Vibrational properties and the temperature-induced phase transition mechanism have been studied in [NH4][Zn(HCOO)3] and [ND4][Zn(DCOO)3] metal organic frameworks by variable-temperature dielectric, IR, and Raman measurements. DFT calculations allowed proposing the detailed assignment of vibrational modes to respective motions of atoms in the unit cell. Temperature-dependent studies reveal a very weak isotopic effect on the phase transition temperature and confirm that ordering of ammonium cations plays a major role in the mechanism of the phase transition. We also present high-pressure Raman scattering studies on [ND4][Zn(DCOO)3]. The results indicate the rigidity of the formate ions and strong compressibility of the ZnO6 octahedra. They also reveal the onset of a pressure-induced phase transition at about 1.1 GPa. This transition has strong first-order character, and it is associated with a large distortion of the metal formate framework. Our data indicate the presence of at least two nonequivalent forma...

http://pubs.acs.org/doi/pdf/10.1021/ic501074x

Temperature- and Pressure-Induced Phase Transitions in the Metal Formate Framework of [ND4][Zn(DCOO)3] and [NH4][Zn(HCOO)3]

Numerous nuclear magnetic resonance (NMR) investigations of the KH2PO4 family of hydrogen-bonded compounds that display ferroelectric behavior have reported results related to structure, molecular motion, and phase transitions. The current study looks at how high magnetic field and high-resolution solid-state techniques affect the measured NMR parameters for ammonium dihydrogen phosphate (NH4H2PO4). Under magic-angle spinning (MAS), the ammonium and acid protons do not achieve a common spin temperature as previously stated in the literature [i.e., the spin–lattice relaxation times (T1) are different]. While the relaxation behavior in the static sample can be well-described with a single exponential at ambient temperature and below, the behavior becomes increasingly nonexponential as the sample is taken above ambient temperature. This behavior may arise from the motion of the ammonium moiety or from ionic conduction. The three principal values of the 1H shielding tensors for the different species of proton...

Revisiting NH4H2PO4: 1H MAS, CRAMPS, and Spin–Lattice Relaxation

Ammonium cyanate is well-known to undergo a solid-state reaction to form urea. Knowledge of fundamental physicochemical properties of solid ammonium cyanate is a prerequisite for understanding this solid-state chemical transformation, and this paper presents a comprehensive study of the dynamic properties of the ammonium cation in this material. The techniques used—incoherent quasielastic neutron scattering (QENS) and solid-state 2H NMR spectroscopy—provide insights into dynamic properties across a complementary range of time scales. The QENS investigations (carried out on a sample with natural isotopic abundances) employed two different spectrometers, allowing different experimental resolutions to be probed. The 2H NMR experiments (carried out on the deuterated material ND4+OCN−) involved both 2H NMR line shape analysis and 2H NMR spin−lattice relaxation time measurements. The results of both the QENS and 2H NMR studies demonstrate that the ammonium cation exhibits reorientational dynamics across a wide ...

Dynamic Properties of Solid Ammonium Cyanate

A calorimetric study of urea/n-nonadecane, CO(NH(2))(2)/C(19)H(40), and the deuterated derivatives, CO(ND(2))(2)/C(19)D(40) and CO(NH(2))(2)/C(19)D(40), around the structural phase transition temperature is presented. For this purpose differential scanning (DSC), temperature-modulated (AC) and adiabatic calorimetry have been used and the obtained results are compared. Leaving apart the noticeable peak associated with the main phase transition at 158.5, 149.4 and 154 K respectively, small anomalies of the specific heat are found at lower temperatures and their corresponding entropic and enthalpic changes are reported. Heating and cooling experiments show the influence of the temperature rate and the thermal history on the detailed profile of the specific heat traces. The presence of thermal hysteresis and latent heat as a way to characterize the order of the phase transitions is discussed. Finally, a tentative approach to the urea and the alkyl chain contributions to the specific heat and their influence on the phase transition mechanisms is presented.

https://iopscience.iop.org/article/10.1088/0953-8984/19/18/186221/pdf

Phase transitions in the urea/n-nonadecane system by calorimetric techniques.

