Raman and IR studies of pressure- and temperature-induced phase transitions in [(CH2)3NH2][Zn(HCOO)3]

Temperature and volumetric effects on structural and dielectric properties of hybrid perovskites

Three-dimensional organic-inorganic perovskites are rapidly evolving materials with diverse applications. This study focuses on their two representatives - acetamidinium manganese(II) formate (AceMn) and formamidinium manganese(II) formate (FMDMn) - subjected to varying temperature and pressure. We show that AceMn undergoes atypical pressure-induced structural transformations at room temperature, increasing the symmetry from ambient-pressure P21/n phase II to the high-pressure Pbca phase III. In turn, FMDMn in its C2/c phase II displays temperature- and pressure-induced ordering of cage cations that proceeds without changing the phase symmetry or energy barriers. The FMD+ cations do not order under constant volume across the pressure-temperature plane, despite similar pressure and temperature evolution of the unit-cell parameters. Temperature and pressure affect the cage cations differently, which is particularly pronounced in their relaxation dynamics seen by dielectric spectroscopy. Their motion require a rearrangement of the metal-formate framework, resulting in the energy and volumetric barriers defined by temperature-independent activation energy and activation volume parameters. As this process is phonon-assisted, the relaxation time is strongly temperature-dependent. Consequently, relaxation times do not scale with unit-cell volume nor H-bond lengths in formates, offering the possibility of tuning their electronic properties by external stimuli (like temperature or pressure) even without any structural changes.

Spectroscopic studies of temperature induced phase transitions in metal-organic complex trans-PtCl2(PEt3)2

Tuning of molecular and electronic properties of Pt(II)-organic complexes have a profound effect on their applications in the fields of technology, pharmaceuticals and crystal engineering. Here, we present combined infrared and Raman spectroscopic investigations on trans-PtCl₂(PEt₃)₂ systematically carried out at various temperatures from 300 to 4.2 K in a wide spectral range. The studies suggest drastic orientational changes of different moieties around 180 K and 130 K in the ligand groups attached to the central Pt atom. This is accompanied by a systematic strengthening of C-H⋯Cl hydrogen bonds in the 180-130 K temperature range. A discontinuous change in intensity, peak variations of modes and emergence of new modes across 180 K and 130 K in the lattice region are suggestive of a possible structural phase transition. It is interesting to note that the spectral signatures of the low temperature phase are different from those reported recently for the high pressure phase in this compound. These studies will be useful in better understanding the physico-chemical properties of metal-organic complexes in order to exploit their applications in various bio-chemical and technological fields.

Palladium-catalyzed oxidative homocoupling of pyrazole boronic esters to access versatile bipyrazoles and the flexible metal–organic framework Co(4,4′-bipyrazolate)

A facile catalytic protocol achieves the homocoupling of pyrazole boronic esters, enabling access to the structurally-flexible metal–organic framework Co(bpz).

Hydrogen-bond-mediated structural variation of metal guanidinium formate hybrid perovskites under pressure

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, their phase transformations under pressure are more complex and less well understood. We have studied the structures of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Under pressure, these materials transform to a rhombohedral phase isostructural to cadmium guanidinium formate. This transformation accommodates the reduced cell volume while preserving the perovskite topology of the framework. Using density-functional theory calculations, we show that this behaviour is a consequence of the hydrogen-bonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression within their plane. Our results demonstrate more generally that identifying suitable host-guest hydrogen-bonding geometries may provide a route to engineering hybrid perovskite phases with desirable crystal structures. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.

Pressure-induced reversible framework rearrangement and increased polarization in the polar [NH4][Cd(HCOO)3] hybrid perovskite

The polar [NH₄][Cd(HCOO)₃] hybrid perovskite displays an unprecedented structural arrangement and an electric polarization enhancement under applied pressure.

Novel bimetallic MOF phosphors with an imidazolium cation: structure, phonons, high- pressure phase transitions and optical response

We report the synthesis, high-pressure phase transitions and optical properties of novel bimetallic perovskite-type MOFs with imidazolium ions.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni)

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH₃)₂NH₂] M(HCOO)₃ ( M = Mn²⁺, Co²⁺, Ni²⁺) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH₃)₂NH₂]Mn(HCOO)₃ takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across T_C reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH₃)₂NH₂]Co(HCOO)₃ reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co²⁺ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Remarkable resilience of the formate cage in a multiferroic metal organic framework material: dimethyl ammonium manganese formate (DMAMnF)

Dimethyl ammonium (DMA) metal formate, an important member of the dense metal organic framework (MOF) family, is known to exhibit a low temperature ferroelectric transition, caused by the ordering of the hydrogen bonds. In this study, we probed the effect of pressure on the disordered hydrogen bond and the HCOO^- linkers of DMA manganese formate, with the help of XRD, IR and Raman spectroscopic studies up to ∼20 GPa. We observed that though a phase transition was initiated at ∼3.4 GPa, it was complete only by 6 GPa, indicating its first order nature. Beyond 7 GPa, this compound becomes highly disordered and shows an almost amorphous character, indicating a total collapse of the formate network. The reversibility of the initial structure of DMAMnF on the release of pressure from 20 GPa (i.e. from a highly disordered phase) shows the remarkable resilience of the formate cage. At the first crystal to crystal transition at 3.4 GPa, the distortion of the formate cage causes the ordering of the dynamically disordered hydrogen bond, resulting in a rearrangement of the DMA⁺ cation. Lifting of the mutual exclusivity of the Raman and IR modes (C-H out of plane and O-C-O bending modes) of HCOO^- linkers, at this transition, indicates that the high pressure phase may be non-centro-symmetric.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn)

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH₂NH₃][M(HCOO)₃] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna2₁, P6₃ to P2₁2₁2₁ and P6₃ to P2₁. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Heterometallic perovskite-type metal–organic framework with an ammonium cation: structure, phonons, and optical response of [NH4]Na0.5CrxAl0.5−x(HCOO)3 (x = 0, 0.025 and 0.5)

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic perovskite-type AmNaCr and AmNaAl MOFs.

