Effect of BaO addition on the durability of iron phosphate glasses that contain molybdenum oxide

Abstract

Iron phosphate (IP) glasses containing both 10 mol % BaO and 6–18 mol % MoO₃ consist mainly of Q¹ PO₄ structural units and small amounts of Q⁰ and Q² species, where n in Qⁿ is the number of bridging oxygen atoms per PO₄ tetrahedron. The PO₄ units are connected to Mo⁵⁺O₆ distorted octahedra with Mo⁵⁺–O–P bonds and to Fe-related polyhedra with Fe–O–P bonds to minimize the formation of P–O–P bonds. The IP glasses that contained both Ba and Mo (BaMo-IP) exhibited superior water durability than those containing only Mo (Mo-IP). After immersion tests in ultrapure water at 120 °C for 72 h, the weight loss per specific area in BaMo-IP glasses showed values that were one-fifth to one-tenth smaller than those in Mo-IP glasses. This result was related to a reaction layer that formed at the sample surface during the immersion process. Microstructural observations revealed that this layer was ∼50 nm in thickness and composed of six-line ferrihydrite and phosphorus-containing noncrystalline phases, and it exhibited a homogeneous nanogranular structure. The reaction layer served to protect against water corrosion.