Electronic paramagnetic resonance study of Cu 2+ ions in copper ion-exchanged layers of lithium niobate crystals
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C. Sada and F. Segato Physics Department, INFM-University of Padova, via Marzolo 8, 35131 Padova, Italy
L.D. Bogomolova, V.A. Jachkin, and N.A. Krasil’nikova Institute of Nuclear Physics, Moscow State University, 119899 Moscow, Russia (Received 28 April 2000; accepted 22 February 2001)
Copper-doped LiNbO3 layers prepared by an Cu–Li ion-exchange process are characterized by electronic paramagnetic resonance. It is found that the majority of Cu2+ ions are coupled by strong exchange interactions which is characteristic of short distances between paramagnetic ions. Such ions are accumulated in a thin layer near the crystal surface and can enter in new crystalline phases formed as a result of the Cu–Li ion exchange. A small amount of Cu2+ ions is incorporated into weakly distorted LiNbO3 crystal lattice inside the diffusion layer.
I. INTRODUCTION
Crystalline lithium niobate (LiNbO3) has been the subject of increasing interest as a ferroelectric material due to its specific piezoelectric, electro-optical, and photorefractive properties, for the development and fabrication of integrated optical devices. Ion exchange is a very attractive process for the local doping of LiNbO3 due to the fact that it is performed at relatively low temperatures ( 60 K, an almost isotropic EPR line with g ⳱ 2.2 was observed. In this case, the results were explained by dynamic JTE. Similar complicated behavior of the EPR spectrum of Cu2+ ions in trigonal crystalline field was predicted earlier.15 It is also important to point out that the HFS spectra obtained for Cu2+ ions in copper ion-exchange layers of LiNbO3 are found to be completely different from
J. Mater. Res., Vol. 16, No. 6, Jun 2001
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F. Caccavale et al.: Electronic paramagnetic resonance study of Cu2+ ions in copper ion-exchanged layers of lithium niobate crystals
Cu2+ spectra in bulk crystals. As a matter of fact, they are detected at room temperature and do not vary at 77 K and their spectral parameters differ from those given in Ref. 10. This permits one to eliminate trigonal symmetry for Cu2+ centers in ion-exchange LiNbO3 crystals. Moreover, the positions of Cu2+ inside the diffusion layers differ from those in bulk LiNbO3. As a consequence, on the basis of SIMS data,5 we assume that during Li–Cu ion-exchange process the Cu2+ ions occupy Li+ sites and that Li+ vacancies are formed, too. V. CONCLUSIONS
Computer simulation of the EPR spectra of Cu2+ ions observed for powder samples prepared by crushing copper ion-exchange LiNbO3 single crystals showed that these spectra are on a superposition of a single Lorentzian line with g ⳱ 2.15 and HFS spectra. It was found that in the samples obtained under different processing conditions the majority of Cu2+ are coupled by strong exchange interactions which is characteristic of short distances between paramagnetic ions. The comparison with SIMS and x-ray diffraction data5 indicates that the ions responsible for the g ⳱ 2.15 line are accumulate
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