OH Groups and Fe Valence in Hydrothermal and Flux-Grown YIG

Abstract

In the transparent region of Yttrium Iron Garnet (YIG) in the infrared, it has been shown that Fe²⁺ and Fe⁴⁺ both give broad optical absorption accompanied by electrical conductivity. The conductivity is n‐type for Fe²⁺ ions and p‐type for Fe⁴⁺ ions, both involving a hopping mechanism. Heat treatment of YIG in a reducing atmosphere produces Fe²⁺ at rather low temperatures, and in hydrogen gas the equilibrium is controlled by temperature rather than the diffusion rate. At the same time the Fe²⁺ is formed, OH groups appear in the structure, and their optical absorption near 3 μ affords another way of monitoring the reversible changes in iron valence for this treatment. Hydrothermal YIG is formed with OH groups already present, but with all Fe ions trivalent. The amount of OH present in crystals grown from KOH is much less than for crystals grown from NaOH, and the primary mechanism for admitting the proton into the structure involves substitution of an alkali or a vacancy for the Fe³⁺ ion. Removal of the OH by heat treatment in oxygen produces Fe⁴⁺ ion, and p‐type conductivity. Fourteen distinct lines in the 3 μ region are seen in the infrared spectra of crystals containing protons introduced in various ways, including a small amount even in flux‐grown crystals. These lines arise from OH groups in different environments in the lattice.