Structural Transitions of Silicate Nanocrystals in the Glass

Abstract

The temperature dependences of resistivity and thermopower measurements reveal that nanocrystals of silicates in lead-silicate glass of various compositions undergo structural transitions in the temperature range of 800 -1000 K. The diameter of these nanocrystals estimated from Scherrer's formula is about 0.8 -1.6 nm and each nanocrystal consists of 8 -64 unit cells. Structure transitions are detected as sharply maxima of resistivity ρ and thermopower S at temperature T = 800 -100 K. Lead-silicate glass was doped by RuO 2 up to 16 weight% to facilitate the measurements of the ρ and the S. The doped lead-silicate glass has a metal-like behavior at the room temperature: the temperature dependences ρ(T) and S(T) are very slow, the value of the S is typical for metals (few and tens of μV/K). Beyond the maximum of the resistance (temperature T > 1000 K) doped lead-silicate glass turns into typical semiconductor having energy gap about 0.05 -1.5 eV depending on the composition of the glass. Anomalous thermal expansion of the RuO 2 relict crystals is detected at the temperatures of 800 -1000 K as well.

Some Problems of Physics of the Thermoelectric Phenomena

“…The silicate glass doped by ruthenium oxide [23][24][25] stands by itself among the materials which thermoelectric properties have investigated. This glass has metallic properties (S ≈ 10−20 µV/K) near room temperature, and the maximum of S(T)≈1.7 mV/K is observed not at low temperatures, but at T ≈ 970−1000 K (Fig.…”

“…2k). Such behaviour, being accompanied similar change of resistance R(T), is a consequence of structural transformations in silicate nanocrystals leading to change of location of the impurity band relative to valence band of glass [23,26].…”

Impurity Bands and Density of State in Doped Silicate Glasses with Metal Oxides (RuO2, CuO, MnO2)

“…(7) In many semiconductors (E) has been studied well [9], but in doped silicate glass information on the density of states has not been found. According to the formula for the first energy level of ruthenium, the hydrogen-like model of energy levels [10] gave a value of 0.124 eV, where m* = 330m 0 is the effective mass of charge carriers in DSG, m o is the mass of a free electron, ε st = 190 is the measured static dielectric constant of DSG, R ꝏ = 13.605 eV -Rydberg constant.…”

“…As a result, J = 23 meV. Under the influence of heat, the impurity band moves towards the valence band of the glass with a speed of about 10 -4 eV/K, and around 300 K they combine to form "metallic" conductivity [8][9][10][11][12][13][14][15]. Accordingly, all impurity atoms are ionized at room temperature.…”

“…But there was no information about this in DSG. Therefore, it is necessary to estimate the location of the impurity band and the density of states in the lead-silicate glass doped with copper and ruthenium oxides with the help of the measuring method used in [10] and the data in it.…”