Nuclear magnetic resonance study of superionic conductors with tysonite structure

Abstract

The fluorine mobility in single crystals of the anion-deficient solid solution La1-xSrxF3-x (x=0, 0.001, 0.003, 0.01, 0.03, 0.05 or 0.16) with the tysonite structure has been studied in the temperature range of 130-520 K by 19F NMR. Different modes of ionic motion were identified and the leading mechanism of ionic diffusion has been suggested. The influences of temperature and fluorine vacancy concentration on the ionic mobility in tysonite have been investigated. In all crystals, fluorine diffusion begins in the F1 sublattice. On increase in the temperature, ionic exchange between the F1 and F2,3 sublattices becomes observable. Addition of small amounts of Sr (x<0.003) significantly increases the ionic mobility between the sublattices. For 0.003<x<0.03 a further increase in x has no significant influence on the ionic exchange. Around x approximately=0.01 the beginning of cluster formation is noticeable. At higher concentrations (0.03<x<0.16) it is seen that ionic mobility decreases again and the guest-induced vacancies are preferentially located in the F1 sublattice. At these doping concentrations the influence of large clusters seems to play a significant role.

Frequency-dependent spin-lattice relaxation study of transport processes in superionic conductors

“…The single crystals of La,_ X SrF3 _x (x = 0; 0.03) were the same as studied in earlier works [9,16]. Their quality was checked optically and using Laue X-ray diffraction.…”

“…This is particularly important when more than one motional process are activated, a situation found in superionic conductors of the type La 1 _XSrF3 _x with tysonite structure [9].…”

“…They are not only structurally but also dynamically nonequivalent. For the temperature range between 240 and 400 K the motion is restricted mainly to the F -ions in the F 1 sublattice, while F -ions in the F2 or in the F3 sublattices are slow, i.e., the corresponding correlation times are longer than a half of a millisecond [9]. Hence, the spectral density of local field fluctuations in the frequency range above 50 kHz is determined by the F 1 ions.…”

Field-dependent nuclear relaxation of spins 1/2 induced by dipole–dipole couplings to quadrupole spins: LaF3 crystals as an example

“…The particular dipole-dipole interactions are modulated by jumps of the fluorine ions between equivalent (F A -F A , F B -F B ) and non-equivalent (F A -F B ) lattice sites. Important information on the dynamics of the fluorine ions in LaF 3 has been gained from the analysis of the fluorine lineshape [16,17], fluorine diffusion measurement by NMR gradient techniques [18] and some earlier relaxation studies performed at high magnetic field [14,19]. In [16,17] the two different fluorine subsystems have been clearly identified in the NMR spectrum by their chemical shifts.…”

“…Important information on the dynamics of the fluorine ions in LaF 3 has been gained from the analysis of the fluorine lineshape [16,17], fluorine diffusion measurement by NMR gradient techniques [18] and some earlier relaxation studies performed at high magnetic field [14,19]. In [16,17] the two different fluorine subsystems have been clearly identified in the NMR spectrum by their chemical shifts. It has been observed that fast fluorine motion within the F A sublattice leads to a narrowing of the corresponding resonance line, while the F B line stays broad until it collapses with the faster one.…”