Chemical exchange in NH₄HSeO₄single crystals studied by two-dimensional²H nuclear magnetic resonance

Abstract

Processes of chemical exchange of deuterons in partially deuterated ammonium hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of 2H nuclear magnetic resonance (NMR) experiments over a wide temperature range. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed line-shape changes which are characteristic for chemical exchange processes. A detailed study of these exchange processes in AHSe is achieved by means of two-dimensional 2H NMR experiments. In the temperature range investigated, a chemical exchange occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (- and -positions). It was established that the rates of exchange between all types of hydrogen-bound deuteron are approximately the same. Exchange between these positions and the deuterons in the ND4 groups could not be detected. On the basis of our findings, we finally discuss a model for the microscopic mechanism of hydrogen transport in AHSe.

Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

“…Such crystals serve as very appropriate model objects in order to verify the various assumptions about the microscopic mechanisms of ionic conductivity. Our 2-D-2H-NMR (nuclear magnetic resonance) exchange spectroscopy experiments [5][6][7] prove the presence of chemical exchange processes between deuterons of hydrogen bonds in the structure of ammonium hydrogen selenate NH4HSeO 4 (AHSe), betaine phosphate (BP), betaine phosphite (BPI) and mixed crystals BPI_xBPIx, containing quasi 1-D chains of SeO4-, PO4-, or PO3-tetrahedra connected by hydrogen bonds. A close relationship between chemical exchange and ionic conductivity processes was proved [7].…”

“…This is caused on the one hand by the important practical applications of these crystals in energy storage and fuel cells and on the other hand by the possibility to study the fundamental problem of the microscopic mechanism of the electric conductivity ha superionic crystals [1][2][3][4][5][6]. From this point of view of particular interest are crystals with the structural peculiarity of quasi one-dimensional (l-D) chains of atomic groups bound by hydrogen bonds showing proton conductivity.…”

Study on nuclear magnetic resonance of superionic conductor NH4HSeO4in rotating frame

“…The crystal then transits to the incommensurate phase (III) at 261 K (=T i ) and to the ferroelectric phase (IV) at 250 K (=T C1 ). When the temperature drops below 98 K (=T C2 ), the crystal commences its low temperature phase and becomes non-ferroelectric [12][13][14]. Previous studies of NH 4 HSeO 4 with 1 H nuclear magnetic resonance (NMR) spectra, nuclear spin-spin relaxation times T 2 , in the laboratory frame and conductivity measurements in the vicinity of T SI concluded that the diffusions of protons and NH 4 + cations in the crystals were isotropic [15,16].…”

“…Previous studies of NH 4 HSeO 4 with 1 H nuclear magnetic resonance (NMR) spectra, nuclear spin-spin relaxation times T 2 , in the laboratory frame and conductivity measurements in the vicinity of T SI concluded that the diffusions of protons and NH 4 + cations in the crystals were isotropic [15,16]. In addition, the microscopic mechanism of deuteron transport in ND 4 DSeO 4 crystals was studied by means of 2 H NMR [13,[17][18][19]. A detailed analysis of the behavior of the ammonium and HSeO 4 deuterons in ND 4 DSeO 4 was discussed by Ivanov et al [13].…”

“…In addition, the microscopic mechanism of deuteron transport in ND 4 DSeO 4 crystals was studied by means of 2 H NMR [13,[17][18][19]. A detailed analysis of the behavior of the ammonium and HSeO 4 deuterons in ND 4 DSeO 4 was discussed by Ivanov et al [13]. Early 1 H NMR investigations of NH 4 HSeO 4 only described the average behavior of protons belonging to the hydrogen-bond and NH 4 groups.…”