88 Chapter IV MIXED ALKALI EFFECT IN NASICON GLASSES 4.1 Introduction The majority of the known methods for calculating the particular properties of oxide glasses from composition are based on additive formulae that represent the calculated property as a linear function of oxide concentrations. The evolution of the glass structure according to the composition provides an overview of the behavior of each species. However the physical properties of oxide glasses cannot generally be related to the composition accurately by means of linear functions of the amounts of each component. Linear factors may be used, to a first approximation, and many such sets of aspects have been invoked for the guidance of glass technologists in developing or modifying glass compositions to meet particular specifications [1]. One of the important exceptions to this approximate linearity is the effect of changing the relative proportions of the alkali oxides in glasses containing more than one alkali. When one alkali is progressively substituted for another, the variation of physical properties with the amount substituted is often so non-linear that the initial trend is later reversed, giving rise to a maximum or a minimum. This extreme departure from linearity is called the mixed alkali effect (MAE) [2-5]. The use of mixed alkalis has been exploited in many commercial compositions to give glasses having superior combinations of properties that could be obtained with the incorporation of any one alkali alone. This effect has a significant application [6-8] and makes the mixed alkali glasses of special interest, for instance, low dielectric loss glasses can easily be obtained by incorporating two different alkali. The challenge of the mixed alkali effect arises from its universal occurrence and from the systematic way in which it increases with the difference in sizes of the alkali ions. An adequate theory must be applicable to any oxide glass, simple or complex, and must relate the effect only to the ionic sizes. Many authors has put forward theories to explain the effect as far as a particular property is concerned, more especially the electrical conductivity, but the mixed alkali effect is noticeable on the majority of properties and it is essential for the success of a theory that it agrees, at least qualitatively, with all the experimental facts.
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Chapter IV
MIXED ALKALI EFFECT IN NASICON GLASSES
4.1 Introduction
The majority of the known methods for calculating the particular properties of
oxide glasses from composition are based on additive formulae that represent the
calculated property as a linear function of oxide concentrations. The evolution of the
glass structure according to the composition provides an overview of the behavior of each
species. However the physical properties of oxide glasses cannot generally be related to
the composition accurately by means of linear functions of the amounts of each
component. Linear factors may be used, to a first approximation, and many such sets of
aspects have been invoked for the guidance of glass technologists in developing or
modifying glass compositions to meet particular specifications [1]. One of the important
exceptions to this approximate linearity is the effect of changing the relative proportions
of the alkali oxides in glasses containing more than one alkali. When one alkali is
progressively substituted for another, the variation of physical properties with the amount
substituted is often so non-linear that the initial trend is later reversed, giving rise to a
maximum or a minimum. This extreme departure from linearity is called the mixed alkali
effect (MAE) [2-5].
The use of mixed alkalis has been exploited in many commercial compositions to
give glasses having superior combinations of properties that could be obtained with the
incorporation of any one alkali alone. This effect has a significant application [6-8] and
makes the mixed alkali glasses of special interest, for instance, low dielectric loss glasses
can easily be obtained by incorporating two different alkali. The challenge of the mixed
alkali effect arises from its universal occurrence and from the systematic way in which it
increases with the difference in sizes of the alkali ions. An adequate theory must be
applicable to any oxide glass, simple or complex, and must relate the effect only to the
ionic sizes. Many authors has put forward theories to explain the effect as far as a
particular property is concerned, more especially the electrical conductivity, but the
mixed alkali effect is noticeable on the majority of properties and it is essential for the
success of a theory that it agrees, at least qualitatively, with all the experimental facts.
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The MAE in glasses gives rise to large changes in many dynamic properties,
particularly those related to ionic transport such as electrical conductivity, ionic diffusion,
dielectric relaxation and internal friction, when a fraction of the mobile ions is substituted
by another type of mobile ions [1, 2, 5, 9]. Macroscopic properties such as molar volume
and density, refractive index, thermal expansion coefficient, and elastic moduli usually
change linearly or only slowly with composition. Properties related to structural
relaxation, such as viscosity and glass transition temperature, usually exhibit similar
deviations from linearity as other mixed glass-forming systems which do not contain any
cations [2-4]. The reduced diffusivity in mixed alkali glasses as compared to single alkali
glasses cannot be explained by any major structural alteration upon the mixing of alkali
ions. Rather, experimental results show [10-13] that the alkali ions tend to preserve their
local structural environment regardless of the glass composition. Furthermore, the two
types of alkali ions are randomly mixed in the glass [13-15]. Similar conclusions have
been drawn from computer simulations of mixed alkali glasses [16-18].
