Abstract
The 35Cl nuclear quadruple resonance (NQR) frequencies and spin-lattice relaxation times were measured in the temperature range of 4.2–420 K, of the above-room-temperature ferroelectric 5,6-dichloro-2-methylbenzimidazole, the ferroelectricity of which is thought to arise from the positional ordering of protons along chain of N-H...N hydrogen bonds. The first-order phase transition was clearly detected at around 400 K, as a discontinuity in the temperature dependence of NQR frequencies. The two NQR lines observed showed a much more significant difference in frequency in the high-temperature phase. This is consistent with the recently reported symmetry-lowering of crystal structure on warming. The relaxation mechanism in the low-temperature phase is discussed.
Highlights
DC-MBI (5,6-Dichloro-2-methylbenzimidazole) has been reported by Horiuchi et al [1] to exhibit ferroelectricity above room temperature
For the nuclear quadruple resonance (NQR) measurements, the frequency ranges of 36.1 ± 1 MHz and 35.7 ± 3 MHz were swept at 77 K and room temperature, respectively
high-temperature phase (HTP). distortion mode analysis, the structure of HTP was shown to belong to the symmetry monoclinic polar space group Pc, while the low-temperature phase (LTP) belongs to the orthorhombic polar space group
Summary
DC-MBI (5,6-Dichloro-2-methylbenzimidazole) has been reported by Horiuchi et al [1] to exhibit ferroelectricity above room temperature. The ferroelectricity is thought to arise from the positional ordering of protons along chain of N-H...N hydrogen bonds. The relaxation was considered to be relating to a ferroelectric domain-wall motion. 4 × 10−5 s at low temperatures, may be explained by quantum relaxation as reported for the domain-wall motion in ferromagnetics [4]. It was challenged to detect the domain-wall motion due to quantum tunneling, by NQR spin-lattice relaxation time measurements. The spin-lattice relaxation time T1Q was determined by the inversion recovery method. 1H nuclear magnetic resonance (NMR) spin-lattice relaxation time T1 was measured by use of a Thamway’s spectrometer (PROT3100MR, Thamway Co., Ltd, Fuji, Shizuoka, Japan) at 45.08 MHz by the saturation recovery method using a comb-τ-90°(x)-τe-90°(y) pulse sequence The signal intensity was monitored by the echo height. 1H nuclear magnetic resonance (NMR) spin-lattice relaxation time T1 was measured by use of a Thamway’s spectrometer (PROT3100MR, Thamway Co., Ltd, Fuji, Shizuoka, Japan) at 45.08 MHz by the saturation recovery method using a comb-τ-90°(x)-τe-90°(y) pulse sequence
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