Uskova N I, Charnaya E V, Podorozhkin D Yu, Baryshnikov S V, Milinskiy A Yu, Egorova I V
St. Petersburg State University, St. Petersburg 198504, Russia.
J Phys Condens Matter. 2019 Dec 18;31(50):505404. doi: 10.1088/1361-648X/ab40eb.
The diisopropylammonium chloride (CHClN, DIPAC) and diisopropylammonium bromide (CHBrN, DIPAB) molecular crystals are recently discovered ferroelectrics with sufficiently high spontaneous polarization and Curie temperature. We performed first studies of these crystals by C NMR. CP MAS spectra were collected within large temperature ranges covering the Curie points. The reconstructive phase transition from the initial orthorhombic P222 structure of DIPAB to the monoclinic ferroelectric P2 structure leads to an abrupt alteration in the C spectrum. The C spectra for DIPAC and DIPAB in the ferroelectric P2 phase are quite similar with four lines at lower frequencies, which correspond to the CH groups, and two lines with close chemical shifts, which correspond to two CH groups. The transition into the paraphase leads to gradual reduction of the interline distances in the low-frequency quadruplet and in the doublet. The step-like changes in the interline frequency shifts at this transition indicating its first order. The analysis of the spectrum evolution in the paraphase shows that only a CH group lays in the reflection plane above the P2 → P2/m transition, while the second CH group only moves closer to the reflection plane upon further heating.
二异丙基氯化铵（CHClN，DIPAC）和二异丙基溴化铵（CHBrN，DIPAB）分子晶体是最近发现的具有足够高自发极化和居里温度的铁电体。我们通过碳核磁共振（C NMR）对这些晶体进行了首次研究。在覆盖居里点的大温度范围内收集了交叉极化魔角旋转（CP MAS）光谱。从DIPAB最初的正交P222结构到单斜铁电P2结构的重构相变导致碳谱发生突然变化。铁电P2相中的DIPAC和DIPAB的碳谱非常相似，低频处有四条线，对应于CH基团，还有两条化学位移相近的线，对应于两个CH基团。向顺电相的转变导致低频四重峰和双峰中线间距离逐渐减小。此转变时线间频移的阶梯状变化表明其为一级相变。对顺电相光谱演化的分析表明，在P2→P2/m转变以上，只有一个CH基团位于反射平面内，而另一个CH基团在进一步加热时只是向反射平面靠近。
