Mantas Šimėnas

Electron paramagnetic resonance and electric characterization of a [CH3NH2NH2][Zn(HCOO)3] perovskite metal formate framework

We present a combined continuous-wave (CW) and pulse electron paramagnetic resonance (EPR), pulse electron-nuclear double resonance (ENDOR), pyrocurrent as well as broadband dielectric study of a [CH3NH2NH2][Zn(HCOO)3] dense perovskite metal–organic framework (MOF). The pyroelectric current of a single crystal sample reveals two structural phase transitions at Tc1 = 325 and Tc2 = 173 K that are related to the ordering of CH3NH2NH2+ cations. The dielectric permittivity exhibits a small kink at Tc1 implying improper ferroelectric phase transition, while much stronger anomaly is observed at Tc2. The dielectric spectra of the intermediate phase reveal a Cole–Cole relaxation process that is assigned to the hopping motion of the CH3NH2NH2+ cations. EPR and ENDOR experiments are performed on powder MOF samples doped with small amounts of paramagnetic Mn2+ and Cu2+ probe ions. CW EPR spectra reveal the successful incorporation of these ions at the Zn2+ lattice sites, while ENDOR measurements indicate several proton species that are in excellent agreement with the X-ray diffraction data. The CW EPR linewidth and intensity of the Mn2+ spectra demonstrate anomalies at the phase transition points. The direct measurements of the phase memory time Tm of the Mn2+ centers indicate a second motional process of CH3NH2NH2+ cations below Tc2. The measurements of the longitudinal relaxation time T1 of the low-temperature phase reveal a coupling between the electron spins and a hard optical phonon mode which undergoes a damping due to the coupling with the relaxational mode as Tc2 is approached. The temperature dependent Mn2+ and Cu2+ spectra reflect the structural changes of the metal–oxygen octahedra. The fine structure splitting of Mn2+ ions is increasing as the temperature is decreased reflecting a distortion of the MnO6 octahedra. The Cu2+ hyperfine interaction demonstrates a first-order character close to the tricritical limit of the phase transition at Tc2.

Antiferromagnetic triangular Blume-Capel model with hard-core exclusions.

Using Monte Carlo simulation, we analyze phase transitions of two antiferromagnetic (AFM) triangular Blume-Capel (BC) models with AFM interactions between third-nearest neighbors. One model has hard-core exclusions between the nearest-neighbor (1NN) particles (3NN1 model) and the other has them between the nearest-neighbor and next-nearest-neighbor particles (3NN12 model). Finite-size scaling analysis reveals that in these models, the transition from the paramagnetic to long-range order (LRO) AFM phase is either of the first order or goes through an intermediate phase which might be attributed to the Berezinskii-Kosterlitz-Thouless (BKT) type. The properties of the low-temperature phase transition to the AFM phase of the 1NN, 3NN1, and 3NN12 models are found to be very similar for almost all values of a normalized single-ion anisotropy parameter, 0 < δ < 1.5. Higher temperature behavior of the 3NN12 and 3NN1 models is rather different from that of the 1NN model. Three phase transitions are observed for the 3NN12 model: from the paramagnetic phase to the phase with domains of the LRO AFM phase at T(c), from this structure to the diluted frustrated BKT-type phase at T(2), and from the frustrated phase to the AFM LRO phase at T(1). For the 3NN12 model, T(c) > T(2) > T(1) at 0 < δ < 1.15 (range I), T(c) ≈ T(2) > T(1) at 1.15 < δ < 1.3 (range II), and T(c) = T(2) = T(1) at 1.3 < δ < 1.5 (range III). For the 3NN1 model, T(c) ≈ T(2) > T(1) at 0 < δ < 1.2 (range II) and T(c) = T(2) = T(1) at 1.2 < δ < 1.5 (range III). There is only one first-order phase transition in range III. The transition at T(c) is of the first order in range II and either of a weak first order or a second order in range I.

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.

Screening of point defects in methylammonium lead halides: a Monte Carlo study

We present a theoretical modeling of the point defect screening by the CH3NH3+ dipoles in CH3NH3PbX3 (X = I, Br, Cl) perovskites. The organic sublattice of these materials is believed to be related to the exceptional performance of the perovskite solar cells. Our work is based on a statistical model with the short-range and dipolar interactions between the CH3NH3+ cations. We extend the model to account for the long-range charge–dipole interactions between the defects and organic cations. The model is studied by the Monte Carlo simulations on a three-dimensional lattice. We investigate the charge screening in different structural phases of CH3NH3PbI3 perovskite for various values of the charge–dipole interaction energy. We demonstrate that a substantial interaction disturbs the antipolar long-range order of the CH3NH3+ cations giving rise to a multidomain phase with a small electric polarization. We also discuss the screening of two neighboring charges which might be important in a context of the photogenerated electron–hole pair separation.

Effect of lattice coarsening and exclusion on phase-transition properties of the Bell–Lavis model

The temperature and order of the phase transition are studied for a less-coarsened Bell–Lavis model. The interactions exist between triangular molecules being on third-neighbour (3NN) sites, while the nearest-neighbour (1NN) interactions are subject to a hard core exclusion. Using Monte Carlo calculations, we obtain the temperature, and chemical potential phase diagram for a whole range of values, where the honeycomb phase exists. We find that the entropy gain prevails at low molecular densities (high ). Therefore, the first-order disordered-to-honeycomb phase transition found in the 3NN model has lower temperature than that of the 1NN model. At higher molecular densities (low ), below the tricritical point, the exclusions start to play some role and the entropy decreases. This leads to the occurrence of the intermediate short-range ordered phase in between disordered and honeycomb phases. The upper phase transition is found at higher temperature, while the lower one at the same temperature as that of the 1NN model. The critical exponents obtained for the lower phase transition demonstrate the values typical for the three-state Potts model universality class.