EPR Study of Structural Phase Transition in Manganese-Doped [(CH₃)₂NH₂][Zn(HCOO)₃] Metal–Organic Framework

We present an electron paramagnetic resonance (EPR) study of [(CH₃)₂NH₂]­[Zn­(HCOO)₃] metal–organic framework (MOF) powder doped with a small amount of paramagnetic Mn²⁺ ions. Our EPR measurements indicate a successful incorporation of local Mn²⁺ probes into the structure allowing us to detect and investigate an order–disorder structural phase transition in the studied MOF. The temperature-dependent continuous wave (CW) X- and Q-band EPR measurements reveal a sudden change in the spectra at a phase transition temperature <i>T</i>₀ = 163 K. Simulations were performed to determine the spin Hamiltonian parameters of the spectra which reflect the local symmetry of the Mn²⁺ probes in the disordered and ordered phases. The temperature dependence of the axial zero-field splitting parameter <i>D</i> demonstrates that the observed phase transition at <i>T</i>₀ is discontinuous. Additionally, this dependence follows the prediction of the Landau theory. We also performed preliminary pulse EPR measurements which reveal a rather long phase memory time sufficient to detect a spin echo even in the high-temperature disordered phase. The modulation with the proton and nitrogen Larmor frequencies of the electron spin echo was observed as well

Adsorption and Desorption of HD on the Metal–Organic Framework Cu_2.97Zn_0.03(Btc)₂ Studied by Three-Pulse ESEEM Spectroscopy

Cu_2.97Zn_0.03(btc)₂ is a structural analogue of the well-known HKUST-1 metal–organic framework. In this compound 1% of the Cu²⁺ ions in the paddle-wheel units are substituted by Zn²⁺, resulting in the formation of Cu/Zn paddle-wheel units in low concentration. The paramagnetic Cu²⁺ ions of these mixed Cu/Zn pairs allow to perform pulsed electron paramagnetic resonance experiments at low temperatures. Here we report on the three-pulse electron spin echo envelope modulation (3p ESEEM) study of the deuterated hydrogen gas HD adsorption and desorption in Cu_2.97Zn_0.03(btc)₂. The HD adsorption sites in this modified compound were identified by precisely simulating experimentally observed 3p ESEEM time domain pattern. To elucidate the HD desorption process, the 3p ESEEM experiments were performed at different temperatures. Employing this method, the detachment of HD from the Cu²⁺ binding sites is found to already occur slightly above 6 K temperature. Hereby 3p ESEEM spectroscopy reveals to be a powerful method to study adsorption of small molecules in the local environment of Cu²⁺ ions

Exploring the Antipolar Nature of Methylammonium Lead Halides: A Monte Carlo and Pyrocurrent Study

The high power conversion efficiency of the hybrid CH₃NH₃PbX₃ (where X = I, Br, Cl) solar cells is believed to be tightly related to the dynamics and arrangement of the methylammonium cations. In this Letter, we propose a statistical phase transition model which accurately describes the ordering of the CH₃NH₃⁺ cations and the whole phase transition sequence of the CH₃NH₃PbI₃ perovskite. The model is based on the available structural information and involves the short-range strain-mediated and long-range dipolar interactions between the cations. It is solved using Monte Carlo simulations on a three-dimensional lattice allowing us to study the heat capacity and electric polarization of the CH₃NH₃⁺ cations. The temperature dependence of the polarization indicates the antiferroelectric nature of these perovskites. We support this result by performing pyrocurrent measurements of CH₃NH₃PbX₃ (X = I, Br, Cl) single crystals. We also address the possible occurrence of the multidomain phase and the ordering entropy of our model

Q-band EPR cryoprobe

Following the success of cryogenic EPR signal preamplification at X-band, we present a Q-band EPR cryoprobe compatible with a standard EPR resonator. The probehead is equipped with a cryogenic ultra low-noise microwave amplifier and its protection circuit that are placed close to the sample in the same cryostat. Our cryoprobe maintains the same sample access and tuning which is typical in Q-band EPR, as well as supports high-power pulsed experiments on typical samples. The performance of our setup is benchmarked against that of existing commercial and home-built Q-band spectrometers, using CW EPR and pulsed EPR/ENDOR experiments to reveal a significant sensitivity improvement which reduces the measurement time by a factor of about 40× at 6 K temperature at reduced power levels.ISSN:1090-780

Synthesis, Structure, and Electron Paramagnetic Resonance Study of a Mixed Valent Metal–Organic Framework Containing Cu₂ Paddle-Wheel Units

