Structural changes, thermodynamic properties, 1H magic angle spinning NMR, and 14N NMR of (NH4)2CuCl4·2H2O

The structural changes and thermodynamic properties of (NH4)2CuCl4·2H2O were studied by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. In addition, the chemical shift, line width, and spin-lattice relaxation time of the crystals were also investigated by 1H magic angle spinning nuclear magnetic resonance (MAS NMR), focusing on the role of NH4 and H2O near the phase transition temperature. The change at TC2 (=406 K) and TC3 (=437 K) seems to be a chemical change caused by thermal decomposition rather than a physical change such as a structural phase transition. The changes in the temperature dependence of these data near TC2 are related to variations in the environments surrounding NH4 and H2O. The 14N NMR spectrum is also measured in order to investigate local phenomena related to the phase transition.


I. Introduction
A number of compounds with the chemical formula A 2 BX 4 $2H 2 O, where A ¼ NH 4 , K, Rb, Cs and B ¼ Cu, Mn, Ca, Ni are monovalent and divalent metal ions, respectively, and X ¼ Cl, Br is a halide ion, crystallize as perovskite-type twodimensional layered structures. [1][2][3][4][5][6][7] Crystals with this arrangement can be divided into two classes according to their symmetry and structure. [8][9][10] The rst class includes compounds containing Cu 2+ ions that crystallize with tetragonal symmetry with the space group P4 2 /mnm at room temperature. The tetrahedrons surrounding the divalent metal ions placed at the corners of the unit cell are rotated by exactly 90 with respect to the tetrahedron surrounding the ion at the center of the cell. The A + ions are placed in the almost cubic cavities formed by the tetrahedrons. 11,12 Crystals containing Mn 2+ , Ca 2+ , and Ni 2+ ions form the second of these two classes with triclinic symmetry and the space group P 1 . The (NH 4 ) 2 CuCl 4 $2H 2 O crystal, which is an example of the former, exhibits a structural phase transition from the point group 4/mmm to the point group 4(bar)2m at 200 K. [13][14][15] In addition, two different phase transitions at approximately 383 K and 413 K were observed in the DC electric conductivity measurements reported by Narsimlu et al. 16 At room temperature, (NH 4 ) 2 CuCl 4 $2H 2 O forms a tetragonal structure with space group P4 2 /mnm, as shown in Fig. 1. 14 The unit cell contains two formula units and has the lattice 17 The unit cell contains two Cu 2+ ions at equivalent positions (0, 0, 0) and (1/2, 1/2, 1/2) each surrounded by an approximate octahedron of four Cl À ions and two water molecules. The line connecting the water molecules is parallel to the crystallographic c-axis. 1 The water molecule is trigonal coordinated and forms two equivalent O-  17 The purpose of this study is to investigate the structural changes and thermodynamic properties of (NH 4 ) 2 CuCl 4 $2H 2 O single crystals using differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. Further, the chemical shi, line width, and spin-lattice relaxation time T 1r in the rotating frame of (NH 4 ) 2 CuCl 4 $2H 2 O are measured by 1 H magic angle spinning/nuclear magnetic resonance (MAS/NMR) near the phase transition temperature, focusing on the role of NH 4 and H 2 O. In addition, the 14 N NMR spectrum in the laboratory frame is measured as a function of the temperature, in an attempt to understand the structural geometry near the phase transition temperature. Our ndings represent the rst report on the thermodynamic properties and NMR characteristics of (NH 4 ) 2 CuCl 4 $2H 2 O, and are useful for understanding the phase transitions.

