Liu, W., Yang, X. X., Chen, H. H., Huang, Y. X., Schnelle, W., & Zhao, J. T. (2004). A mixed cation nickel diphosphate with a layered intergrowth structure: synthesis, structural and magnetic characterization of Na(NH4)[Ni3(P2O7)2(H2O)2]. Solid State Sciences, 6(12), 1375-1380. doi:10.1016/j.solidstatesciences.2004.07.035.