Isomorphic impurity distribution in garnets as a function of the solubility of the tri-valent oxides in molten salts and of the method of crystal growth

Abstract

The solubility of Y 2O 3, Fe 2O 3, Al 2O 3, Nd 2O 3, Cr 2O 3 oxides and Y 3Al 5O 12 and Y 3Fe 5O 12 garnets in PbO-0.35 B 2O 3-1.2 PbF 2 changes in the temperature range of 1000–4500°C as follows: C r 2 O 3 < Y 2 O 3 < Y 3 A l 5 O 3 < A l 2 O 3 < Y 3 F e 5 O 12 < N d 2 O 3 < F e 2 O 3 . The temperature range between 1370 °C and 1300 °C (when the solution contains a rather large number of Cr 3+ ions) represents the most favourable temperature conditions for Cr 3+ entering the Y 3Al 5O 12 crystal. Below this temperature only a very limited (minimum) number of Cr 3+ ions enter the crystal. The extent of Nd 2O 3 solubility is greater than that of Y 3Al 5O 12, Nd 2O 3 solubility considerably changing with the change of temperature. This difference increases at high temperatures while both curves draw together at low temperatures. As a consequence, Nd 3+ ions enter the Y 3Al 5O 12 crystal even at low temperatures. In order to reveal the temperature conditions for the impurities entering the crystal two methods for Y 3Al 5O 12 growing on seed crystals were used. 1) The method of smooth temperature decrease (from 1300 °C to 1000 °C). 2) The method of substance transfer from a high-temperature region to a cooler zone using a constant temperature gradient. The two experiments carried out in solution with the equal initial admixtures of Nd 3+ and Cr 3+ concentration showed the effect of the selected method of crystal growth on the Nd 3+ and Cr 3+ impurities' distribution. The method of smooth temperature decrease is characterized by a large temperature range of growth which leads to a rather inhomogeneous distribution of impurities in a crystal at the initial and subsequent stages of growth. In the transfer method the growth proceeds in the high-temperature region (1375 °C to 1350 °C) within a small temperature interval (20 °C to 30 °C). This permits one to obtain crystals with a uniform impurity distribution along the length of the crystal. Thus, the Nd content ranges from 0.9 to 1.2 wt % whereas the Cr 3+ content is equal to 0.45-0.35 wt %.