II. Experimental method
Single crystals of (NH 4 ) 2 CuCl 4 $2H 2 O were grown by slowly evaporating aqueous solutions containing a stoichiometric mixture of NH 4 Cl and CuCl 2 $2H 2 O in the molar ratio of 2 : 1 at room temperature. The obtained crystals were light blue. The phase transition temperature was determined using a Dupont 2010 DSC instrument. The rate of temperature change during heating was 10 C min À1 .
The 1 H MAS NMR spectra of (NH 4 ) 2 CuCl 4 $2H 2 O in a rotating frame were obtained using the Bruker DSX 400 FT NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. The static magnetic eld used was 9.4 T, and the central radio frequency was set at u 0 /2p ¼ 400.13 MHz for the 1 H nucleus. The powder sample was placed in a 4 mm MAS probe, and the MAS rate was set to 5 kHz to minimize spinning sideband overlap. The spin-lattice relaxation times in the rotating frame were measured using a saturation recovery pulse sequence called sat-t-p/2; the nuclear magnetizations of the 1 H nuclei at time t aer the sat pulse, a combination of one hundred p/2 pulses applied at regular intervals, were determined following the p/2 excitation pulse. The width of the p/2 pulse was 3.45 ms below 410 K and 6.7 ms above 420 K for 1 H.
In addition, the 14 N NMR spectra of the (NH 4 ) 2 CuCl 4 $2H 2 O single crystals in the laboratory frame were measured using a Unity INOVA 600 NMR spectrometer at the Korea Basic Science Institute, Western Seoul Center. The static magnetic eld was 14.1 T, and the Larmor frequency was set to u 0 /2p ¼ 43.342 MHz. The 14 N NMR experiments were performed using the solid-state echo sequence: 3.7 ms-t-3.7 ms-t. The NMR measurements were obtained in the temperature range of 180-430 K. Unfortunately, the chemical shi and resonance frequency could not be measured above 430 K because the NMR spectrometer did not have adequate temperature control at high temperature. The temperatures of all the samples were maintained at constant values by controlling the helium gas ow and heater current, which yielded an accuracy of AE0.5 K.

III. Results and discussion
The structure of the (NH 4 ) 2 CuCl 4 $2H 2 O crystals at room temperature was determined by X-ray diffraction (XRD) (PANalyical, X'pert Pro MPD) with a Cu-Ka (l ¼ 1.5418Å) radiation source at the Korea Basic Science Institute, Western Seoul Center. Measurement was taken in a q-2q geometry from 10 to 60 at 45 kV and 40 mA tube power. The (NH 4 ) 2 CuCl 4 $2H 2 O crystals were determined to have a tetragonal structure with the lattice constants This result is consistent with those of previous studies. 1,17 The DSC analysis of the (NH 4 ) 2 CuCl 4 $2H 2 O crystals revealed three endothermic peaks during heating, as shown in Fig. 2. The mass of the powdered samples used in the DSC experiment is 6.6 mg. The endothermic peak enlarged in Fig. 2 near 200 K (¼T C1 ) is consistent with the phase transition temperature reported previously, and is very small relative to the other endothermic peaks. The endothermic peaks near 406 K (¼T C2 ) and 437 K (¼T C3 ) are related to the thermal dehydration according to the below-mentioned loss of H 2 O. TG analysis was used to determine whether these high-temperature transformations were structural phase transitions or melting temperatures. The TG curve of (NH 4 ) 2 CuCl 4 $2H 2 O is shown in Fig. 3  Optical polarizing microscopy showed that the crystals are light blue in color at room temperature and that they undergo color changes as the temperature increases. As the temperature increases, the color of the crystal varies from light blue (295 K and 373 K) to light green (386 K) to green (393 K) to dark yellow (433 K) and nally to brown (448 K), as shown in the inset of Fig. 3. This color change may be related to the partial loss of H 2 O. The DSC peaks at 406 K and 437 K are related to chemical changes through thermal dehydration, based on the TG and optical polarizing microscopy results. The weight loss evidenced by the TG curve suggests that T C2 and T C3 in (NH 4 ) 2 -CuCl 4 $2H 2 O are not related to physical changes such as structural phase transitions. Rather, they are related to a chemical change through thermal dehydration.
The 1 H MAS NMR spectra of (NH 4 ) 2 CuCl 4 $2H 2 O, which were obtained as a function of temperature, only exhibit one peak ascribed to chemical shi, as shown in Fig. 4. The spinning sidebands of the peak are marked with asterisks. There are two kinds of protons in (NH 4 ) 2 CuCl 4 $2H 2 O: ammonium protons and water protons. The current experiment was unable to distinguish the signals resulting from these two types of protons, because the eight protons from ammonium and the four protons from water are expected to yield two superimposed lines. Thus, the signal generated by the ammonium protons might include the signal caused by the water protons.
