“…The relaxation rate for the 39 K nucleus was found to increase with increasing temperature, and is proportional to T 2 . These results are consistent with the T 2 reported in the 39 K nuclei in KHSO 4 and K 3 H(SO 4 ) 2 crystals [24]. For the 27 Al nucleus, the relaxation rate abruptly decreases below T c as the temperature is increased, and exhibits a remarkable change above 360 K. The jumps in the 39 K and 27 Al spin-lattice relaxation rates near 360 K indicate the phase transition of the crystal.…”
The spin–lattice relaxation processes of the
39K and
27Al nuclei
in KAl(SO4)2·12H2O
crystals were studied; we found that these processes can be described by linear
combinations of two and three exponential functions, respectively. From these results, we
conclude that the discontinuity in the curve of the spin–lattice relaxation rate near
Tc
(= 360 K) corresponds to the first-order phase transition of the crystal. In the case of the
39K
nucleus, the relaxation rate increases as the temperature increases. The relaxation rate of the
27Al nucleus decreases below
Tc as the temperature
increase, while above Tc
its relaxation rate increases with increasing temperature. The temperature dependences of
these spin–lattice relaxation rates can be described with the power law . Least square fits of the relaxation rates at temperatures below
Tc indicate
that k = 2
for the 39K
nucleus and k = 7
for the 27Al
nucleus. From these results, we conclude that the
39K
and 27Al
spin–lattice relaxations occur from Raman relaxation processes.
“…The relaxation rate for the 39 K nucleus was found to increase with increasing temperature, and is proportional to T 2 . These results are consistent with the T 2 reported in the 39 K nuclei in KHSO 4 and K 3 H(SO 4 ) 2 crystals [24]. For the 27 Al nucleus, the relaxation rate abruptly decreases below T c as the temperature is increased, and exhibits a remarkable change above 360 K. The jumps in the 39 K and 27 Al spin-lattice relaxation rates near 360 K indicate the phase transition of the crystal.…”
The spin–lattice relaxation processes of the
39K and
27Al nuclei
in KAl(SO4)2·12H2O
crystals were studied; we found that these processes can be described by linear
combinations of two and three exponential functions, respectively. From these results, we
conclude that the discontinuity in the curve of the spin–lattice relaxation rate near
Tc
(= 360 K) corresponds to the first-order phase transition of the crystal. In the case of the
39K
nucleus, the relaxation rate increases as the temperature increases. The relaxation rate of the
27Al nucleus decreases below
Tc as the temperature
increase, while above Tc
its relaxation rate increases with increasing temperature. The temperature dependences of
these spin–lattice relaxation rates can be described with the power law . Least square fits of the relaxation rates at temperatures below
Tc indicate
that k = 2
for the 39K
nucleus and k = 7
for the 27Al
nucleus. From these results, we conclude that the
39K
and 27Al
spin–lattice relaxations occur from Raman relaxation processes.
“…Until very recently, 39 K NMR has seen very few applications in the solid state because of difficulties in obtaining spectra, as both isotopes, 39 K and 41 K (natural abundance of 93.7 and 6.3%, respectively), are spin- 3 / 2 quadrupolar nuclei with low magnetogyric ratios (absolute resonance frequencies of Ξ = 4.6664 MHz for 39 K and Ξ = 2.5613 MHz for 41 K). The total number of publications involving the more easily observed 39 K nucleus is relatively small and involves results on potassium halides (cubic lattice = zero quadrupole interactions), several studies of single crystals, − and powders. − Most of the studies were performed in magnetic fields of intermediate strengths of 7−11 T, with a single study reporting 39 K NMR in some minerals at 21 T . Two recent high-field studies (19.6 T) were devoted to examination of potassium tetraphenylborates and the detection of potassium cations bound to G-quadruplex structures found in telomeric DNA…”
39K Solid State NMR spectra (static and magic angle spinning (MAS)) on a set of potassium salts measured at 21.14 T show that the chemical shift range for K(+) ions in diamagnetic salts is well in excess of 100 ppm contrary to previous assumptions that it was quite small. Inequivalent potassium sites in crystals can be resolved through differences in chemical shifts, with chemically similar sites showing differences of over 10 ppm. The quadrupolar coupling constants obtained from MAS and solid echo experiments on powders cover the range from zero for potassium in cubic environments in halides to over 3 MHz for the highly asymmetric sites in K2CO3. Although the quadrupolar effects generally dominate the 39K spectra, in several instances, we have observed subtle but significant contributions of chemical shift anisotropy with values up to 45 ppm, a first such observation. Careful analysis of static and MAS spectra allows the observation of the various chemical shift and quadrupole coupling tensor components as well as their relative orientations, thereby demonstrating that high-field 39K NMR spectroscopy in the solid state has a substantial sensitivity to the local environment with parameters that will be of considerable value in materials characterization and electronic structure studies.
“…The compound was subjected to careful optical observation followed by DSC/DTA experiments, which suggest that even though there is no phase transition at high temperatures, the weight loss is primarily due to partial dehydration on the surface of the single crystals of K 3 H(SO 4 ) 2 . A recent study , on single crystals grown by the slow evaporation method shows the presence of a phase transition at a T c of 480 K. 1 (a) Packing diagram of Rb 3 H(SO 4 ) 2 at 293 K. (b) Packing diagram of Rb 3 H(SO 4 ) 2 at 100 K. (c) Packing diagram of Rb 3 H(SO 4 ) 2 at 350 K. (d) Packing diagram of Rb 3 H(SO 4 ) 2 at 425 K. 2 Difference Fourier maps of Rb 3 H(SO 4 ) 2 at (a) 293 K and (b)100 K. …”
Evaluation of phase transitions in a series of hydrogen sulfates (Rb3H(SO4)2, (NH4)3H(SO4)2, K3 H(SO4)2, and Na3H(SO4)2) based on the single-crystal structure analysis has revealed the exact nature of such transitions and has sorted out the various ambiguities involved in earlier literature. Rb3H(SO4)2 at 293 K is C2/c. It is isostructural to its ammonium analogue, (NH4)3H(SO4)2, at room temperature. However, the variable temperature single-crystal diffraction studies indicate that the phase transition mechanism is different. When cooled to 100 K, the structure of Rb3H(SO4)2 remains C2/c. When heated to 350 K, it transforms to C2/m (with double the volume at room temperature), which changes to C2/c (with 4 times the volume at room temperature) at 425 K. The high-temperature (420 K) structural phase transition in (NH4)3H(SO4)2 is shown to be Rm. The structure of Na3H(SO4)2 remains invariant (P21/c) throughout the range of 100-500 K except for the usual contraction of the unit cell at 100 K and expansion at 500 K. The structural phase transitions with temperature for the compound K3H(SO4)2 are very different from those claimed in earlier literature. The hydrogen atom participating in the crucial hydrogen bond joining the two sulfate tetrahedra controls the structural phase transitions at low temperatures in all four compounds. The distortion of the SO4 tetrahedra and the coordination around the metal atom sites control the phase evolution in the Rb compound, while the Na and K analogues show no phase transitions at high temperature, and the NH4 system transforms to a higher symmetry space group resulting in a disorder of the sulfate moiety.
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