Synthesis, properties, and structure of potassium titanyl phosphate single crystals doped with chromium
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CTURE OF INORGANIC COMPOUNDS
Synthesis, Properties, and Structure of Potassium Titanyl Phosphate Single Crystals Doped with Chromium E. I. Orlovaa, E. P. Kharitonovaa, N. E. Novikovab, N. I. Sorokinab, and V. I. Voronkovaa a Moscow
b Shubnikov
State University, Moscow, 119991 Russia Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: [email protected] Received February 26, 2015
Abstract—Chromium-doped potassium titanyl phosphate single crystals have been synthesized and their physical properties have been studied. Doping with chromium barely affects the temperature of the ferroelectric phase transition but significantly reduces conductivity (by almost two orders of magnitude). A precise Xray diffraction study of K1.00Cr0.02Ti0.98ОPO4 and K0.996Cr0.005Ti0.995ОPO4 crystals has shown that chromium atoms are located near sites of titanium atoms. DOI: 10.1134/S1063774515060243
INTRODUCTION The vast family of potassium titanyl phosphate crystals KTiOPO4 (KTP) is a promising object of study due to the unusual combination of their ferroelectric, superionic, and nonlinear optical properties [1–3]. This unique combination of the characteristics of KTP crystals is determined by their crystal structure of tunnel type [4]. At room temperature it is a rigid three-dimensional framework, formed by alternating TiO6 octahedras and PO4 tetrahedra with shared vertices. TiO6 octahedra are connected so as to form infinite chains oriented parallel to the 011 and 0 11 crystallographic directions. There are wide spiral channels, elongated along the c axis, in the threedimensional structural framework; these channels contains potassium cations occupying two independent crystallographic sites, K1 and K2 (with coordination numbers of 9 and 8, respectively), at room temperature. The KTP structure demonstrated splitting of K atomic sites into the main site and several additional sites [5]. Note that the KТР crystal growth at high temperatures is accompanied by the formation vacancies in potassium sites; their presence explains the high ion conductivity of KТР crystals. The conductivity of the crystal along the c axis is 10–4–10–8 S/cm [6, 7] at room temperature; it increases to 10–2 S/cm near the ferroelectric phase transition point. The conductivity is implemented via hops. The existence of ferroelectricity in KTP crystals was proven for the first time in [8]. The structural aspect of the ferroelectric phase transition in these crystals was investigated for the first time by Harrison et al. [9], who showed that the displacements of Ti+4
cations from the centers of TiO6 octahedra and alkali metal cations along the c axis play a key role in the occurrence of spontaneous polarization in crystals of potassium titanyl phosphate family. Doping of KTiOPO4 crystals with impurities of different valences is mainly performed to search for crystals with improved nonlinear optical characteristics and to gain a deeper inside into the nature of these and other physical properties of the cryst
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