Tuning the structure and properties of a multiferroic metal–organic-framework via growing under high magnetic fields

High magnetic field-induced synthesis has been demonstrated to tune the structure and properties of the multiferroic metal–organic framework [(CH₃)₂NH₂][Mn(HCOO)₃].

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and β = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and β = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

Growth of centimeter-sized [(CH3)2NH2][Mn(HCOO)3] hybrid formate perovskite single crystals and Raman evidence of pressure-induced phase transitions

The fewer the number of the nucleation sites formed in the vessel, the larger the size of the obtained crystals.

Structural, optical and phonon properties of formate-based MOF phosphors with ethylammonium cations

We report the structural and spectroscopic properties of metal–organic phosphors.

The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of novel heterometallic DMAKCr and EtAKCr MOFs.

Spin-Lattice Coupling in [Ni(HF2)(pyrazine)2]SbF6 Involving the HF2- Superexchange Pathway

Magnetoelastic coupling in the quantum magnet [Ni(HF₂)(pyrazine)₂]SbF₆ has been investigated via vibrational spectroscopy using temperature, magnetic field, and pressure as tuning parameters. While pyrazine is known to be a malleable magnetic superexchange ligand, we find that HF₂^- is surprisingly sensitive to external stimuli and is actively involved in both the magnetic quantum phase transition and the series of pressure-induced structural distortions. The amplified spin-lattice interactions involving the bifluoride ligand can be understood in terms of the relative importance of the intra- and interplanar magnetic energy scales.

Insight into Understanding Dielectric Behavior of a Zn-MOF Using Variable-Temperature Crystal Structures, Electrical Conductance, and Solid-State 13C NMR Spectra

A Zn-based metal-organic framework (MOF)/porous coordination polymer (PCP), (EMIM)[Zn(SIP)] (1) (SIP^3- = 5-sulfoisophthalate, EMIM⁺ = 1-ethyl-3-methylimidazolium), was synthesized using the ionothermal reaction. The Zn²⁺ ion adopts distorted square pyramid coordination geometry with five oxygen atoms from three carboxylates and one sulfo group. One of two carboxylates in SIP^3- serves as a μ₂-bridge ligand to link two Zn²⁺ ions and form the dinuclear SBU, and such SBUs are connected by SIP^3- ligands to build the three-dimensional framework with rutile (rtl) topology. The cations from the ion-liquid fill the channels. This MOF/PCP shows two-step dielectric anomalies together with two-step dielectric relaxations; the variable-temperature single-crystal structure analyses disclosed the dielectric anomaly occurring at ca. 280 K is caused by an isostructural phase transition. Another dielectric anomaly is related to the dynamic disorder of the cations in the channels. Electric modulus, conductance, and variable-temperature solid-state ¹³C CP/MAS NMR spectra analyses revealed that two-step dielectric relaxations result from the dynamic motion of the cations as well as the direct-current conduction and electrode effect, respectively.

Raman scattering studies of pressure-induced phase transitions in perovskite formates [(CH3)2NH2][Mg(HCOO)3] and [(CH3)2NH2][Cd(HCOO)3]

Pressure-dependent Raman studies were preformed on two dimethylammonium metal formates, [(CH3)2NH2][Mg(HCOO)3] (DMMg) and [(CH3)2NH2][Cd(HCOO)3] (DMCd). They revealed three pressure-induced transitions in the DMMg near 2.2, 4.0 and 5.6 GPa. These transitions are associated with significant distortion of the anionic framework and the phase transition at 5.6 GPa has also great impact on the DMA+ cation. The DMCd undergoes two pressure-induced phase transitions. The first transition occurred between 1.2 and 2.0 GPa and the second one near 3.6 GPa. The first transition leads to subtle structural changes associated with distortion of anionic framework and the later leads to significant distortion of the framework. In contrast to the DMMg, the third transition associated with distortion of DMA+ cation is not observed for the DMCd up to 7.8 GPa. This difference can be most likely associated with larger volume of the cavity occupied by DMA+ cation in the DMCd and thus weaker interactions between anionic framework and DMA+ cations.

Temperature- and pressure-dependent studies of niccolite-type formate frameworks of [NH3(CH2)4NH3][M2(HCOO)6] (M = Zn, Co, Fe)

Structural changes occurring in [H₃N(CH₂)₄NH₃][M₂(HCOO)₆] (M = Zn, Co, Fe) niccolites due to temperature- and pressure-induced transitions were monitored by dielectric, IR and Raman spectroscopy.

Phase transitions and chromium(iii) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5−x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties

We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C₂H₅NH₃][Na_0.5Cr_xAl_0.5−x(HCOO)₃] (x = 0, 0.025, 0.5).

Experimental and theoretical studies of structural phase transition in a novel polar perovskite-like [C2H5NH3][Na0.5Fe0.5(HCOO)3] formate

We report the synthesis and studies of a novel heterometallic formate [C₂H₅NH₃][Na_0.5Fe_0.5(HCOO)₃].

A first-principles study of pressure-induced phase transformation in a rare-earth formate framework

The reversible pressure-induced phase transformation associated with bond rearrangement in a rare-earth formate framework was explored using first-principles calculations, which complement and help understand its experimental observations.