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Large single crystal of Na2Ni (SO4)2.4H2O (NNSH) was grown for the first time using method of isothermal decrease of supersaturation. The crystal structure of NNSH is determined by single crystal X-ray diffraction method, which is far different from the structure of known nickel sulfate hexahydrate series compounds. The growth habit, optical transmission spectrum and thermal stability are first reported in this paper. Similar optical transmission spectrum with nickel sulfate hexahydrate (NSH) crystal shows that NNSH crystal can also been used as ultraviolet light filter (ULF). The dehydration temperature (tdh) is 132º, which is much higher than commercial available ultraviolet filters, this is correlate with the unique crystal structure of NNSH crystal.

Single crystal structures of M2Ni (SO4)2.XH2O (M=Na, X=4; M=K, NH4, Rb, Cs, X=6) were investigated using the single crystal X-ray diffraction method. They crystallize isotypic (space group P2 (1)/c). Many structure features exhibit regularity with the relative size of radius, such as a, b, c, V. But to ANSH, the regularity of average bond lengths of violets from others. Their optical transmission spectrums were recorded on PE-lambda 900 with performing wavelengths ranging from 200 to 1100 nm, they have the similar optical characters in tested wavelength range. This phenomenon arises from the electron transition of d orbits in the octahedral field. The similarity in optical spectrum is consistent with the comparability of structures. We also investigated the thermal-gravimetric quality of above single crystals under same experiment conditions. We found that the dehydration temperatures increase with increasing of radius of the M+ when M =K, Rb, Cs. When M=Na, the dehydration temperature (tdh) is above the others pronouncedly due to the specialty of structure. To NH4-, the so-called compound is also with exception of the regularity.

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