We report synthesis and composite study of a novel metal–organic framework (MOF) compound of chemical formula _∞³[Cu₂^ICu₂^II{H₂O}₂{(Me–trz–<i>m</i>ba)₂thio}₂]­Cl₂, where (Me–trz–<i>m</i>ba)₂thio^2– stands for 3,3-(5,5-(thiophene-2,5-diyl)­bis­(3-methyl-4H-1,2,4-triazole-5,4-diyl))­dibenzoate. This coordination polymer was synthesized by solvothermal synthesis. The crystal structure was determined using single crystal X-ray diffraction. The main building block of this compound is a so-called Cu₂ paddle-wheel (PW) unit, which contains two Cu²⁺ ions connected via four carboxylate groups. Magnetic properties of the investigated MOF were studied by continuous-wave electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies in a wide temperature range. Mononuclear Cu²⁺ ions were observed in the EPR spectra and characterized by spectral simulations. In addition, the obtained EPR data allowed us to detect and investigate three distinct magnetic interactions related to the Cu²⁺ pairs. At higher temperatures the fine structure pattern was observed in the EPR spectra and the spin–spin interaction tensor <i><b>D</b></i> was determined. The origin of this pattern was assigned to the thermally populated excited triplet states of the Cu²⁺ pairs. It was found that two Cu²⁺ ions within a single PW unit couple antiferromagnetically with the exchange coupling constant <i>J</i> = −258 cm^–1. Moreover, the EPR spectra of dehydrated MOF samples show a broad, poorly resolved spectral feature, the origin of which is an exchange of the spin triplets between neighboring Cu₂ PW units. By simulating the powder pattern of this interdinuclear exchange line, we estimated the exchange coupling between neighboring PW units (|<i>J</i>′| = 4.9 cm^–1). It was also found that the interdinuclear exchange gradually disappears, if the dehydrated samples are allowed to interact with air, demonstrating that this exchange can be rather easily manipulated in the investigated MOF

Single Crystal Electron Paramagnetic Resonance with Dielectric Resonators of Mononuclear Cu²⁺ Ions in a Metal–Organic Framework Containing Cu₂ Paddle Wheel Units

Dielectric resonator aided sensitivity-enhancing electron paramagnetic resonance was successfully applied to small single crystals of the previously reported metal–organic framework compound _∞³[Cu₂^ICu₂^II(H₂O)₂L₂Cl₂] in a conventional X-band EPR spectrometer at 7 K sample temperature to reveal the nature of mononuclear Cu²⁺ ion defect species. We found that these paramagnetic defects are not related to an impurity phase or extraframework species of the parent metal–organic framework material but are formed within the framework. Novel angular resolved single crystal continuous wave electron paramagnetic resonance supported by powder measurements and single crystal X-ray diffraction on this metal–organic framework compound identified defective copper paddle wheel units with one missing Cu²⁺ ion as the observed mononuclear paramagnetic species in this compound. The sensitivity enhancement by an estimated factor of 8.6 for the single crystal electron paramagnetic resonance spectroscopy is required to efficiently record the Cu²⁺ ion signals in single crystals of typical sizes of 200 × 50 × 50 μm³ at X-band frequencies. The results demonstrate that conventional electron paramagnetic resonance spectrometers operating at X-band frequencies and equipped with dielectric resonators can successfully be used to perform single crystal studies of these porous, low density materials with very small volume samples at low temperatures

Single Crystal Electron Paramagnetic Resonance of Dimethylammonium and Ammonium Hybrid Formate Frameworks: Influence of External Electric Field

We present a continuous wave electron paramagnetic resonance (EPR) study of a Mn²⁺ doped [(CH₃)₂NH₂]­[Zn­(HCOO)₃] hybrid dense metal–organic framework (MOF) that exhibits an order–disorder structural phase transition at <i>T</i>_c = 163 K. The W-band EPR measurements of a powder sample are performed to verify the previously reported spin Hamiltonian parameters of the Mn²⁺ centers in the low-temperature phase. The temperature dependent single crystal X-band EPR experiments reveal that Mn²⁺ probe ions are susceptible to the phase transition, as the spectrum changes drastically at <i>T</i>_c. The angular dependent EPR spectra of Mn²⁺ centers are obtained by rotating the single crystal sample about three distinct directions. The simulation of the determined angular dependences reveals six MnO₆ octahedra in the ordered phase that originate from a severe crystal twinning of the [(CH₃)₂NH₂]­[Zn­(HCOO)₃] MOF. The possible ferroelectric origin of the crystalline twins is investigated by single crystal EPR measurements with an applied external electric field. No significant effect of the electric field on the spectra is observed. The EPR results are supported by the measurements of the electric field dependence of the macroscopic electric polarization. Analogous EPR measurements are performed on a single crystal sample of ferroelectric Mn²⁺ doped [NH₄]­[Zn­(HCOO)₃] MOF. Contrary to the dimethylammonium framework, the EPR signal and electric polarization of the ammonium compound demonstrate clear ferroelectric behavior

Pulse EPR and ENDOR Study of Manganese Doped [(CH₃)₂NH₂][Zn(HCOO)₃] Hybrid Perovskite Framework

We present a pulse electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) study of a manganese-doped [(CH₃)₂NH₂]­[Zn­(HCOO)₃] dense metal–organic framework which exhibits a structural phase transition at 163 K. The echo-detected field sweep Mn²⁺ EPR spectrum of the low-temperature phase is in a perfect agreement with the previous continuous-wave EPR results, while the spectrum of the disordered phase reveals a significant EPR transition-dependent relaxation. The ¹H ENDOR pattern indicates several protons in the vicinity of the Mn²⁺ ion. The experimental ENDOR spectrum is successfully simulated using the proton hyperfine tensors calculated by the density functional theory. A multifrequency electron spin echo envelope modulation (ESEEM) spectroscopy shows a peculiar signal which is unaffected by the external magnetic field. The modulation depth of this signal starts to decrease above 40 K, coinciding with the temperature at which the methyl groups of the (CH₃)₂NH₂⁺ cations start to rotate. We also relate the methyl group motion to the decrease of the phase memory time of the Mn²⁺ ions. The temperature dependence of the longitudinal relaxation time indicates a coupling between the Mn²⁺ electron spins and a hard optical phonon mode. This mode undergoes a damping at the phase transition point