Based on the experimental findings, a few theoretical models have also been
developed to understand the MAE [19-24]. These models consider either based upon
structural features e.g., conduction pathways [19, 21, 22] or based upon differing cation
interactions resulting from differences in the mass and/or size of the cation [23, 24].
However, these models are more or less unverified assumptions, such as site relaxation, a
selective hopping mechanism, or a crucial role of Coulomb interactions between the
mobile ions. The promising model which takes into account the two features of the MAE
is the dynamic structure model (DSM) reported by Bunde et al., and Maass et al., [19,
22]. In these models the reduced ion diffusivity in mixed alkali glasses has been
explained in terms of a site relaxation and memory effect, where each type of mobile
cation is able to adapt the glassy nature according to its spatial and chemical
requirements. Swenson et al., have predicted MAE and its relevant alkali conduction
pathways for the mixed alkali glass (LixRb1-xPO3) through reverse Monte Carlo structural
models by bonds valence model [12]. While all these models yield a qualitative
composition dependence of the ionic diffusivity, none of them is able to account for the
mixed alkali effect in the frequency response of the ionic conductivity. This present study
explores the conductivity and relaxation mechanism in mixed alkali NASICON glasses in
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the system (LixNa(1-x))5TiP3O12 and (LixNa(1-x))4NbP3O12) in order to understand the
dynamics of charge carriers in such oxide systems. The ac conductivity and relaxation
mechanisms have been analyzed in the framework of the conductivity and the modulus
formalism. In the present work it has been shown that the conductivity formalism
accounts for the same qualitative variation of relaxation parameters with composition as
the modulus formalism. In this chapter the electrical properties of the glasses have been
studied for NASICON glasses with varying compositions in (LixNa(1-x))5TiP3O12
(LNTPx) and (Lix Na(1-x))4NbP3O12 (LNNbPx).
4.2 Synthesis and Characterization
The mixed alkali NASICON glasses were synthesized by the conventional melt
quenching method. Stoichiometric amount of analytical grade Li2CO3, Na2CO3, Nb2O5,
TiO2 and NH4H2PO4 were used as starting materials. All the compositions form glasses
when cast onto a steel mould; these glasses were subjected to X-ray diffraction studies
and no crystalline phases were detected. FTIR spectrum shows similar six main peaks at
~1200 , 1080, 983, 900, 741, 544 cm-1 for Niobium based glasses and five main peaks at
~ 1150, 1050, 920, 741, 571 for titanium based glasses. The assigns of these bands are
mostly from the contribution of various phosphate vibration and very few from Nb and Ti
vibration which has been discussed in chapter II. There is no deviation in vibration
frequency when alkali atom is replaced, which insists that there is no structural changes
in the glasses due to MAE.
The density (ρ) and the molar volume (V) for these glasses are shown in
Table 4.1. When Li2O is replaced by Na2O, it can be noted that the measured density as
well as the molar volume increases. These variation shapes are similar to those of mixed
Li2O and Na2O alkalis in the Li2O–Na2O–MoO3–P2O5 system [25]. Since the values of
the density and the molar volume are consistent with the ionic size, atomic weight of
lithium and sodium elements and their amount in these glasses, there is no MAE in these
parameters. Glassy nature was confirmed in DSC for all the samples. The glass transition
temperature Tg, the onset of the crystallization temperature Ts, the peak crystallization
temperature Tc, and melting temperature Tm, and the thermal stability parameters (∆T,S)
[26, 27] and Hurby’s parameter, Kgl [28] were determined and listed in Table 4.1. All the
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critical temperature is low for x=0.6 insisting MAE in thermal properties of the sample.
The strength of the MAE in the composition for the glass transition temperature is
defined as,
∆Tg=Tg,lin – Tg (4.1)
where Tg,lin is the linear interpolation between the experimentally determined Tg values of
the two end members (the single alkali NASICON glasses) at the composition which
corresponds to Tg. The ∆Tg,min for (NaxLi(1-x))5TiP3O12 and (NaxLi(1-x))4NbP3O12 is 47
and 44 respectively.
Table 4.1: Glass transition temperature Tg in K, onset of crystalline temperature Ts
in K, crystalline temperature Tc in K, melting temperature Tm in K, thermal
stability parameters (∆T, S), Hruby parameter Kgl and strength of MAE ∆Tg for NASICON glasses.