Structure of lead silicate glasses and its correlation with photoelastic properties
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ORIGINAL PAPER
Structure of lead silicate glasses and its correlation with photoelastic properties A Khanna1*
, A Kaur1, M Fa´bia´n2, Hirdesh1 and A Kaur1
1
Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India 2
Center for Energy Research, Konkoly Thege St 29-33, Budapest 1121, Hungary Received: 14 December 2019 / Accepted: 14 May 2020
Abstract: Short-range structures of xPbO–(100 - x)SiO2 (x = 50, 60 and 65 mol%) glasses were studied by neutron diffraction and Reverse Monte Carlo modeling. Si–O atomic pair correlation distributions are symmetrical and show peaks ˚ . Si4? are tetrahedrally co-ordinated with oxygen, whereas the Pb–O and O–O atomic pair in the range: 1.60–1.64 ± 0.02 A correlation distributions are broad and asymmetrical due to the existence of wide range of Pb–O and O–O distances in the ˚ on glass network. The peak positions in the Pb–O atomic pair correlations shift from 2.60 ± 0.05 to 2.42 ± 0.05 A 2? increasing PbO concentration from 50 to 65 mol%. Pb exist in PbOx (x = 3, 4, 5 and 6) structural units, and the average Pb–O coordination is constant and is in the range of 4.08 ± 0.11 to 4.14 ± 0.08. The O–Si–O bond angles distributions are broad and asymmetrical with peak values in the range of 91° to 109°, and deviate significantly from the value of 109.5° in the ideal tetrahedral structural units. The short-range structural properties of glasses i.e. the cation-oxygen coordination numbers and bond lengths were used to predict the photoelastic properties of the glasses by the Zwanziger model, and it is concluded that xPbO–(100 - x)SiO2 (x = 50, 60 and 65 mol%) glasses should exhibit the properties of zero-stress birefringence. Keywords: Silicate glasses; Pb2? in glass; Structure; Neutron diffraction; RMC simulations; Stress-induced birefringence
1. Introduction PbO has a wide glass forming range with several oxide materials such as B2O3 [1, 2], P2O5 [3], SiO2 [4, 5], TeO2 [6] and GeO2 [7]. PbO acts as a network former at concentrations higher than 40 mol%, and as a network modifier at lower concentrations in xPbO–(100 - x)SiO2 system [8]. The increase of PbO concentration in xPbO– (100 - x)SiO2 system decreases the melting temperature and viscosity of the melt, while the density and refractive index of glasses increase significantly [9, 10]. The structural models of lead silicate glasses show that Pb2? exist in PbOx polyhedral units that are connected to each other and to SiO4 tetrahedra, further it is known that the incorporation of PbO produces non-bridging oxygens (NBO) in the silicate network [11, 12]. Extended X-ray Absorption Fine Structure and molecular dynamics simulation studies by Rybicki and coworkers found that PbO4 is the dominant
Pb–O structural unit in lead silicate glasses [13, 14]. Pb–O bonds are significantly weaker than Si–O bonds, and the glass network can be easily depolymerized, resulting in a steady decrease of the glass transition temperature (Tg) with increase in PbO concentration [15]. Kohara et al. [16] studied the structure of lead si
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