Probing time dependent phase transformation in a flexible metal-organic framework with nanoindentation

Phase transformation in a flexible metal-organic framework, {[Zn₄(1,4-NDC)₄(1,2-BPE)₂]·xSolvent}_n, which loses guest molecules rapidly at room temperature, leading to several phase transitions, is examined using the nanoindentation technique. Nanoindentation results revealed that the time dependent transformation of an open to a closed phase happens gradually, through multiple intermediate phases, with the mechanical properties (elastic modulus and hardness) increasing as the transformation progresses from an open to a closed phase.

Elastic Properties and Energy Loss Related to the Disorder-Order Ferroelectric Transitions in Multiferroic Metal-Organic Frameworks [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3]

Elastic properties are important mechanical properties which are dependent on the structure, and the coupling of ferroelasticity with ferroelectricity and ferromagnetism is vital for the development of multiferroic metal-organic frameworks (MOFs). The elastic properties and energy loss related to the disorder-order ferroelectric transition in [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] were investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The DSC curves of [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] exhibited anomalies near 256 K and 264 K, respectively. The DMA results illustrated the minimum in the storage modulus and normalized storage modulus, and the maximum in the loss modulus, normalized loss modulus and loss factor near the ferroelectric transition temperatures of 256 K and 264 K, respectively. Much narrower peaks of loss modulus, normalized loss modulus and loss factor were observed in [(CH3)2NH2][Mg(HCOO)3] with the peak temperature independent of frequency, and the peak height was smaller at a higher frequency, indicating the features of first-order transition. Elastic anomalies and energy loss in [NH4][Mg(HCOO)3] near 256 K are due to the second-order paraelectric to ferroelectric phase transition triggered by the disorder-order transition of the ammonium cations and their displacement within the framework channels, accompanied by the structural phase transition from the non-polar hexagonal P6322 to polar hexagonal P63. Elastic anomalies and energy loss in [(CH3)2NH2][Mg(HCOO)3] near 264 K are due to the first-order paraelectric to ferroelectric phase transitions triggered by the disorder-order transitions of alkylammonium cations located in the framework cavities, accompanied by the structural phase transition from rhombohedral R3¯c to monoclinic Cc. The elastic anomalies in [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3] showed strong coupling of ferroelasticity with ferroelectricity.

Order-disorder, ferroelasticity and mobility of domain walls in multiferroic Cu-Cl boracite

Domain walls in Cu-Cl boracite develop as a consequence of an improper ferroelastic, improper ferroelectric transition, and have attracted close interest because some are conductive and all can be mechanically written and repositioned by application of an electric field. The phase transition and its associated dynamical properties have been analysed here from the perspective of strain and elasticity. Determination of spontaneous strains from published lattice parameter data has allowed the equilibrium long-range order parameter for F [Formula: see text]3c → Pca2₁ to be modelled simply as being close to the order-disorder limit. High acoustic loss in the cubic phase, revealed by resonant ultrasound spectroscopy, is consistent with the presence of dynamical microdomains of the orthorhombic structure with relaxation times in the vicinity of ∼10^-5-10^-6 s. Low acoustic loss in the stability field of the orthorhombic structure signifies, on the other hand, that ferroelastic twin walls which develop as a consequence of the order-disorder process are immobile on this time scale. A Debye loss peak accompanied by ∼1% elastic stiffening at ∼40 K is indicative of some freezing of defects which couple with strain or of some more intrinsic freezing process. The activation energy of ⩾∼0.01-0.02 eV implies a mechanism which could involve strain relaxation clouds around local ferroelectric dipoles or freezing of polarons that determine the conductivity of twin walls.

Stress-Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5-diBr-sal2 (323))]BPh4 , 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition.

Disorder in the hydrogen-atoms uninvolved in hydrogen bonds in a metal–organic framework

Detailed investigations based on single-crystal neutron diffraction discloses disorder in the hydrogen atoms that do not participate in hydrogen bonding.

High pressure behaviour and elastic properties of a dense inorganic-organic framework

The high pressure behaviour of a cubic dense inorganic-organic framework [DABCOH2(2+)][K(ClO4)3] (DABCOH2(2+) = diazabicyclo[2.2.2]octane-1,4-diium) has been systematically studied via synchrotron X-ray powder diffraction, over the range of 0-3.12 GPa. The framework [DABCOH2(2+)][K(ClO4)3] shows a more rigid response, with a bulk modulus of 30(1) GPa and an axial compressibility of 7.6(4) × 10(-3) GPa(-1), compared with ZIF-8 and the dense hybrid solar cell perovskite CH3NH3PbI3. Density functional theory calculations reveal that the structural change in [DABCOH2(2+)][K(ClO4)3] is attributed to the contraction of the KO12 polyhedra, which consequently results in the rotation of the perchlorate linkers and synergistic movement of the DABCOH2(2+) guest. Further extensive theoretical calculations of full elastic tensors give full mapping of the Young's moduli, shear moduli and Poisson's ratios of [DABCOH2(2+)][K(ClO4)3], which are in the range of 31.6-36.6, 12.3-14.6 GPa and 0.2-0.32, respectively. The Young's and shear moduli of [DABCOH2(2+)][K(ClO4)3] are larger than those of cubic MOF-5, ZIF-8 and CH3NH3PbI3. In addition, the narrow range of Poisson's ratios in [DABCOH2(2+)][K(ClO4)3] indicates its very isotropic nature in response to biaxial stress.

Strain tuning of ferroelectric polarization in hybrid organic inorganic perovskite compounds

Metal-organic frameworks (MOFs) are hybrid crystalline compounds comprised of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic perovskites have been shown to possess interesting properties, e.g., coexistence of ferroelectric and magnetic ordering. Using first-principles density functional theory, we have investigated the effect of strain on the compounds C(NH2)3Cr(HCOO)3 and (CH3CH2NH3)Mn(HCOO)3. Here, we show that compressive strain can substantially increase the ferroelectric polarization by more than 300%, and we discuss the mechanism involved in the strain enhancement of polarization. Our study highlights the complex interplay between strain and organic cations' dipoles and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing.

Role of hydrogen-bonding and its interplay with octahedral tilting in CH3NH3PbI3

Computed Kohn–Sham energies as a function of torsion angle for three rotational modes of the methylammonium group in orthorhombic CH₃NH₃PbI₃.

Probing ferroic transitions in a multiferroic framework family: a neutron diffraction study of the ammonium transition metal formates

The ferroic transitions of the NH₄M(HCO₂)₃ frameworks were probed using neutron diffraction, finding a unique alternation in their magnetic structures.