A survey of transition-metal-containing phosphate glasses

Abstract

Oxidation–reduction equilibrium and coordination of transition metals ions (TM) in meta or more depolymerised phosphate glasses are reviewed. For small concentrations of metal ions, the basicity of the glass host plays an important role in determining the valency of the transition metal (TM) ion. If one considers the molten glass as a solvent, the phosphates being more acceptor of O 2– than silicates and borates glasses, the oxidoreduction equilibrium is shifted towards the reduced state. More generally, the oxidoreduction (redox) reaction of TM m + /TM ( m +1)+ is related to the glass basicity and the possible complex ion formation by the following reaction: TMO p ( m –2 p ) + ( n – p – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:mn>1</mml:mn> <mml:mn>2</mml:mn> </mml:mfrac> </mml:math> ) O 2– ↔ TMO n ( m +1–2 n ) – <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mfrac> <mml:mn>1</mml:mn> <mml:mn>4</mml:mn> </mml:mfrac> </mml:math> O 2 . The oxygen ion activity (O 2– ) is thus related to the basicity and to the possibility of complex ion formation in the glass melt. For glasses containing large concentrations of transition metals, the valency of the metal ions depends mainly on the atmosphere, on the temperature of melting, and on the transition-metal concentration. According to the metal transition nature, the evolution of the redox ratio with the transition-metal concentration is different. TM cations present a large diversity of environments in glasses, which depend on their concentration and valency state. The octahedral (more and less deformed) coordination appears to be frequent for the transition metals in phosphate glasses. Exception is encountered with zinc metaphosphate. The Zn(IV) ions combine with Q 2 P sites to form a tetrahedral network, reminiscent of three-dimensional network glasses such as silica. Besides this simple cation coordination we found other cationic organisations: ( i ) complex oxide formation is sometimes described – VO 2+ , for example corresponds to a compressed octahedral site –; ( ii ) the progressive addition of TM may even leads to condensation of TM polyhedra – it is the case during the addition of Nb 2 O 3 in sodium borophosphate glasses; ( iii ) ‘invert’ glasses may also be observed – for example, the titanophosphates are based on linkages between small phosphate anions and distorted octahedral titanate entities charge balanced by alkali. Spectroscopic methods used to characterise the local surrounding of transition metals are mainly optical absorption, electron paramagnetic resonance, Mössbauer effect, X-ray absorption spectroscopy, and wide-angle neutron scattering with isotopic substitution. The phosphoric anions network is investigated by Raman and IR spectroscopy and particularly with 31 P and 17 O solid-state NMR. New developments of NMR enable to characterise homoconnectivities between the phosphate groups and will allow one in the near future to undertake studies of heteconnectivities between phosphates and TM polyhedra.