α-甘氨酸晶体中NH_3~+扭曲振动的变温拉曼光谱研究

1999年,Chilcott研究了α-甘氨酸单晶电阻抗随温度的变化,发现晶体在304 K开始导电。为了了解其导电机理,本文研究90 K到340 K的拉曼光谱,发现在α-甘氨酸单晶中NH3+扭曲振动模式分裂,表现为N-H(6)…O(1)(491 cm-1)和N-H(7)…O(2)(483 cm-1)模式,以及CO2-摇摆模式(503cm-1),在304 K均有不连续性变化,并与变温中子衍射晶体结构层内的氢键N-H(6)…O(1)键角和N-H(7)…O(2)长度出现转折点一致。由于α-甘氨酸晶胞中NH3+和CO2-基团构成的电偶极子在变温下重新定向,出现两性离子电荷重心变化致使晶体极化,引起晶体在304 K左右发生了铁电相变。

Temperature-dependent Raman Spectroscopic Study of NH+3 Torsional Mode in α-glycine Crystal

In 1999,Chilcott found that electrical impedance measurements of single crystal glycine revealed anomalous temperature dependence of the conductance and capacitance around 304 K.This unusual electric behavior was not explained by the conduction mechanism.The present paper investigates temperature-dependent Raman spectra of α-glycine from 90 K to 340 K.We find a discontinuous frequency variation of the NH+3 torsional mode of α-glycine crystal around 304 K.It is consistent with the sharp peak of the hydrogen bonds N-H(6)…O(1) and N-H(7)…O(2) around 304 K,which was assumed due to the NH+3 torsion mode splitting and deformations.The electrical dipoles of NH+3CH2COO-are reoriented and the crystal had to undergo a ferroelectric transition around 304 K.