Synthesis, structure, and spectroscopic properties of bismuth citrate compounds. 1. Crystal structures of K5-x(NH4)x[Bi2(cit)2(Hcit)](H2O)y (x = 0.25, y = 13) and (NH4)8[Bi2(cit)2(Hcit)2(H2O)4](H2O)2