2020同步年報

076 ACTIVITY REPORT 2020 with nominal composition Pb 2 (Pb,Li,Na,K,Rb,Re) 1‑ x Nb 5 O 15 (0 ≤ x ≤ 0.15, Re = rare earth). Interestingly, many of these compounds also exhibit a large SHG response. These workers proposed a new parameter, called local distortion index m , to evaluate the degree of local distortion of these compounds. Please refer to the Supporting Information of Ref. 1 for the definition of local distortion index m . Figure 2 shows the distribution of the SHG response and local distor- tion index m of several NLO materials. The SHG response evidently tends to be strengthened with the enhancement of local distortions. In summary, the analysis of the local structure unveiled that the structural vacancies at the A1-site strengthen the local distortion and local dipole moments of nearby NbO 6 octa- hedra and significantly improve the NLO effect. Lin’s work is a perspective for the search and design of new materials with an effective NLO performance. (Reported by Chin-Wei Wang) This report features the work of Kun Lin and his collaborators published in J. Am. Chem. Soc. 142 , 7480 (2020). ANSTO ECHIDNA – High-resolution Powder Diffractometer • NPD, XRD, Total Scattering • Materials Science, Chemistry Reference 1. K. Lin, P. Gong, S. Chu, Q. Li, Z. Lin, H. Wu, Q. Wang, J. Wang, M. J. Kim, K. Kato, C.-W. Wang, X. Liu, Q. Huang, J. Chen, H. Zhu, J. Deng, X. Xing, J. Am. Chem. Soc. 142 , 7480 (2020). N eutron diffraction is a powerful tool to study magnetic phenomena. It is the best method to determine the magnetic structure and can detect the short-range magnetic correlation between magnetic spins. It is known that competing an- tiferromagnetic interactions among the magnetic spins on a regular lattice could lead to geometrically magnetic frustration, resulting in the absence of long-range magnetic order or extremely low ordering temperatures. In such systems, the diffuse scattering and unconventional spin dynamics could be observed in a neutron-scattering experiment. Spinel compounds of formula AB 2 O 4 have been studied for a long time. The B-site (16b of space group F d -3 m ) cations form a regular corner- sharing tetrahedral network, which is potentially geometrically frustrated. Hsiung Chou (National Sun Yat-sen University) and his coworkers maintain a great interest in spinel compounds and recently conducted the powder neutron experiments on Mn 1.5 Cr 1.5 O 4 , employing both high-resolution diffractometer ECHIDNA and high-intensity diffractometer WOMBAT of ANSTO. This work determined the cation distribution and magnetic structure of Mn 1.5 Cr 1.5 O 4 , using the high-resolution NPD data ( Fig. 1(a) and 1(b) ). The magnetic structure is collinear ferrimagnetic; the uncompensated A- and B-site sublattice produces a net magnetization. The magnetic order parameter obtained from neutron diffraction ( Fig. 1(a) ) matches satisfactorily with the spontaneous magnetization for the transition temperature and the critical exponent ( Fig. 1(a) ), but the deviation from the Curie-Weiss behavior occurs at a much higher temperature, T ’ ~ 170 K, indicating that diffuse scattering might be detectable in neutron-diffraction experiments. Detailed temperature-dependent measurements were hence conducted employing HIPD WOMBAT ( Fig. 2(a) ). Figure 2(a) shows that modulation of the background intensities develops below T ’ before it eventually collapses onto the magnetic Bragg reflections at T N . The diffuse magnetic scattering weakens but continues to co-exist with the ferrimagnetic phase below T N . The difference between the diffraction patterns collected at 163 K and 63 K clearly reveals the modulation ( Fig. 2(b) ). Because of the Q dependence of the instrument resolution, the oscillation at higher q is smeared out and becomes shallower, but the oscillation persists. Chou suggested that the liquid-like short-range order develops be- cause of the presence of magnetic frustration in the system. This observation of diffuse scattering above the magnetic-ordering temperature is neither the first time reported nor a rare case. For example, magnetic diffuse scattering of the textbook example, MnO, was reported in the 1940s. 2 The broad diffuse Magnetic Frustration Induces Liquid-Like Short-Range Ordering in Cubic Mn 1.5 Cr 1.5 O 4 Short-range magnetic correlation manifested by the liquid-like structure factor is observed at temperatures well above T N in the title compound.