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

Abstract

Orientational disorder of the molecular C O 3 2 − 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 ( m 3 ¯ m ) 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.

Disordered Aragonite: The New High-Pressure, High-Temperature Phase of CaCO3

“…At such high values, the average distribution of oxygen atoms of the CO 3 atomic group actually loses the flat form and became elliptical. This sufficient deviation from the flat distribution of C–O bond orientations was earlier highlighted by Cai et al based on their results of MD simulations with empirical potentials . Meanwhile, according to the refinements performed based on X-ray diffraction experiments with calcite, the inclination of the orbital of oxygen atoms from the flat form is within ±7.7° .…”

“…This type of disorder is strictly bound to the disorder of the orientation of the CO 3 group. In the MD simulation of witherite BaCO 3 , the transition to the hexagonal symmetry has been noticed by Cai and co-authors . Assuming rotational disorder of the carbonate group and its average position midway between cp layers, the symmetry of the disarag structure will be also hexagonal, same as that of NiAs.…”

Orientational order/disorder and network flexibility in deuterated methylammonium lead iodide perovskite by neutron total scattering

“…Whereas in the analysis of crystallographic data the coefficients might be fitted to data for the structure factors of Bragg reflections, 46 the orthonormality of the functions means that we can obtain the values of directly from the distribution of atoms in the RMC configurations rather than by fitting, making use of the relationshipwhere the averages are over all molecules in a set of configurations. This approach has been used recently to analyse the RMC configurations of the cubic phases of KCN, 18 BaCO 3 , 19 methane 20 and SF 6 . 21…”

“…The RMC method is used to generate supercell congurations of atomseffectively instantaneous imagesthat are simultaneously consistent with the long-range order revealed by the information contained in the Bragg scattering and with the uctuations of the local structure revealed by the PDF. The congurations can therefore give information about the distributions of orientations of molecules and molecular ions in ordered, partially ordered and disordered phases, [15][16][17][18][19][20][21] and about uctuations within inorganic crystal networks. 6,22,23 We believe that the total scattering method coupled with RMC analysis gives a unique view of disordered crystalline materials.…”

“…This is exactly the sort of question that total scattering methods analysed by the RMC method can tackle and provide useful, and unique, new insights. [15][16][17][18][19][20][21] 3 Methods…”