Spectral Analysis on the Nickel Hydroxyl and Deuteroxyl Nitrate Powders

Infrared absorption (4000-400 cm−1) and Raman (4000-50 cm−1) spectra of the magnetic geometrically frustrated compound nickel hydroxyl nitrate (Ni2(OH)3NO3), and incompletely deuterized nickel hydroxyl/deuteroxyl nitrate (Ni2(OH/D)3NO3) powders, which were synthesized successfully by the hydrothermal methods, have been measured at room temperature and analyzed in four spectral regions (the [OH/D] and [NO3] functional group regions, the [OH/D] and [NO3] correlation peak region and the Ni-related fingerprint region) by referring to the spectral assignments of the layered β-Ni(OH)2 and β-Ni2(OH)3Cl. The spectral analysis can be beneficial to analyzing their low temperature spectral properties, which can help to understand the underlying physics of their exotic quantum phenomena at low temperatures from the spectroscopic point of view.


Introduction
Transition metal hydroxyl halides with the chemical formula M2(OH)3X ( M = Mn, Fe, Co, Ni, Cu, and X = Cl, Br, I) have been found to possess unconventional magnetic transitions at low temperatures originated from the geometric frustration property [1][2][3], and besides the conventional magnetic methods, these quantum magnetic phenomena, such as in γ-Cu2(OH)3Cl, can be studied by use of the temperature-dependent vibration spectral methods (Raman and mid-IR) because of the spin lattice/phonon coupling effect [4,5]. Therefore the assignment of appearing spectral bands/lines of these inorganic coordination compounds is a fundamental task at room temperature. For a similar but relatively complicated geometric frustration material series where X = NO3, such as Cu2(OH)3NO3, is also investigated in the magnetic respect, and the IR and Raman spectra at room and low temperatures are studied only in the pure spectral respect [6][7][8].
As to the nickel hydroxyl nitrate with the molecular formula Ni2(OH)3NO3, like its counterpart Ni2(OH)3Cl [9][10][11], its IR and Raman spectra at room temperature are surely needed to be studied carefully. Because till now the IR and Raman spectral analysis of Ni2(OH)3Cl is not comprehensive and not completely correct [12], here the IR and Raman spectral analysis of Ni2(OH)3NO3 and its deuteride Ni2(OD)3NO3 is reported by referring to the spectral assignments of the layered Ni2(OH)3NO3 and β-Ni2(OH)3Cl [13,14], in order to help to understand the underlying physics of their exotic quantum phenomena at low temperatures from the spectroscopic point of view.
The preparation of Ni2(OD)3NO3 sample is slightly more complicated. Ni(NO3)2· 6H2O powder was first dehydrated in drying oven for two days at 60 º C. The incompletely dehydrated Ni(NO3)2 (1.827 g, 0.01mol) was dissolved into the heavy -water (deuteroxide, 10 ml) and 30 wt% NaOD-D2O solution (0.41g, 0.01 mol NaOD in the 1.366 g solution) was diluted by 5 ml heavywater. Then the diluted NaOD-D2O solution was gradually dropped into the Ni(NO3)2-D2O solution and the flocculation-like precipitate Ni2(OD)3NO3 was gradually generated. The precipitate Ni2(OD)3NO3-D2O solution was put into a sealed reaction vessel and kept at 100 º C for one day. After a filtration process and a dry procedure for one days at 60 º C, the Ni2(OD)3NO3 powder was obtained. In fact the powder was incompletely deuterized, therefore the sample was written as Ni2(OH/D)3NO3 ever after.
The IR spectra were measured with Nicolet iS50 (Thermo Fisher, USA) FTIR spectrometer whose measurement region was between 4000 -400 cm -1 , by using of the general KBr disc technology.
Raman spectra were obtained using XploRA PLUS Laser confocal micro-Raman microscopy (Horiba, Japan) excited by an Nd: YAG laser (532.0 nm) with the 1200 g/mm grating between 4000 -50 cm -1 . The Raman spectrum was automatically baseline corrected (fluorescence subtracted partly), moderately smoothed and normalized according to the largest counting peak value. Figure 1 shows the TG curves of Ni2(OH)3NO3 and Ni2(OH/D)3NO3 samples at 40 º C-600 º C. From the following thermal decomposition equation (1), one can determine that Ni2(OH)3NO3 sample is almost pure, because the weight percentage at 600 º C is 66.0 % that is very close to the theoretical 64.8 % at > 800 º C.

Results and Discussion
Theoretically the weight percentage of Ni2(OD)3NO3 sample at 600 º C should be about 64.0 %, but from figure 1, the weight percentage is high to 77.4 % and thermal decomposition are conducted by two The Ni2(OH/D)3NO3 sample may also contain some Ni(OH)2 and Ni(OD)2. Figures 2a and 2b show their IR and Raman spectra in the ranges of 4000-400 cm -1 , and 4000-50 cm -1 , respectively. In the spectral analysis process, we will refer to the well-established crystal structure information, the analysis results of the β-Ni(OH)2 [12], Ni2(OH)3NO3 [13], and β-Ni2(OH)3Cl [14], and other experiences.
[   [NO3] and [OH/D] CP region (1000-600 cm -1 ). According to the normal mode analysis on two correlation peak modes of the   FP region (600-50 cm -1 ). According to our analysis on the β-Ni2(OH)3Cl [14] with the viewpoint that the Cl atomic quantity is roughly equivalent to that of [NO3] unit, there maybe three sets of complicated IR/Raman modes: (1)  (v[Ni3(NO3)) are observed, but for Ni2(OH/D)3NO3, only 188/101 cm -1 (v[Ni3(NO3)) appear in the Raman spectrum, which can not be explained by us till now. Table 1 collects the IR and Raman data of two samples with listing the suggested assignment and the published spectral data of Ni2(OH)3NO3 [13] and β-Ni2(OH)3Cl [14] as assigning references.

Summary
We have prepared and measured infrared and Raman spectra of the nickel hydroxyl and deuteroxyl nitrate Ni2(OH)3NO3 to find out its whole really existing vibration spectral bands. Except for two hydroxyl stretching modes vs [ Table 1. IR/Raman spectral data (cm -1 ) of Ni2(OH)3Cl with those of β-Ni2(OH)3 and β-Ni2(OH)3Cl as assigning references (only strong or some moderately strong spectral lines are considered).