Protonic conductivity and ferroelastic instability in triammonium hydrogen disulphate: a dielectric and neutron diffraction study

Abstract

The protonic conductivity of has been measured using the complex-admittance method in the frequency range 30 Hz-200 MHz and the temperature interval 290-500 K covering both the ferroelastic and the paraelastic phases. The dc conductivity shows quasi-two-dimensional behaviour and in the trigonal paraelastic phase its values in the (001) plane are typically super-protonic with low activation enthalpy . The temperature dependence of the monoclinic superstructure reflection 120 has been studied using elastic neutron diffraction. It was found that the observed anomalies of the macroscopic and microscopic quantities, such as the morphic birefringence and the high-frequency dielectric constant on the one hand and the diffusion proton dynamics as well as the integrated intensity of the reflection 120 on the other hand, show pronounced differences in their temperature evolution below the ferroelastic phase transition temperature. The neutron scattering results as well as the dielectric measurements indicate precursor effects above . The results are discussed on the basis of a phenomenological two-order-parameter model for the - ferroelastic phase transition. It is argued that properties which originate from the disorder of the oxygen and proton subsystem can be described by irreducible representations of the wave vector at the point and the L point of the Brillouin zone, while properties which originate from displacements of the heavy atoms (the displacement mode) are solely described by the wave vector at the L point of the Brillouin zone of the paraphase.

Proton Dynamics in Letovicite, (NH4)3H(SO4)2: A 1H and 14N NMR Spectroscopic Study

“…However, these narrowing effects lie truly in the phase transition temperature range. Moreover, at T c the proton conductivity in the ab-plane increases from 10 −3 to 10 −2 S cm −1 [8]. Theor.…”

“…In the trigonal paraelastic phase the centers of the three SO· · · H-OS bonds are distributed over the 9e Wyckoff positions forming a dynamically disordered hydrogen bond network. This effect enables a superprotonic conductivity of σ dc a = σ dc b = 10 −2 S cm −1 in the (001) plane [8][9][10]. Substantial conductivity exists also perpendicular to the (001) plane along the trigonal c-axis (σ dc c = 10 −4 S cm −1 ).…”

“…The thermodynamic behavior can be described by means of higher terms in the Landau potential of the Gibbs free enthalpy appropriate to a Slater instability [7]. Neutron diffraction and dielectric studies show the formation of precursor phenomena above the phase transition temperature, which means that remaining domains of the ferroelastic phase still exist in the paraelastic phase [8]. Details of these effects have not been investigated extensively.…”

The Disordered High-Temperature Structure of (NH4)3H(SO4)2 and Its Relationship to the Room-Temperature Phase

“…Other layers composed of isolated [NH 4 + ] tetrahedra alternate with them and provide the necessary charge balance. 2 The high-temperature phase I of (NH 4 ) 3 H(SO 4 ) 2 is of special interest due to its elevated protonic conductivity (9). Two independent structure determinations have been carried out.…”

EPR Study of VO2+Center in Fast Proton Conductor K3H(SO4)2

“…Crystals, which belong to family M 3 H(XO 4 ) 2 , where M = NH 4 , Rb, Cs, K; X = S, Se, exhibit a high proton conductivity [1][2][3][4][5][6][7]. The crystal structure of K 3 H(SO 4 ) 2 (KHS) was determined by X-ray diffraction [8,9].…”