Dielectric and Optical Properties of Stoichiometric Magnesium Aluminate Spinel Single Crystals

Abstract

Stoichiometric magnesium aluminate spinel false(MgAl2O4false) single crystals grown by the flux, Czochralski, and Verneuil methods have been characterized for their dielectric and optical properties. Dielectric properties were determined from capacitance measurements in the frequency range 102–105 Hz. The dielectric constant and loss tangent of MgAl2O4 spinel crystals grown by the various methods are comparable. At 1 kc the MgAl2O4 spinel has a dielectric constant of 8.4 and a loss tangent of 10−4. Optical transmission and reflection measurements were made in the wavelength range uv to IR (0.2–40 µm). Optical characteristics of the MgAl2O4 crystals are highly dependent on the method of growth. Chromium‐doped MgAl2O4 spinel single crystals have also been grown by a flux technique. Optical emission and absorption characteristics of Cr+3 in flux‐grown MgAl2O4 spinel have been studied. The results are compared with those of Cr+3 in natural spinel and synthetic spinel crystals prepared by the Czochralski and Verneuil methods.

Magnesium Aluminum Spinel (MgAl2O4)

“…The low-frequency real index of refraction (dielectric constant ~n 2 ) of spinel was measured by Wang and Zanzucchi [40], Westphal and Iglesias [41], Stead and Simonis [42], and Ho [43]. These values lie between 2.88 and 2.90, which are slightly higher that the value of 2.86 determined from Table II.…”

“…Table I lists locations of the IR modes (transverse optical frequency) observed by several researchers [37][38][39]. In particular, an absorption peak near 818 cm -1 becomes prominent as the alumina concentration is increased, as demonstrated by Hafner and Laves [8] and Wang and Zanzucchi [40]. Also, other weak modes not predicted by group theory may be present because of disorder or lack of stoichiometry [35].…”

“…Measurements of Stead and Simonis [42] and others [40,41,43] are used to represent the low-frequency characteristics of spinel. In this region, onephonon-model contributions [39] to k are 70 to 100% of measured values [42], and index predictions are 0.7 to 1.5% low compared with measured values [40][41][42][43].…”

“…Wavenumber (cm-1 ) O'Horo et al [37] 4-pole fits Thomas [40]). O'Horo et al [37] give the following Raman-active mode locations: 311, 492, 611 (F 2 g );…”

Discovering low-permittivity materials: Evolutionary search for MgAl2O4 polymorphs

“…(1) and (2) demonstrate that as the permittivity e increases, then R also increases, and thus, T becomes smaller. MgAl 2 O 4 spinel has much higher permittivity in the microwave band (8.38, 14 8.42 15 ) than in the infrared band (2.96, 15 2.98 5 ). Therefore, it is an intriguing prospect to make MgAl 2 O 4 spinel transparent in microwave band by reducing its permittivity.…”