Further adventures of the perovskite family

The perovskites are an intensely studied class of materials, with a breadth of possible compositions made even wider by the possibility of incorporating molecular ions. Here the context is discussed of a newly reported metal-free perovskite with the H(3)O(+) ion on the B site

Neutron scattering study of the orientational disorder and phase transitions in barium carbonate.

Orientational disorder of the molecular carbonate anions in BaCO3, which occurs naturally as the mineral witherite, has been studied using a combination of neutron total scattering analysed by the Reverse Monte Carlo method and molecular dynamics simulations. The primary focus is on the phase transition to the cubic phase, which assumes a rocksalt structure (Strukturbericht type B1) with highly disordered orientations consistent with the mismatch between the site (m3m) and molecular (3/m) symmetries. Both experiment and simulation show a high degree of disorder, with the C-O bond orientation distribution never exceeding 25% variation from that of a completely uniform distribution, although there are differences between the two methods regarding the nature of these variations. Molecular dynamics simulations are also reported for the analogous phase transitions in the very important mineral calcite, CaCO3. The combination of the simulations and comparison with BaCO3 shows that the properties of calcite at all temperatures within its stability field are affected mostly by the onset of orientational disorder associated with the high-temperature cubic phase, even though this lies outside the stability field of calcite. This is a new understanding of calcite, which previously had been interpreted purely in terms of the phase transition to an intermediate partially-disordered phase. Finally, we also found that witherite itself appears to support the development of orientational disorder on heating, with the simulations showing a sequence of phase transitions that explain the much larger thermal expansion of one axis

First-principles study of structure and magnetism in copper(Ii)-containing hybrid perovskites

We report a first-principles study of hybrid organic–inorganic perovskites with formula [A]Cu(H2 POO)3 (A = triazolium (Trz) and guanidinium (Gua), and H2 POO− = hypophosphite), and [HIm]Cu(HCO2)3 (HIm = imidazolium cation, HCO−2 = formate). The triazolium hypophosphite and the formate have been suggested as possible ferroelectrics. We study the fully relaxed structures with different magnetic orderings and possible phonon instabilities. For the [Trz]Cu hypophosphite, the Trz cation is shown to induce large octahedral distortions due to the Jahn-Teller effect, with Cu-O long-bond ordering along two perpendicular directions, which is correlated with antiferromagnetic ordering and strongly one-dimensional. We find that the structure is dynamically stable with respect to zone-center distortions, but instabilities appear along high symmetry lines in the Brillouin zone. On the other hand, for the [HIm]Cu formate, large octahedral distortions are found, with large Cu-O bonds present in half of the octahedra, in this case along a single direction, and correspondingly, the magnetism is almost two-dimensional

Short-time photodissociation dynamics of A-band and B-band bromoiodomethane in solution: An examination of bond selective electronic excitation

We have obtained resonance, Raman spectra and absolute Raman cross section measurements at eight excitation wavelengths in the A-band and B-band absorptions of bromoiodomethane in cyclohexane solution. The resonance Raman intensities and absorption spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode vibrational descriptions were used with.the results of the calculations to find the short-time photodissociation dynamics in terms of internal coordinates. The A-band short-time photodissociation dynamics indicate that the C-I bond becomes much longer, the C-Br bond becomes smaller, the I-C-Br angle becomes smaller, the H-C-Br angles become larger, the H-C-I angles become smaller, and the H-C-H angle becomes a bit smaller. The B-band short-time photodissociation dynamics indicate the C-Br bond becomes much longer, the C-I bond becomes slightly longer, the I-C-Br angle becomes smaller, the H-C-I angles become larger, the H-C-Br angles become smaller, and the H-C-H angle becomes slightly smaller. Both the A-band and B-band short-time photodissociation dynamics appear to be most consistent with an impulsive "semi-rigid" radical model qualitative description of the photodissociation with the CH 2Br radical changing to a more planar structure in the A-band and the CH 2I radical changing to a more planar structure in the B band. We have carried out a Gaussian deconvolution of the A-band and B-band absorption spectra of bromoiodomethane, as well as iodomethane and bromomethane. The absorption spectra, resonance Raman intensities, and short-time photodissociation dynamics sueeest a moderate amount of coupling of the C-I and C-Br chromophores. © 1996 American Institute of Physics.published_or_final_versio

Dynamics in the ordered and disordered phases of barocaloric adamantane

High-entropy order–disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K1 kg1 . Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this difference in the dynamics to the contrast between an ‘interlocked’ state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials

Magnetic structure and spin-wave excitations in the multiferroic magnetic metal-organic framework (CD3)(2)ND2[Mn(DCO2)(3)]

The authors thank STFC for the award of beam time at ISIS Neutron and Muon Source. A.E.P. is grateful to EPSRC for funding (EP/L024977/1

Control of Multipolar and Orbital Order in Perovskite-like [C(NH2)(3)]CuxCd1-x(HCOO)(3) Metal-Organic Frameworks

We study the compositional dependence of molecular orientation (multipolar) and orbital (quadrupolar) order in the family of perovskite-like metal–organic frameworks [C(NH2)3]CuxCd1–x(HCOO)3. On increasing the fraction x of Jahn-Teller-active Cu2+, we observe first an orbital disorder/order transition and then a multipolar reorientation transition, each occurring at distinct critical compositions xo = 0.45(5) and xm = 0.55(5). We attribute these transitions to a combination of size, charge distribution, and percolation effects. The transitions we observe establish the accessibility in for-mate perovskites of novel structural degrees of freedom beyond the familiar dipolar terms responsible for (an-ti)ferroelectric order. We discuss the symmetry implica-tions of cooperative quadrupolar and multipolar states for the design of relaxor-like hybrid perovskites

Colossal Pressure-Induced Softening in Scandium Fluoride

The counter-intuitive phenomenon of pressure-induced softening in materials is likely to be caused by the same dynamical behaviour that produces negative thermal expansion. Through a combination of molecular dynamics simulation on an idealised model and neutron diffraction at variable temperature and pressure, we show the existence of extraordinary and unprecedented pressure-induced softening in the negative thermal expansion material scandium fluoride, ScF$_3$, with values of the pressure-derivative of the bulk modulus $B$, $B^\prime = \partial B / \partial P$, reaching as low as $-40 \pm 1$

Colossal Reversible Barocaloric Effects in Layered Hybrid Perovskite (C10H21NH3)2MnCl4 under Low Pressure Near Room Temperature

Barocaloric effects in a layered hybrid organic–inorganic compound, (C10H21NH3)2MnCl4, that are reversible and colossal under pressure changes below 0.1 GPa are reported. This barocaloric performance originates in a phase transition characterized by different features: A strong disordering of the organic chains, a very large volume change, a very large sensitivity of the transition temperature to pressure and a small hysteresis. The obtained values are unprecedented among solid-state cooling materials at such low pressure changes and demonstrate that colossal effects can be obtained in compounds other than plastic crystals. The temperature-pressure phase diagram displays a triple point indicating enantiotropy at high pressure