Three New Niccolites: High‐Temperature Phase Transitions, Prominent Anisotropic Thermal Expansions, Dielectric Anomalies, and Magnetism

Abstract

Three new iso‐structural ammonium metal formates of [dmpnH2][M2(HCOO)6], in which dmpnH22+=N,N′‐dimethyl‐1,3‐propylenediammoium and M=divalent Co, Zn and Mg ions, are reported. They possess niccolite metal formate frameworks with long‐shaped cavities for the accommodation of dmpnH22+ cations. The three materials display reversible phase transitions of similar mechanism from ordered, antipolar or antiferroelectric, low‐temperature phases in space group C2/c, to disordered, paraelectric, high‐temperature phases in space group Ptrue3‾ 1c, with quite high critical temperatures of 366, 370, and 334 K for Co, Zn, and Mg members, respectively. On warming, the dmpnH22+ cation experiences an ordered state with gradual increase of the local vibration motions of the central CH2 and terminal CH3 groups, a partially disordered state with gradually enhanced flipping motion between the major and minor orientations, and finally a twisting or rotating motion after the phase transition, accompanied by prominent anisotropic thermal expansions and dielectric anomalies/relaxations. The phase transition characters and relevant properties also exhibit a subtle metal‐dependence. The Co member shows spin‐canted antiferromagnetism below the Néel temperature of 16.1 K, with unusual large spontaneous magnetization and coercive field.

Reversible Phase Transition, Switchable Second‐Order Nonlinear Optical and Dielectric Properties in a Bismuth(III)‐Based Compound: (R)‐3‐(Fluoropyrrolidinium)2BiBr5

“…[7] Therefore, manipulation of the dynamics of the dipoles is the key to build moleculebased stimuli-responsive materials by motional changes of polar components between static and dynamic phases. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Inclusion compounds or host-guest compounds can well exemplify this concept by assembling both static and dynamic components together. [22] The host can imposes internal (chemical) pressure upon the guest.…”

“…[24] Intriguing properties, such as ferroelectric, antiferroelectric, and dielectric transitions, have been extensively investigated. [3,6,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Besides, the H/F substitution, as a supplement to the isotopic effect, has also demonstrated a particularly useful idea in designing halogenometallate hybrid compounds with a switchable phase transition behavior. [24,26,27] Due to the different electronegativities between the H and F atoms, the H/F substituted products reduce the crystal symmetry while the structure remains basically unchanged and increase the possibility of the significant changes in the physical responses.…”

“…It is known that dipolar reorientation contributes greatly to switchable properties in molecular‐based materials, compared with the other two intrinsic polarizability origins (electronic and ionic displacements) . Therefore, manipulation of the dynamics of the dipoles is the key to build molecule‐based stimuli‐responsive materials by motional changes of polar components between static and dynamic phases . Inclusion compounds or host–guest compounds can well exemplify this concept by assembling both static and dynamic components together .…”

“…Fortunately, Xiong and co‐workers proposed “quasi‐spherical theory” in hybrid perovskite‐like halogenometallates, in which quasi‐spherical organic cations exist disorder dynamically over several sites at a relatively high temperature and stay ordered at low temperature, giving rise to an order‐disorder reversible thermodynamic phase transition . Intriguing properties, such as ferroelectric, antiferroelectric, and dielectric transitions, have been extensively investigated , , . Besides, the H/F substitution, as a supplement to the isotopic effect, has also demonstrated a particularly useful idea in designing halogenometallate hybrid compounds with a switchable phase transition behavior , , .…”

The materials of ammonium metal formate framework: structures, phase transitions and functionalities

“…Figure20 The phase transitions of dmpnM compounds. (a) The Co, Zn, and Mg members (Reproduced from Ref [132]. with permission of John Wiley and Sons); (b) the Mn member (Reproduced from Ref [126].…”