Between 1916 and 1919, Nishikawa stayed in the United States and worked at Cornell University, where he was a mentor of the then graduate student Ralph Wyckoff.
[3][6] Nishikawa pioneered in the application of space groups in crystal structure determination, using spinel compounds as examples.
[7] His other significant scientific contributions include the analysis of the phase transformation of quartz and the experimental evidence of deviations from Friedel's law for certain crystal structures.
The first son, Tetsuji Nishikawa (1926–2010), was one of the founding fathers of The High Energy Accelerator Research Organization (also known as KEK) and was its general director from 1977 to 1989.
A younger son, Kyōji Nishikawa (born 1934), is an emeritus professor at the Hiroshima University specialized in nuclear fusion and plasma science.