6. R. Chen, J. F. Wang*, Z. W. Ouyang*, Z. Z. He, S. M. Wang, L. Lin, J. M. Liu, C. L. Lu, Y. Liu, C. Dong, C. B. Liu, Z. C. Xia, A. Matsuo, Y. Kohama, and K. Kindo, “Magnetic field induced ferroelectricity and half magnetization plateau in polycrystalline R2V2O7 (R = Ni, Co)”, Phys. Rev. B 97, 184404 (2018)