Cathodoluminescence from mixed KTaO₃‐KNbO₃ crystals

Abstract

Cathodoluminescence spectra are shown for a sequence of mixed crystals of KTaO 3 :KNbO 3 . The luminescence emission bands are broadly similar across the spectrum from 250 to 850 nm, although their relative strengths are sensitive to composition and temperature. A UV band near 330 nm is only well resolved in some compositions and a broad line near 700 nm may arise from Cr ions. The presence of low temperature phase transitions within the crystals is evidenced by intensity steps and spectral changes of the cathodoluminescence. Hysteresis is seen, as are domain effects and electric field shifts of the transition temperatures. The cathodoluminescence of the surface material reveals more transitions than are reported from earlier bulk studies. ] pre-phase softening, precursor effects, plus additional phases [4,7] and existence of dipolar glassy states [8]. In part some differences may result from inaccuracies in the measurement of the Nb concentration, since there are significant differences between compositions of the growth melt and the crystal, as well as sample uniformity.The KTN materials offer a useful framework to explore phase transitions by variations in optical properties. Optical absorption and second harmonic generation have also revealed extra sub-phases [7]. Luminescence properties similarly reveal phase transitions, as briefly reviewed [9], with changes in spectra between phases (as for NH 4 Br [10]), minor wavelength shifts [11], charge release and intense emission events at the phase transition for KNbO 3 [12], irreversible phase changes [13], or those which require a high temperature regeneration of the starting phase [14] and hysteresis effects between cooling and heating [12]. There can be differences between the spectra and the number of detectable phase transitions when recording luminescence with X-ray excitation (RL, radioluminescence) or electrons (CL, cathodoluminescence) [15]. Such differences exist since surface relaxation processes sensed by CL can proceed more readily than those in the bulk material, as for RL. Surface damage as the result of polishing and/or contaminants may additionally result in local changes in phase, composition or dislocation density. Unexpected events can occur with dramatic intensity steps during heating or cooling at temperatures

Growth of Incipient Ferroelectric KTaO3 Single Crystals by a Modified Self-Flux Solution Method

“…Because of the very good lattice match between KT ( a = 3.9877 Å) and the superconducting material, epitaxial films with optimized performance can be grown. − In optical applications, single crystals are used to minimize scattering or absorption of energy. KTaO 3 -based crystals have been already tested in order to be adopted for nonlinear and electro-optics, or as solid immersion lens made for a near-field optical disk system. − Although crystals of suitable size are required for most of the applications, , the growth of high-quality single crystals (i.e., with a high homogeneity of chemical and physical properties) still remains challenging.…”

“…KTaO 3 -based crystals have been already tested in order to be adopted for nonlinear and electro-optics, or as solid immersion lens made for a near-field optical disk system. [15][16][17] Although crystals of suitable size are required for most of the applications, 18,19 the growth of high-quality single crystals (i.e., with a high homogeneity of chemical and physical properties) still remains challenging.…”