1H and 87Rb nuclear magnetic resonance study of the order–disorder phase transition of RbHSeO4 single crystals
Introduction
Protonic conductors are required for progress in the development of energy storage and fuel cells. MeHXO4 (Me: Cs, Rb, NH4; X: S, Se) materials exhibit high levels of proton transport and electrical conductivities of 10−3–10−2 Ω−1 cm−1. The electrical conductivity of these materials drastically changes with increase in temperature from that of an insulator to that of a semiconductor due to a structural phase transition, the so-called “superionic phase transition”. Rubidium hydrogen selenate, RbHSeO4, belongs to this family of compounds, which exhibit phase transitions connected with alterations in proton ordering in their hydrogen-bonded structures. RbHSeO4 undergoes two-phase transitions at TC1=446 K and TC2=371 K [1], [2], [3]. The three crystal phases are denoted as I, II, and III. Phase I (T>TC1) is superionic and has high protonic conductivity. Phase II (TC2<T<TC1) is paraelectric. Below TC2, RbHSeO4 simultaneously shows ferroelectric and ferroelastic properties [4], [5], [6].
The conductivity mechanism of RbHSeO4 crystals has previously been studied near the superionic phase transition temperature [7]. Their properties have been studied using dielectric measurements [8], [9], electro-optic effect [10], [11], [12], [13], [14], Brillouin scattering [15], [16], [17], elastic constants [18], [19], and domain structure [20], [21]. Further, high-resolution 77Se nuclear magnetic resonance (NMR) spectra were obtained for phases II and III of RbHSeO4 by Rozanov et al. [22]. Their analysis of the magnetic shielding tensors for the Se nuclei, including the directions of the tensor principal axes, showed that the ferroelectric phase transition is due to the ordering of hydrogens in only one of the two types of hydrogen bonds. They also reported that the SeO42− ions undergo rapid isotropic reorientational motion in the high-temperature phase, phase I [23]. Rosenberger et al. [24] reported the 1H chemical shifts of polycrystalline RbHSeO4. They have been able to distinguish the signals from protons of hydrogen atoms taking part in hydrogen bonds differing in their lengths.
The spin–lattice relaxation time T1 and spin–spin relaxation time T2 of a crystal can be used as a measure of its dynamics. As protons play a dominant role in the physical properties of these hydrogen-bonded crystals, probing their proton motions with 1H NMR is expected to be a powerful means of studying their microscopic nature. The association between the motions of protons and structural phase transitions has also been the subject of keen research interest.
In this study, the temperature dependences of the relaxation times T1 and T2 for the 1H and 87Rb nuclei in RbHSeO4-single crystals were investigated near the phase transition temperatures using a pulse NMR spectrometer. The aim of this study is to investigate the atomic mechanisms of the ferroelectric phase transition in RbHSeO4. The phase transition induced by the ordering of the protons in the hydrogen bonds is discussed; the present study is very different from previous studies, especially with regard to the roles of the α- and β-type protons, and of the rubidium nuclei.
Section snippets
Crystal structure
The RbHSeO4 crystal has three phases, denoted as I, II, and III, in order of decreasing temperature. Phase II is monoclinic with space group (Z=6) [3], [20] and its cell parameters at 383 K are as follows: a=19.962 Å, b=4.634 Å, c=7.611 °, γ=102.76° [24]. At 293 K, the unit cell parameters of phase III with space group B1 (Z=6) (or P1 (Z=6)) are as follows: a=19.852 Å, b=4.622 Å, c=7.575 Å, α=90.63 °, β=90.04 °, γ=102.75 ° [25], [26], [27], [28]. The structures in phases II and III consist of SeO4
Experimental method
Single crystals of RbHSeO4 with good optical quality were grown with the slow evaporation method from an aqueous solution of H2SeO4 and Rb2SeO4 in an equimolar ratio. The crystals are transparent and consist of polygonal plates parallel to the (1 0 0) cleavage plane.
The NMR signals of the 1H and 87Rb nuclei of the RbHSeO4 single crystals were measured using the Varian 200 FT NMR and Bruker 400 FT NMR spectrometers at the Korea Basic Science Institute. The static magnetic field and the central radio
Results and discussion
The inversion recovery traces of the 1H (I=1/2) nuclei showed a single exponential behavior, and T1 was determined by fitting the recovery pattern with the following equation [29], [30]:where S(t) is the nuclear magnetization at time t, and W is the transition probability corresponding to Δm=±1, which is given by T1=1/W.
The saturation recovery traces for the central line of 87Rb (I=3/2) with dominant quadrupole relaxation can be represented by a combination of two
Conclusion
The phase transitions and proton dynamics of RbHSeO4 single crystals were studied by determining the relaxation times T1 and T2 of the 1H and 87Rb nuclei. The H distributions of the α- and β-type bonds differ significantly. The α-type protons in the ferroelectric phase lead to a noticeable change in the proton magnetic resonance spectra. There is indeed a change in the proton NMR spectrum as a result of the transition from the ferroelectric to the paraelectric phase. The intensity of the signal
Acknowledgment
This work was supported by the Korea Research Foundation Grant Funded by the Korea Government (MOEHRD, Basic Research Promotion Fund) (KRF-2007-531-C00022).
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