STRUCTURE, ELECTRON AND OSCILLATORY PROPERTIES OF ZINC NITRATE AND ITS CRYSTAL HYDRATES
Abstract and keywords
Abstract (English):
Within the generalized gradient aproximation of the Density Functional Theory (DFT) with the PBE exchange-correlation functional in the basis of localized atomic orbital of CRYSTAL14 program code, the study is conducted to evaluate the structural, electronic and oscillatory properties of zinc nitrate and its crystal hydrates Zn(NO3)2 • nH2O (n = 2,4,6), with its tested method using the zinc oxide. The first-principle structural study is performed at the full optimization of the lattice distance and atomic positions for the zinc nitrate in the cubic lattice and that of crystal hydrates - in monoclinic lattice. Elastic properties of the nitrate are studied and the mechanical stability is approved using the Born criteria. Electronic properties of rated structures are assessed by energetic (energy-band picture, full and partial density of states) and spatial electron distribution (electronic and deformation density, population density of atomic membranes and density of their overlapping). Crystal hydrates show the electrostatic pattern of nitrogroup interaction and water molecules, availability of localized valence bands and areas of vacant state of anion and cation origin. Oscillatory properties are studied by calculation of frequencies and intensity of IR-active normal long-wave oscillation. In crystal hydrates, the appearance of additional oscillation frequency O-H in terms of nitrate 3000 cm-1 above the IR-spectrum in water molecules and within the area 1200÷1600 cm-1 - of hybrid with nitrogroups.

Keywords:
Zinc nitrate, crystal hydrates, zinc oxide, crystalline structure, chemical bonding, atomic charge, band structure, density of states, IR spectra
Text
Zinc nitrate is the inorganic compound with the chemical formula Zn(NO3)2 (NO3)2 (further - ZN). This white crystalline solid substance is quite hygroscopic and is normally seen as crystal hydrates of Zn(NO3)2•nH2O, where n = 1, 2, 4, 6 and 9. Most common are the zinc nitrate dihydrate Zn(NO3)2 • 2H2O (further ZNH2), tetrahydrate Zn(NO3)2 • 4H2O (further ZNH4) and hexahydrate Zn(NO3)2 • 6H2O (further ZNH6). Zinc nitrate is not commonly used rather than the precursor to synthesize a variety of nanostructures based on ZnO and coordination polymers [1-4]. In turn, due to its high radiation, chemical and thermal stability, zinc oxide is widely used to manufacture gas sensors, light-emitting diodes, photodetectors, varistors and etc. [5, 6]. Moreover, ZnO lms produced by chemical deposition are the promising material to produce anti-reection, conductive and window layers of solar cells for large areas [7]. The thermal decomposition of crystalline zinc nitrate is one of main mechanisms to produce zinc oxide. Processes of Zn(NO3)2 • 6H2O thermal decomposition may be divided into two stages: dehydration and pyrolysis [8, 9]. The chemical dehydration occurs at the temperature within 100-350°C. At this temperature, the chemical reaction can be represented as: Zn(NO3)2 • 6H2O → Zn(NO3)2 + 6H2O, (1)Above 350°С, zinc nitrate is decomposed to ZnO with the abundant evolution of NO2. At this stage, chemical decomposition is given by the equation 2Zn(NO3)2 → 2ZnO + 4NO2 + O2, (2)The thermal decomposition of zinc nitrate results in zinc oxide. [6] The crystalline structure of the zinc crystal hydrates is studied in [10-13]. Since ZNH2 has the monoclinic lattice with the space group P121/c1 with two formula units in the unit cell and density of 2.544 g/cm3 [10]. These data were later proved by the work [11], that, however, had no reference for hydrogen atom sites. The zinc nitrate tetrahydrate also forms the monoclinic lattice with symmetry group P121/n1 and four formula units [12]. The hexahydrate has the orthorhombic lattice with four formula units though hydrogen atom sites are not dened in [13]. Places of all atoms, including the hydrogen, are known for the most common magnesium nitrate hexahydrate which is monoclinic [14]. The crystalline structure of zinc nitrate to date is not determined due to its high hygroscopic property. The same is reported for magnesium nitrate and calcium. They are however known to