35 Cl nuclear quadrupole resonance was applied to study molecular dynamics in (ND4)2PtCl4. The spin-lattice relaxation time T1Q, the quadrupole resonance frequency νQ, the line width δν and the integrated intensity T∫F(ν) d ν of the resonance line F(ν) multiplied by the temperature T to compensate for the Curie law effect were measured in a wide temperature range 12.4 K K . Effects due to cation and anion mobility were clearly seen and their contributions are disentangled. Domain structure and diverse mobility of ammonium ions in the ordered phase are the basic assumptions underlying the fitting of the experimental data. An order–disorder phase transition takes place at 148 K and appears visible in all measured quantities. Unrestricted mobility of ammonium ions dominates above the phase transition. At higher temperatures, mobility of the planar PtCl42− ions, first torsional oscillations and then reorientational jumps, becomes detectable.

35Cl nuclear quadrupole resonance (NQR) study of ion mobility in (ND4)2PtCl4

The effect of deuteration on the motion of ammonium ions and/or the structural phase transition in NH4AuCl4 was studied by means of 35Cl nuclear quadrupole resonance measurements. The transition temperature shifted by 5 K from 28.5 K for NH4AuCl4 to 33.5 K for ND4AuCl4. Spin-lattice relaxation caused by the modulation of the electric field gradient due to the motion of nearby ammonium ions was observed. The activation energy of the motion of the ammonium ions was determined to be 0.8 and 1.8 kJ mol-1 for NH4AuCl4 and ND4AuCl4, respectively. By the use of the simple harmonic oscillator model, the barrier height V0 and the number of wells n of the potential for the reorientation of the ammonium ions were determined as V0 = 4.2 kJ mol-1 and n = 4 from the above activation energies for the NH4+ and ND4+ reorientations.

http://iopscience.iop.org/article/10.1088/0953-8984/11/26/322/pdf

The effect of deuteration on the motion of ammonium ions and the structural phase transition in NH4AuCl4

Cl NQR measurements revealed two phase transitions at 14.5 K and 70 K in NH4ICl4. By deuteration of the ammonium ions, the respective transition temperature increased to 27 K and 74 K. In the lowest-temperature phase of NH4ICl4, by the deuteration a remarkable change of the temperature dependence of the Cl NQR frequencies was observed. The NQR spin-lattice relaxation-time minimum, ascribable to the modulation effect of the electric field gradient (EFG) due to a motion of the ammonium ions, was observed at 7 K and 11 K in NH4ICl4 and ND4ICl4, respectively. The activation energy of the ammonium motion and the fluctuation fraction q0=q of the EFG were determined to be 0.32 kJ mol 1 and 1.3 10 3 for NH4ICl4, and 0.30 kJ mol 1 and 0.88 10 3 for ND4ICl4. The experimental result suggests that the inter-ionic interaction between the ammonium ion and the ICl 4 complex anion is weakened by the deuteration. However, the frequency shift observed in the lowest-temperature phase is much larger in ND4ICl4 than in NH4ICl4. This suggests that the lattice distortion in NH4ICl4 is suppressed to a great deal by the tunneling rotation of NH4 ions. The effective inter-ionic interaction responsible for the lattice distortion, the transition temperature and the enhancement of tunneling of NH4 as compared with ND4 is discussed for NH4ICl4 as well as for previously reported NH4AuCl4 and -NH4HgCl3.

/pdf/effect-of-deuteration-on-ammonium-motion-and-structural-3dpswdgreo.pdf

Tomohiro Ishizaka

Papers

The effect of deuteration on the motion of ammonium ions and the structural phase transition in NH4AuCl4

Effect of Deuteration on Ammonium Motion and Structural Phase Transition Studied by Nuclear Quadrupole Resonance