Ion exchange of Rb, Ba, and Sr in KTiOPO4

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Abstract
The potassium ions of potassium titanyl phosphate (KTiOPO4, or KTP) are relatively mobile and may be exchanged with other ions from molten salts. Distribution coefficients for potassium, rubidium, barium, and strontium between nitrate melts and titanyl phosphate were measured between 350 and 425 °C. At 425 °C, using melts containing K and a dopant chosen from Rb, Ba, Sr, and with dopant cation mole fractions in the melt of 0.1, Rb is incorporated in the crystal at a site fraction of 0.014, Ba at 0.007, and Sr at 0.002. Introducing 0.008 fraction of Ba increases the low‐frequency dielectric constant and loss of KTP several orders of magnitude, giving conductivities (ωε0ε‘) of 4×10−5 Ω−1 cm−1 at 200 kHz. The results support a potassium vacancy mechanism for ionic conductivity in KTP. Using low concentrations of divalent ions in the melt, flux‐grown KTP has a lower uptake of divalent ions than does hydrothermally grown KTP, an effect attributed to the presence of potassium vacancies created in the high‐temperature flux growth process. Ba or Sr incorporation results in a contraction of the lattice constants of the KTP structure.