have the cubic lattice with the symmetry group Pa3 [15].Physical properties of zinc nitrate and its crystal hydrates are not adequately evaluated whereas there are not any paper works. This is apparently due to Science Evolution, 2017, vol. 2, no. 129Vibration properties of crystal hydratesIR spectra of research targets may be easily identied by characteristic bands available - Fig. 8. So, only one intense band at 1343 сm-1 is in the anhydrous nitrate. The IR spectrum in the zinc nitrate dihydrate is much more abundant. First of all, two intense bands appear in the region at over 3000 cm-1 where vibrations of oxygen and hydrogen atoms of water molecules (O-H region) are intense. This is the symmetry vibration Bu with the intensity of 3962 km/mol (taken as 100 % in ZNH2) with the frequency of 3400 сm-1 and vibration of the same symmetry, less intensity (64%) and higher frequency (3485 сm-1). To note, at such vibrations the shortest spaces between the hydrogen and oxygen atoms of nitrogroups change. In hexahydrate, the number of intense bands increases to six for this region. This is the band with the peak intensity at 8634 km/mol (in ZNH6 taken as 100%) at the frequency of 3366 cm-1 to match vibrations of hydrogen atoms in water molecules of symmetry Au. The frequency of 3657 сm-1 and the intensity 16% match the vibrations of the same symmetry. Vibrations of О-Н atoms in water molecules of symmetry Bu will be consistent with the frequency 3315 сm-1 (69% intensity), 3396 (21%), 3428 (51%), 3613 сm-1 (18%).Within the frequency range 1400-1600 сm-1 in-plane vibration of hydrogen atoms of “scissors” type will be intense (64%) at the frequency of 1458 сm-1 in ZNH2 and the same weak (11%) vibration at 1592 сm-1 frequency. Intense vibrations are not reported in this region for the hexahydrate: 1600 сm-1 (4%) and 1641 сm-1 (6%).Fig. 8. IR spectra of dihydrate (top, ZNH2) and hexahydrate (bottom, ZNH6) of the zinc nitrate.In the region of intra-molecular vibrations within the nitrogroup with frequencies over 1000 сm-1, the symmetry vibration Bu in ZNH2 will be the most intense (90%) at the frequency 1246 сm-1, where oxygen and nitrogen atoms shift along the bond lines in the nitrogroup plane (deformation type). At that, hydrogen atoms in the water molecule also vibrate by the “scissors” type. This results in this vibration shearing within the frequency range by almost 100 сm-1 against the anhydrous nitrate. The relative low (14 %) vibration at the frequency of 1282 сm-1 of symmetry Au is also reported in this region. The symmetry vibration Bu and 4% intensity appear at the frequency of 1033 сm-1, specic for the breathing vibration Ag, so active in the Raman spectra. For ZNH6, four intensive vibrations will be reported in this spectrum bands: two symmetries Au 1333 сm-1 (15%), 1382 (30%), symmetries Bu 1327 (22%) and 1383 сm-1 (22%). Resulting from application of such vibrations, only two peaks will be reported in the IR spectrum at 1380 and 1327 сm-1.In the band of 700-850 сm-1, also typical for out-of-plane in-nitrate vibrations, the bands at the frequency of 809 сm-1 and 728 сm-1 will be apparent in ZNH2 and intensity values will be 19% and 17%, respectively. Hydrogen atoms perpendicularly shifting to the plane of water molecule will also take part. Two vibrations with apparent intensity will be reported in hexahydrate: symmetries Bu at the frequency of 741 сm-1 (6%) and symmetries Au at 770 сm-1 (5%). Finally, the vibration at 449 сm-1 in ZNH2 and 19% intensity (466 сm-1 in ZNH6 and 11% intensity) will correspond to rotational movements of water molecule and the activity in the anhydrous nitrate spectrum in this region is apparently zero. In the region of lattice vibrations ZNH2 will be traceable only at the frequency 203 сm-1 and 4 % intensity (195 сm-1, 6% in ZN, 209 сm-1, 2% in ZNH6), corresponding to swinging nitrogroup movements. Estimations of total lattice energy make it possible to assess the thermal effect of the chemical reaction (1, 2). For crystal hydrates, the reactions are exothermic and the zinc nitrate decomposition is followed by the discharge of 329.6 kJ, ZNH2 - 195.0 kJ, ZNH4 - 384.9 kJ, ZNH6 - 532.3 kJ. In line with the rst law of thermal chemistry, the heat of