H2C(X)- X center dot center dot center dot X- (X = CI, Br) Halogen Bonding of Dihalomethanes
Monte Carlo and molecular dynamics computer simulations using the rigid TIP4P and the flexible BJH intermolecular H2O potentials were carried out for 50 states of supercritical water characterizing a very wide range of thermodynamic conditions, 573 ≤ T ≤ 1273 K; 0.02 ≤ rho ≤ 1.67 g/cm3; 10 ≤ P ≤ 10,000 MPa. Good agreement with available experimental data of the simulated thermodynamic and structural properties give confidence to the quantitative statistical analysis of intermolecular hydrogen bonding under the conditions studied. Energetic, geometric, and angular characteristics of supercritical H-bonds and their distributions at a given temperature remain almost invariant over the entire density range studied from dilute gas-like (~ 0.03 g·cm(-3)) to highly compressed liquid-like (~ 1.5 g·cm(-3)) states. The increase of temperature from ambient to supercritical affects the characteristics of H-bonding in water much more dramatically than the changes in density along any supercritical isotherm. Compared to H-bonds in liquid water under ambient conditions, the H-bonds at 773 K are, on average, 10% weaker, 5% longer, and less linear. Both above and below the H-bonding percolation threshold the fractions of H2O molecules involved in a certain number of H-bonds in liquid and supercritical water closely follow the universal binomial distribution as a function of the average number of H-bonds per one water molecule in the system, as predicted by the independent bond theory. This universal distribution remains intact even when dynamic criteria of H-bonding lifetimes are additionally applied.
Huntite-family nominally-pure and activated/co-activated LnM3(BO3)4 (Ln = La–Lu, Y; M = Al, Fe, Cr, Ga, Sc) compounds and their-based solid solutions are promising materials for lasers, nonlinear optics, spintronics, and photonics, which are characterized by multifunctional properties depending on a composition and crystal structure. The purpose of the work is to establish stability regions for the rare-earth orthoborates in crystallochemical coordinates (sizes of Ln and M ions) based on their real compositions and space symmetry depending on thermodynamic, kinetic, and crystallochemical factors. The use of diffraction structural techniques to study single crystals with a detailed analysis of diffraction patterns, refinement of crystallographic site occupancies (real composition), and determination of structure–composition correlations is the most efficient and effective option to achieve the purpose. This approach is applied and shown primarily for the rare-earth scandium borates having interesting structural features compared with the other orthoborates. Visualization of structures allowed to establish features of formation of phases with different compositions, to classify and systematize huntite-family compounds using crystallochemical concepts (structure and superstructure, ordering and disordering, isostructural and isotype compounds) and phenomena (isomorphism, morphotropism, polymorphism, polytypism). Particular attention is paid to methods and conditions for crystal growth, affecting a crystal real composition and symmetry. A critical analysis of literature data made it possible to formulate unsolved problems in materials science of rare-earth orthoborates, mainly scandium borates, which are distinguished by an ability to form internal and substitutional (Ln and Sc atoms), unlimited and limited solid solutions depending on the geometric factor.