High-temperature Hf-site-interchange chemistry in LiNbO 3 and LiTaO 3
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Gary L. Catchen Department of Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 17 September 1992; accepted 3 February 1993)
We explain observed Hf-site interchange in LiNbO3 and LiTaO3 at high temperatures using solid-state defect chemistry reactions. The model takes into account fully the effects of nonstoichiometry on the ferroelectric-to-paraelectric phase transition. Specifically, we use this model to interpret the temperature dependence of the Hf-site interchange that we measured using perturbed-angular-correlation (PAC) spectroscopy. In this context, the site interchange is an equilibrium, thermodynamic process that involves the partitioning of Hf ions between Li and group-V (Nb and Ta) sites. The Hf ions replace group-V ions by pushing them from their normal sites to the Li sublattice. Based on the temperature dependence of the site occupancy, this reaction requires approximately 2.2 to 2.3 eV.
I. INTRODUCTION Lithium niobate, LiNbO 3 , and lithium tantalate, LiTaO3, are isostructural, electro-optic, and piezoelectric materials that have interesting and important technological applications. These materials are used, for example, to produce optical-waveguide switching devices,1 acoustic-optic modulators, 23 and holographic optical recording devices.4 For many of these applications, dopant atoms are important for determining the final properties of the material. For example, optical waveguides are formed in lithium niobate by diffusing titanium atoms into the surface.5"7 Also, for holographic storage applications, lithium niobate is often doped with iron.8 Because the dopant atoms strongly affect the material properties, it is necessary to understand the dissolution and transport mechanisms of the dopants in the crystal lattice, which includes understanding the associated temperature variations. The PAC technique has been used successfully via the 181Ta probe to investigate phase transitions in several ferroic ternary metal oxides,9"12 including both LiNbO 3 and LiTaO 3 . 1314 Because this probe via its site occupancy shows some interesting and unusual crystal chemistry in LiNbO 3 and LiTaO3,13'14 we focus on hafnium (Hf 4 + ) site-occupation and site-interchange chemistry at high temperatures in these two crystals. The defect chemistry of Hf in these materials, for which we provide a model, presents a paradigm for dopant behavior in other ternary metal-oxide crystals. Moreover, investigating the solid-state chemistry of Hf specifically in LiNbO 3 and LiTaO3 may yield a better understanding of the general dopant chemistry in these two materials. J. Mater. Res., Vol. 8, No. 6, Jun 1993 http://journals.cambridge.org
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To integrate the subjects of dopant crystal chemistry and PAC spectroscopy, we present a review of the former and a brief description of the latter. Dopant incorporation into ternary metal-oxide crystals often has unforeseeable implications on properties of the crystal. For example, when Ti is diffused into lithium nioba
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