Pierre Hudon

The nature of phase separation in binary oxide melts and glasses. I. Silicate systems

Abstract An exhaustive review of compositional and thermal extents of miscibility gaps in 41 binary silicate systems permits identification of three groups of cations exhibiting different immiscibility behaviours. The first group comprises network-modifier cations with an ionic radius larger than about 87.2 pm. They have coordination numbers equal to, or higher than, 5 and their miscibility gap size increases linearly with increasing ionic potential. The second group involves cations with an ionic radius larger than 26 pm and smaller than about 87.2 pm (in octahedral coordination). They have at least two coordination numbers: the first one is always 4 and the other 5 (or more). For this reason they are called amphoteric. Their miscibility gap sizes do not increase linearly with an increase of the ionic potential, but follow curves. The third group includes cations with variable crystal field stabilization energies. They are characterized by larger miscibility gap sizes than expected when they are compared with cations with similar ionic radii despite the fact that some of them (e.g. Cr3+) may behave as an amphoteric element because their ionic radii in octahedral coordination are smaller than about 87.2 pm. The origin of phase separation in binary silicate systems is due to coulombic repulsions between poorly screened cations bounded by bridging oxygen strongly polarized towards the silicon, and by non-bridging oxygen.

The nature of phase separation in binary oxide melts and glasses. III. Borate and germanate systems

A review of immiscibility data in binary borate and germanate systems was performed in order to compare miscibility gap consolute temperatures with ionic potentials and radii of their associated cations. The trends obtained demonstrate that a selective solution mechanism similar to the one identified for the binary silicate systems is present in the borate and germanate binaries. More importantly, the borate and germanate immiscibility data permitted the identification of a new group of cations depicting an immiscibility behaviour different from the ones identified in binary silicate systems. The new group involves highly polarizable cations possessing a lone pair of electrons. This lone pair of electrons together with oxygen bonded by strong covalent bonds to modifier cations provides efficient shielding to the cations’ nuclei which considerably reduces the coulombic repulsions and produces miscibility gaps with very low consolute temperatures. A new group of cations having an homogenizing effect on melts (i.e. a capacity to make immiscible melts single phase) is thus reported. Experimental and spectroscopic data suggest that miscibility gaps associated with cations having a lone pair of electrons exist in binary silicate systems such as TlO1=2–SiO2, PbO–SiO2, SnO–SiO2 and Bi2O3–SiO2. The consolute temperature of their miscibility gaps is expected to be relatively low and metastable. 2002 Elsevier Science B.V. All rights reserved.

The nature of phase separation in binary oxide melts and glasses. II. Selective solution mechanism

A comprehensive review of structural data in binary silicate systems indicates that the tetrahedral critical radius (87.2 pm) of binary silicate melts (or glasses) is associated with the silicon tetrahedral network that defines the structure of the melt. In a binary system, most of the cages present in the melt are made of six and five-membered rings of silicon tetrahedra. Cages bounded by six or more-membered rings can host cations of all sizes. However, cations that enter in cages made of five-membered rings are discriminated by their ionic radius. Cations with ionic radii larger than about 87.2 pm (network modifiers) cannot enter in pentagonal apertures; cations with radii smaller than 87.2 pm (amphoteric cations) can. Cages bounded by pentagonal rings play a key role in phase separation by selecting which cations can fit in them, adopt a four-fold coordination, and reduce the size of miscibility gaps, i.e. the cages permit explaining why some cations are amphoteric. This result is important because it shows that a structural control is exerted by the solvent (here SiO2) upon immiscibility which creates a selective solution mechanism that affects small (<87.2 pm) cations in binary silica-rich melts.

A Coupled Experimental Study and Thermodynamic Modeling of the SiO2-P2O5 System

A coupled key phase diagram experimental study and thermodynamic modeling of the SiO2-P2O5 system was conducted. Equilibration and quenching experiments using sealed Pt crucibles were performed in the SiO2-rich region to resolve inconsistencies found in the literature data about the SiO2 liquidus. Based on new experimental data and all available and reliable phase diagram and thermodynamic data, the thermodynamic optimization of the SiO2-P2O5 system was carried out to obtain a set of thermodynamic equations for liquid and solid phases in the system. The liquidus slope of SiO2 indicates that P2O5 in the SiO2-rich liquid exists mainly in the form of P2O74− instead of PO43−. Thermodynamic properties of the liquid solution can be predicted from the Gibbs energy of the liquid phase.