reaction to form the compound of ordinary substances is equal in absolute magnitude but opposite in signs of the heat effect of its decomposition to ordinary substances. As it is known, the heat of reaction to form one mole of this compound of ordinary substances in the standard state is known as the enthalpy to form this chemical compound. Accordingly, the enthalpies of formation as per our data are the following: ZN: -515.4 kJ/mol, ZNH2: -710.4 kJ/mol, ZNH4: -900.2 kJ/mol, ZNH6: -1047.7 kJ/mol. The formation enthalpies describe the chemical compound stability: the higher (in absolute value) is the negative enthalpy of the chemical compound formation, the more stable it is.CONCLUSIONSWithin the density functional theory, PBE exchange-correlation functional and hybrid PBE0 functionality, the structural, electron and vibrational Science Evolution, 2017, vol. 2, no. 130properties of zinc nitrate and its crystal hydrates Zn(NO3)2 • nH2O (n = 2,4,6) were investigated based on localized atomic orbitals of the CRYSTAL14 software code. The zinc nitrate is estimated in its cubic structure by full optimization of lattice constants and atomic positions whereas the crystal hydrates are evaluated in the monoclinic lattice. The mechanic stability of the nitrate cubic phase as per Born criteria was proved by calculation of elastic constants. Elastic modulus, Poisson’s ratio and the Cauchy pressure indicate the nitrate plasticity. In each structure, the zinc atom has six oxygen atoms in its neighborhood of which n belongs to water molecules and the rest belong to nitrogroups. Within the nitrogroup, oxygen atoms have hydrogen atoms in the neighborhood of water molecules at the space of up to ~1.9 Å. In the dihydrate, these are two atoms (except oxygen atom with the shortest N-O bond) that have one hydrogen atom each. In the tetrahydride, two nitrogroups are in non-equivalent crystallographic positions. For the one group, the hydrogen surround of the oxygen is the same as for dihydrate, and for the other group, two oxygen atoms are surrounded by two hydrogen atoms each. The neighborhood for the hexahydrate is similar to that for the second group of tetrahydrate nitrogroup. Electron clouds of nitrogroup oxygen and hydrogen in water molecules are overlapped with the population density on the О(N)-Н 0.03÷0.05 е line which is less than for О-H in the very water 0.26÷0.28 е, and О-N in nitrogroups 0.28÷0.36 е. This is to tell on low chemical bond between O(N)-H. The electrostatic nature of nitrogroup and water interaction is proved by their charges: of period -0.6 е for nitrogroups and +0.06 е for water.For framing O1s states, the chemical shift is seen up to 0.5 eV due to different oxygen charges both in nitrogroups and in water molecules. In the density of valence energy band states distinct bands are seen formed in the lower part - by s states of oxygen and nitrogen, in the mid-part - by р states of these atoms with the separate peak of 3d zinc. The upper valence band is formed by р states of nitrogroup oxygen of anti-bonding pattern relative to N-O. In crystal hydrates, the energetically separated band of up to 0.3 eV wide is isolated in this region. There are two energy-separate bands of anion and cation nature in vacant state regions. The width of anion band gap is the same in all compounds and is equal to ~ 3.5 eV. Cation bands are formed by s states of zinc, hydrogen and nitrogen, they are known for the considerable variance and the width of the corresponding band gap is 4.8 ÷ 5.0 eV. The continuous conduction band begins with the energy of ~ 8 eV, and it decreases as water molecules increase.The intense peak at 1343 cm-1 is expected in the IR spectrum of anhydrous nitrate. The number of bands increases in crystal hydrates. The number of peaks in the region of О-Н vibrations in water molecules (~3400-3600 сm-1) will be equal to n of water molecules. Hybrid vibrations of nitrogroup atoms and water molecules appear in the range within 1250 сm-1 to ~1450 сm-1 with three bands in spectra. For the region of nitrogroup atom vibrations at the frequency of less than 1000 сm-1, the bands of apparent intensity are expected for the dihydrate only
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