Dielectic properties of binary praseodymium phosphate glasses

Abstract

Interest in the electrical properties of phosphate glasses has been sustained since the discovery by Denton et al. [1] that vanadium phosphate glasses behaved as n-type semiconductors. The vanadate system has been extensively studied [2, 3]. The semiconductor behaviour arises from electron transfer of unpaired d-electrons of a transition metal ion in a low oxidation state. The conduction processes involved have been discussed widely (e.g., [4 6]) as has also the influence on the electrical behaviour of the proportions of reduced to oxidized states [7-9]. A study of the electrical properties of a range of molybdenum phosphate glasses has been reported by Hekmat-Shoar et al. [10], who made d.c. conductivity measurements on a series of binary glasses at various temperatures and showed that the type of conduction corresponded to a hopping process in which the electrical charge transfers from Mo 5+ sites to M o 6+ sites, a small polaron conduction mechanism being applicable; their work was recently complemented by Thorp et al. [11] who made a study of the r.f. permittivity and dielectric loss behaviour of binary molybdenum phosphate glasses of compositions similar to those used by Hekmat-Shoar et a l . We now report a corresponding study of the dielectric properties of praseodymium phosphate glasses. The glasses were of composition 5% Pr6Oll and 10% Pr6Oll , respectively; they were supplied by the Department of Physics, Brunel University, Uxbridge, UK, having been prepared by methods similar to those described by Harani et al. [12]. A variety of techniques (cf. [11]) was used to cover a frequency range extending from 500 Hz to 35 GHz and with some of these it was also possible to examine the temperature dependence of permittivity (e') and dielectric loss (e") up to about 500 ° C, the softening temperature of the glass. For the frequency range from about 500 Hz to 30 MHz a combination of a.c. bridge and Q-meter techniques [l l] using thin discshaped samples, proved to be adequately sensitive. At higher frequencies, up to about l GHz variations of the co-axial line method, again with thin disc-shaped samples, were employed. In extending the measurements to the microwave region, a cavity perturbation method [13] was used at 9 .3GHz (where the cavity dimensions of 2.33cm x 2.33cm x 2.33cm were sufficiently large to enable good accuracy to be obtained after careful selection of appropriate sample sizes) and at the highest frequency, 35 GHz, standing wave measurements were made on a waveguide terminated by a closely fitting block of the glass and a short circuit [11]. The room-temperature permittivity data showed that while e' was almost frequency independent, it was noticeably affected by the praseodymium content, rising, for example, from e' = 6.7 for 5% Pr6Oll to e' = 7.3 for 10% Pr6Ojl at 1 kHz. Using the experimentally measured permittivities at microwave frequencies as the most suitable estimate of e~ the values of (e' e~) were derived and these are shown as a function of frequency in Fig. 1, which also includes, for comparison, the data previously obtained for molybdenum phosphate glass. Fitting the new data to the Universal Law, according to which (e' e~) oc cn (n-l~ shows close agreement and yields a value of n = 0.98; this n value is, within the limits of experimental accuracy, identical for both the 5% and 10% Pr6Ol~ compositions and it is also very similar to the value of n = 0.97 reported previously by the authors for molybdenum phosphate glass. A comparison of the values of e' directly determined experimentally at 9 and at 35 GHz with the extrapolation of Fig. 1 was made (Table I); bearing in mind that three quite separate techniques were involved here, the agreement is good and this suggested that the nearly frequencyindependent behaviour extends at least as far as the beginning of the millimetric wavelength region. The temperature dependence of permittivity was also examined both at low frequencies and at microwave wavelengths. Fig. 2 gives the series of e' against jr plots obtained with successively increasing temperatures up to 500 °C. Inspection shows that the increase in permittivity is somewhat less pronounced at the higher frequencies. Using the 1 kHz data, the