Proton dynamics in Rb₃H(SO₄)₂doped with Mn²⁺studied by EPR and impedance spectroscopy

Abstract

The temperature dependence of the linewidth ΔB(pp) and spin Hamiltonian parameters of Rb(3)H(SO(4))(2) (RHS) crystals were studied by X-band continuous-wave electron paramagnetic resonance. Direction cosines of the zero-field-splitting (ZFS) tensor and the coordination of the Mn(2+) impurities in the crystal lattice were determined in the temperature range 20-450 K. The EPR spectra of crystalline samples and their irreversible changes into pseudo-powder spectra after heating above the superprotonic phase transition temperature were found. Our studies of electric conductivity have shown that, for crystals doped with Mn(2+) ions, the conductivity values increase after heating by about one order of magnitude at intermediate temperatures (below the transition temperature to the superionic phase) in comparison with those of the pure RHS.

Low temperature short-range ordering caused by Mn2 +doping of Rb3H(SO4)2

“…There are two sets of rubidium ions and one type of SO 2− 4 ions in the RHS crystal structure at room temperature. Each Rb 3 H(SO 4 ) 2 formula consists of one Rb + (I)-type ion in a special position on the twofold axis and two Rb + (II)-type ions positioned differently (see [3], figure 2 therein). The Mn 2+ as well as VO 2+ ions replace only the Rb + (I)-type ions in the crystal structure and the resulting excess charge is compensated by proton vacancies [3,19].…”

“…Each Rb 3 H(SO 4 ) 2 formula consists of one Rb + (I)-type ion in a special position on the twofold axis and two Rb + (II)-type ions positioned differently (see [3], figure 2 therein). The Mn 2+ as well as VO 2+ ions replace only the Rb + (I)-type ions in the crystal structure and the resulting excess charge is compensated by proton vacancies [3,19]. This is why the concentrations of Mn 2+ and VO 2+ were calculated per Rb 3 H(SO 4 ) 2 formula unit and that of Cr 6+ per SO 2− 4 ion.…”

“…Our previous studies revealed a significant influence of a small amount of Mn 2+ ions on the electric transport properties and on the temperature of the superionic phase transition in Me 3 H(SO 4 ) 2 compounds [2,3]. An anomalous temperature dependence of the EPR parameters of Rb 3 H(SO 4 ) 2 :Mn 2+ was observed [16] close to the temperature of the antiferroelectric phase transition of Rb 3 D(SO 4 ) 2 (abbreviated as RDS).…”

“…An anomalous temperature dependence of the EPR parameters of Rb 3 H(SO 4 ) 2 :Mn 2+ was observed [16] close to the temperature of the antiferroelectric phase transition of Rb 3 D(SO 4 ) 2 (abbreviated as RDS). However, recent results concerning the dielectric properties confirmed that there are no phase transitions at this or lower temperatures down to 30 K for the Mn 2+ -doped Rb 3 H(SO 4 ) 2 (abbreviated as RHS) compound [3].…”

“…The hydrogen-bonded crystals of the Me 3 A(XO 4 ) 2 family (Me = K, Rb, Tl, NH 4 , Cs; A = H, D; X = S, Se) are of great interest due to superionic properties at high temperature [1][2][3][4][5][6] and an unusually large isotopic effect clearly observed at low temperatures [7][8][9]. In contrast to (NH 4 ) 3 A(XO 4 ) 2 , most crystals of the Me 3 A(XO 4 ) 2 group have rather simple phase diagrams revealing only one or two phase transitions at low temperatures.…”