Elasticity and structure of the compounds in the wollastonite (CaSiO3)-Na 2SiO 3 system: from amorphous to crystalline state.

The elastic properties and structure of four potential bioactive compounds in the CaSiO3-Na2SiO3 system were characterized by Raman and Brillouin spectroscopy at ambient conditions. The increase of Na2O content in the Na2O-CaO-SiO2 glass with the same silica content and hence polymerization was found to lower the elastic moduli with accompanied decrease of Q(0) and Q(2) species, increase of Q(1) species and negligible change of Q(3) species, corresponding to a lower and higher equilibrium constant for the disproportional reactions [Formula: see text] and [Formula: see text] (without balance), respectively. The composition-dependent variation in the shear modulus (G) of the Na2O-CaO-SiO2 glass can be attributed to the concentration change of Q(2) and probably Q(4) species; while bulk modulus (K) ascribed to the cohesion factor. The elastic moduli of the corresponding crystalline phases in this system also lower with the increase of Na2O content following two general criteria as a function of Na2O/CaO molar ratio: (1) K decreases faster than G for both the amorphous and crystalline phases and (2) both K and G decreases faster for the crystals than the glasses.

Exceptionally low shear modulus in a prototypical imidazole-based metal-organic framework.

Using Brillouin scattering, we measured the single-crystal elastic constants (C(ij)'s) of a prototypical metal-organic framework (MOF): zeolitic imidazolate framework (ZIF)-8 [Zn(2-methylimidazolate)(2)], which adopts a zeolitic sodalite topology and exhibits large porosity. Its C(ij)'s under ambient conditions are (in GPa) C(11)=9.522(7), C(12)=6.865(14), and C(44)=0.967(4). Tensorial analysis of the C(ij)'s reveals the complete picture of the anisotropic elasticity in cubic ZIF-8. We show that ZIF-8 has a remarkably low shear modulus G(min) < or approximately 1 GPa, which is the lowest yet reported for a single-crystalline extended solid. Using ab initio calculations, we demonstrate that ZIF-8's C(ij)'s can be reliably predicted, and its elastic deformation mechanism is linked to the pliant ZnN(4) tetrahedra. Our results shed new light on the role of elastic constants in establishing the structural stability of MOF materials and thus their suitability for practical applications.

Effects of CaO/P2O5 ratio on the structure and elastic properties of SiO2-CaO-Na2O-P2O5 bioglasses.

The evolution of elastic properties and structure upon the change of CaO/P(2)O(5) ratio in SiO(2)-CaO-Na(2)O-P(2)O(5) glasses (45S5-derived and 55S4-derived) at ambient conditions has been studied by using both Brillouin and Raman spectroscopy coupled with X-ray diffraction. Under the same SiO(2)/Na(2)O ratio, it is found that a decrease in CaO/P(2)O(5) molar ratio has caused a more-polymerized silicate network via a net consumption of Q(0), Q(1), and Q(2) species yet enriching in Q(3) and Q(4) species. Brillouin experiments revealed that all the bulk, shear and Young's moduli of the glasses studied increases with the increase of CaO/P(2)O(5) molar ratio. The unexpected variation trend in shear modulus can be correlated to the contribution from cohesion, the less-polymerized phosphate Q species, and density. Compared to the 45S5-derived, the more-polymerized 55S4-deived glass has a lower bulk but slightly higher shear modulus at the given CaO/P(2)O(5) ratio.

Directional/acidic dissolution kinetics of (OH,F,Cl)-bearing apatite.

Directional dissolution of a natural (OH,F,Cl)-bearing apatite has been studied at various solution pH values (0-3) and 30 degrees C. This apatite showed abnormally high O--H stretching frequencies due to the substitution of Cl for OH. The advance of dissolution front indicated that steady-state directional dissolution for pH = 0-2 followed an apparent rate law of Rate = ka(H+) (n) (in mole/m(2)h), where the rate constants (k) are 2.15 and 1.61; and the rate orders (n) are 1.44 and 1.30 for [0001] and <1,120> directions, respectively. Previous study, however, indicated a smaller n value (n = 0.55-0.70) for fluorapatite powders at higher pHs. A nonlinear pH dependence of logarithmic dissolution rate at a wide pH range implied that the surface active sites and/or rate-determining steps have changed when the acidity of solution and/or the composition of the apatite were changed. The opening of etch pits on basal planes further indicated that the dissolution rates along the three principal directions have the following relationship: [0001] > <1,120> > <1,010> for pH=0-1, but the order was reversed for pH > 3.