The chemical shis for the 1 H nuclei in (NH 4 ) 2 CuCl 4 $2H 2 O with respect to tetramethylsilane (TMS) at a frequency of 400.13 MHz are presented in Fig. 5 as a function of temperature. The chemical shis of the 1 H nucleus change abruptly near T C2 , whereas those near T C1 are continuous. The change in the chemical shi with temperature indicates that the conguration of the atoms neighboring the 1 H nuclei is undergoing change. The full width at half maximum (FWHM) of the 1 H MAS NMR signal is shown in the inset of Fig. 5 as a function of temperature. As the temperature increases, the FWHM near T C1 is continuous, and that near T C2 decreases in a step-like shape. This stepwise narrowing is generally considered to be caused by internal motions, which have a temperature dependence related to that observed for the chemical shi. The shape of the line changes progressively with increasing temperature from the Gaussian-like shape of a rigid lattice to a Lorentzian shape. Near T C2 the line width undergoes an abrupt drop, aer which the line width becomes considerably narrower.
The nuclear magnetization recovery traces for 1 H MAS NMR are usually represented by a single-exponential function. However, the magnetization recovery traces for 1 H MAS NMR in (NH 4 ) 2 CuCl 4 $2H 2 O could be described by the following doubleexponential function: [18][19][20] M(t)/M(N) ¼ a exp(Àt/T 1r (s)) + b exp(Àt/T 1r (l)), where T 1r (s) and T 1r (l) are the short and long spin-lattice relaxation times, respectively. The magnetization recovery   traces showing the delay time of the 1 H resonance signal at temperatures of 200 K, 420 K, and 430 K are shown in the inset of Fig. 6. The obtained spin-lattice relaxation curves were well tted by the abovementioned double-exponential function, with the slope of the recovery trace decreasing as the temperature increased. Note that the occurrence of a double-exponential spin-lattice relaxation pattern is unusual for a strongly dipolar-coupled proton system, whereas spin diffusion is expected to afford a single-exponential relaxation pattern. 21 Therefore, we concluded that the proton system comprises two spatially well-separated nuclei. The values of T 1r for 1 H in (NH 4 ) 2 CuCl 4 $2H 2 O at several temperatures are shown in Fig. 6. As mentioned above, two different sets of T 1r values were obtained from the double-exponential function: the larger values, T 1r (l), correspond to the longer N-H/Cl chain systems in the NH 4 groups, whereas the smaller ones, T 1r (s), correspond to the shorter O-H/Cl chain systems in the H 2 O groups. Here, T 1r (l) decreases slightly with increasing temperature, as opposed to T 1r (s), which increases with increasing temperature. Furthermore, T 1r (s) and T 1r (l) for 1 H in these groups were continuous in close proximity to T C1 , whereas T 1r (s) and T 1r (l) of 1 Fig. 7. Two resonance lines are expected because of the quadrupole interaction of the 14 N (I ¼ 1) nucleus. 19 With respect to the crystal orientation, the magnetic eld was applied along the crystallographic c-axis. The resonance frequencies of the 14 N signals are plotted in Fig. 8, as a function of temperature. In the vicinity of T C1 , the frequencies of both signals are discontinuous, whereas those near T C2 are continuous, and the resonance frequency increases with increasing temperature. In addition, the splitting of the 14 N resonance lines above 200 K increases slightly with increasing temperature. These temperature-dependent changes in the 14 N resonance frequencies are attributed to changes in the structural geometry of the NH 4 + ion. 22 In all temperature, all nitrogen is physically equivalent, and the 14 N quadrupole parameter is slowly increased. 23 In this case, the electric eld gradient (EFG) tensors at the N sites are varied, reecting the changing atomic congurations around the 14 N nuclei in NH 4 .   This journal is © The Royal Society of Chemistry 2018

IV. Conclusion
The thermodynamic properties and structural mechanisms near the phase transition temperatures in (NH 4 ) 2 CuCl 4 $2H 2 O were studied through DSC, TG, NMR chemical shi, and the spin-lattice relaxation time T 1r . The DSC and TG results indicate that the endothermic peak near 200 K (¼T C1 ) is a structural phase transition, and the endothermic peaks near 406 K (¼T C2 ) and 437 K (¼T C3 ) are related to a chemical change through thermal dehydration due to the escape of H 2 O. On the other hand, the changes in the temperature dependence of the chemical shi, linewidth, and spin-lattice relaxation time T 1r near T C2 are related to variations in the symmetry of the environments of NH 4 and H 2 O. The mechanism of these changes at high temperature is related to hydrogen bond proton transfer involving the breakage of a weak hydrogen bond. The change at T C2 and T C3 seems to be a chemical change caused by thermal decomposition rather than a physical change such as a structural phase transition, and the tetrahedrons formed by the water molecules are probably disrupted by the loss of H 2 O. From the DSC, TG, and NMR data, it is clear that the structural change at high temperature arises due to the loss of the two water molecules coordinated to the Cu 2+ ion along the c-axis.

Conflicts of interest
There are no conicts to declare.