By combining lattice dynamic calculations and high-pressure powder X-ray diffraction, the mechanical properties of a series of dicyanamide-based molecular perovskites are studied.

Bismuth ferrite, BiFeO3, is a multiferroic solid that is attracting increasing attention as a potential photocatalytic material, because the ferroelectric polarisation enhances the separation of photogenerated carriers. With the motivation of finding routes to engineer the band gap and the band alignment, while conserving or enhancing the ferroelectric properties, we have investigated the thermodynamic, electronic and ferroelectric properties of BiCoxFe1 xO3 solid solutions, with 0 < x < 0.13, using density functional theory. We show that the band gap can be reduced from 2.9 eV to 2.1 eV by cobalt substitution, while simultaneously increasing the spontaneous polarisation, which is associated with a notably larger Born effective charge of Co compared to Fe cations. We discuss the interaction between Co impurities, which is strongly attractive and would drive the aggregation of Co, as evidenced by Monte Carlo simulations. Phase separation into a Co-rich phase is therefore predicted to be thermodynamically preferred, and the homogeneous solid solution can only exist in metastable form, protected by slow cation diffusion kinetics. Finally, we discuss the band alignment of pure and Co-substituted BiFeO3 with relevant redox potentials, in the context of its applicability in photocatalysis.

Members of a recently discovered class of two-dimensional materials based on transition metal phosphorous trichalcogenides exhibit an antiferromagnetic ground state and they have potential applications in spintronics. In particular, FePS3 is a Mott insulator with a band gap of ~1.5 eV. In this study, we used Raman spectroscopy and first-principles density functional theoretical analysis to examine the stability of the structure and electronic properties of FePS3 under pressure. Raman spectroscopy detected two phase transitions at 4.6 GPa and 12 GPa, which were characterized by changes in the pressure coefficients of the mode frequencies and the number of symmetry allowed modes. FePS3 transformed from the ambient monoclinic C2/m phase with a band gap of 1.54 eV to another monoclinic C2/m (band gap of 0.1 eV) phase at 4.6 GPa, which was followed by another transition to the metallic trigonal P-31m phase at 12 GPa. Our findings complement those obtained recently in high pressure X-ray diffraction studies. The calculated elastic properties indicated increases in the bulk, shear, and Young's moduli, as well as a significant reduction in the universal elastic anisotropy index as the crystal changed from the ambient monoclinic C2/m phase to the high pressure trigonal P-31m phase.

Mixed-anion mixed-cation perovskites with (FAPbI(3))(1-x)(MAPbBr(3))(x) composition have allowed record efficiencies in photovoltaic solar cells, but their atomic-scale behaviour is not well understood yet, in part because their theoretical modelling requires consideration of complex and interrelated dynamic and disordering effects. We present here an ab initio molecular dynamics investigation of the structural, thermodynamic, and electronic properties of the (FAPbI(3))(0.875)(MAPbBr(3))(0.125) perovskite. A special quasi-random structure is proposed to mimic the disorder of both the molecular cations and the halide anions, in a stoichiometry that is close to that of one of today's most efficient perovskite solar cells. We show that the rotation of the organic cations is more strongly hindered in the mixed structure in comparison with the pure compounds. Our analysis suggests that this mixed perovskite is thermodynamically stable against phase separation despite the endothermic mixing enthalpy, due to the large configurational entropy. The electronic properties are investigated by hybrid density functional calculations including spin-orbit coupling in carefully selected representative configurations extracted from the molecular dynamics. Our model, that is validated here against experimental information, provides a more sophisticated understanding of the interplay between dynamic and disordering effects in this important family of photovoltaic materials.

