chemistry.

In 1957, Prof. Xu Guangxian began to study nuclear fuel extraction chemistry. In 1962, he proposed a new classification method for extraction systems, carried out in-depth research on various extraction systems, and summarized the general rules of the extraction processes, especially the synergistic extraction rules, which is significant for the explanation of the extraction phenomena, and for the design and prediction of new synergistic extraction systems. During the same period, starting with the separation of rare earth elements from the Pr/Nd system, he systematically conducted theoretical, experimental, and industrial research of rare earth element separation. He used coordination chemistry and solution equilibrium theories to design a “push–pull” system that used both extractant and complexing agent, and increased the Pr/Nd separation factor from 1.4 to more than 4, which was the highest value in the world at that time. Aiming to solve the problems of the commonly used Alders liquid–liquid extraction theory for rare earth separation, he proposed a new concept of “mixed extraction ratio” (E_M, which is the total extraction ratio of the mixed rare earth components) and a new assumption of “constant E_M”. He developed the countercurrent extraction theory for rare earth separation, deduced all the theoretical formulas required to calculate the optimal process parameters, and established a mathematical model to simulate the dynamic process of cascade extraction. The theory and method have been widely applied in the extraction and separation of Baotou rare earth ore and southern ion-absorbed rare earth ore. These achievements have made great contributions to the development of China's rare earth industry.

In the field of quantum chemistry, he discovered that the perturbation effect of the quadrupole of the chemical bond on the chromophore is the main factor determining optical rotation, proposed a simple n + 0.7l rule for ordering the energy levels of multi-electron atomic orbitals, and improved the Slater method for calculating the ionization energy of atoms. Through the analysis and summary of structural data and quantum chemistry calculation results, Prof. Xu proposed new concepts in covalency and coordination number, and provided a quantum chemistry definition of covalency. Based on the concept of molecular fragments proposed by Hoffmann et al., the (nxcπ) rule was proposed (where n is the number of molecular fragments, x is the number of molecular excess electrons, c is the number of rings and π is the number of π bonds). This provides a new classification method for all inorganic and organic molecules, is a bridge between inorganic and organic chemistry, and can be used to predict the structure and synthesis of new compounds. At the same time, Prof. Xu performed theoretical studies on rare earths and developed the INDO method and calculation program, which was the first program suitable for the study of the electronic structure of rare earth compounds. The bonding in lanthanide compounds was analyzed in detail, and many concepts were clarified, which is an important contribution to the development of rare earth quantum chemistry and physical inorganic chemistry.

In the field of coordination chemistry, in the 1950s, Prof. Xu proposed and confirmed the coordination of alkali metal ions with ligands, and put forward the theory of “complex equilibrium adsorption” which gave a simple description of the weak coordination equilibrium reaction process in solution. Based on their research on the structure and spectra of complexes, Prof. Xu and coworkers have developed new fields such as luminescent rare earth complexes, magnetic molecular solid materials, and rare earth nanomaterial chemistry.

Prof. Xu Guangxian attached great importance to education and talent training. He has successively launched various courses such as Physical Chemistry, Matter Structure, Quantum Chemistry, Statistical Mechanics, Introduction to Nuclear Physics, Coordination Field Theory, Coordination Chemistry, Extraction Chemistry, Advanced Inorganic Chemistry and Molecular Spectroscopy. The curriculum has established the foundation and promoted the construction of the chemistry discipline in Peking University and even the whole of China. He presided over the compilation of Quantum Chemistry: Basic Principles and ab initio Calculations, Matter Structure, Principles of Extraction Chemistry, Solvent Extraction of Rare Earths, Rare Earths and other textbooks and academic monographs, which played a huge role in the training of talents in the Chinese chemistry community.

Based on his outstanding contributions to scientific research and education, Prof. Xu Guangxian won the Second Prize (1987) and the Third Prize (1987) of the National Natural Science Award, the Special Prize of the National Excellent Textbook Award (1988), the Third Prize (1991) and the Second Prize (1999) of the National Science and Technology Progress Award, the He Liang He Li Science and Technology Progress Award (1994), the Science and Technology Achievement Award (2005) and other awards, and won the National Highest Science and Technology Award in 2008.

November 7, 2020 marks the 100^th anniversary of the birth of Prof. Xu Guangxian. Inorganic Chemistry Frontiers invited scholars in the fields of rare earth chemistry, coordination chemistry, organometallic chemistry, and quantum chemistry to submit research papers to commemorate his achievements and outstanding contributions in academic research and talent training in inorganic chemistry.

Song Gao

Peking University & South China University of Technology

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