Understanding of electron–phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe2 channel based field-effect transistors. While the E2g1 and B2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 1013/cm2, A1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the E2g1 mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe2 that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with E2g1 or B2g modes as compared with the A1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS2 and those observed here in a few-layered MoTe2 demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

Data of a molecular dynamics simulation of the mixed cation and mixed halide perovskite (FAPbI3)0.875(MAPbBr3)0.125 , as well as the end compounds FAPbI3 and MAPbBr3. Related article: J. Mater. Chem. A, 2022,10, 9592-9603, https://doi.org/10.1039/D1TA10860C arXiv:2112.09795 [cond-mat.mtrl-sci] arXiv: 2112.09795 https://doi.org/10.48550/arXiv.2112.09795

Demonstrating both the intrinsic and extrinsic nature of the giant piezoelectric effect (GPE) in complex solid solutions, near the morphotropic phase boundary, has been extremely challenging until now, because such materials exhibit multiple phases on the order of tens of microns across, meaning important information is lost due to averaging when using established high resolution diffraction techniques to extract three dimensional structural information. We have used a different approach proposed by Nisbet et al. [Acta Crystallogr. Sect. A 71, 20 (2015)], which has been adapted to differentiate between spatially adjacent phases and simultaneously track the evolution of those phases in response to electric fields. As a result, we have identified three environment specific GPEs. The first of these is a GPE which is an order of magnitude greater than previously reported for a given change in field. This is observed during a tetragonal-monoclinic transition in a multiphasic environment. A secondary, large GPE is observed in the neighboring, nontransitioning, monoclinic phase due to stress biasing, and a more typical GPE is observed when the system becomes monophasic. Our results demonstrate the simultaneous and complex interplay of intrinsic and extrinsic factors contributing to the GPE which is likely to have implications for device manufacture and miniaturization.

Ta2NiSe5 is an excitonic insulator (EI) exhibiting bound electron-hole pairs condensed at room temperature, which transforms to a small-gap semiconducting state above T = 325 K simultaneously undergoing a phonon-related structural transition. Despite the clear experimental evidence for strong exciton-phonon coupling, its role and the origin of EI state in terms of BCS versus Bose-Einstein condensation mechanisms are unclear. Motivated by the tunability of these mechanisms with pressure, we report Raman experiments under pressure of Ta2NiSe5 and first-principles theoretical analysis of two pressure-induced transitions at 1 and 3 GPa. We present a simple method to derive the exciton-phonon coupling within density functional theory and show using a model Hamiltonian that reducing strength of this coupling relative to electronic gap and phonon frequency destabilizes the EI state with pressure. In addition to connecting with the Raman anomalies observed under pressure, our simple picture explains the recently observed phonon-coupled state of exciton condensate.

With high energy conversion efficiency and low-cost production, hybrid organic–inorganic perovskite solar cells (PSCs) have the potential to be alternative to silicon-based technology. However, there are concerns about their long-term stability and environmental friendliness, which must necessarily be addressed to enable large-scale commercialization of PSCs. Here, we use first-principles theory to determine and understand the effects of humidity on the T-dependent tetragonal to cubic structural transition in CH3NH3PbI3, which can impact the long-term stability of its properties. We show that ferroelectric vs. antiferroelectric structural ordering in CH3NH3PbI3 is influenced by humidity. Within first-principles density functional theory, we determine the lowest energy configurations of dipolar ordering in CH3NH3PbI3·xH2O and effects of their interaction with H2O molecules. Developing a simple effective Hamiltonian to model these configurations, we use Monte Carlo simulations to determine temperature-dependent structural phase transitions in CH3NH3PbI3. We establish ferroelectric ordering in MAPbI3 at low temperature, and demonstrate that it changes to antiferroelectric ordering of MA+ cations at x > 0.2 in CH3NH3PbI3·xH2O.

In this work, we report a lead-free hybrid halide perovskite system with a very high piezoelectric charge density for applications in nanogenerators. We use materials engineering by incorporation of formamidinium tin iodide, FASnI(3), in a soft polymer (polyvinylidene fluoride, PVDF) matrix and demonstrate high-performance large-area flexible piezoelectric nanogenerators. This is achieved by using self-poled thin films of a FASnI(3):PVDF nano-composite. The fabricated devices show an output voltage up to similar to 23 V and power density of 35.05 mW cm(-2) across a 1 M Omega resistor, under a periodic vertical compression, with a release pressure of , similar to 0.1 MPa. Measured values of the local piezoelectric coefficient (d(33)) of these films reach up to 73 pm/V. We provide the microscopic mechanism using first-principles calculations, which suggest that a soft elastic nature and soft polar optic phonons are responsible for the high piezoelectric response of FASnI(3). Our studies open up a route to high-performance organic-inorganic halide perovskite family of materials.