ИСТИНА

Potassium titanyl phosphate KTiOPO4 (KTP) family is an attractive object of investigation due to its unusual property combination: ferroelectric, superionic and nonlinear optical properties. The objects of present research work were the growth of KTP crystals with different contents of chromium (KTP:Cr) and barium and chromium (KTP:Ba:Cr), the study of obtained crystals physical properties and atomic structure since there are some differences regarding the occurrence of chromium in the structure of KTP [1, 2]. Spontaneous flux crystallization was used for growth KTP:Cr single crystals with the addition of 0.05 and 1 mol.% of Cr2O3 to the flux. KTiOPO4 crystals doped with different concentration of Ba (1, 2, 3 mol.%) and Cr (0.1 mol.%) were grown by flux method. Average size of obtained crystals is about 5-6 mm. In these crystals faces {201} and {011} are only well developed. Lattice parameters change lightly. KTP:Ba:Cr and KTP:Cr(0.05 mol.%) single crystals have light green color which indicates the presence of Cr3+ in the structure. KTP:Cr(1 mol.%) is reddish-brown suggesting the presence of chromium ions in the structure in the two valences of 3+ and 6+ [2]. Temperature dependences of ferroelectric permittivity and conductivity of obtained crystals KTP:Cr and KTP:Ba:Cr were measured from 20 to 950°С at the frequency of 1 MHz. Incorporation of chromium has little influence on ferroelectric phase transition temperature for the KTP:Cr crystals but significantly reduces the conductivity lowering it by almost two orders of magnitude (Figure 1). This can be explained by a decrease in the number of vacancies in the positions of potassium atoms due to compensating for differences between charges of trivalent chromium cations and tetravalent titanium cations [3, 4]. So it is an indirect confirmation of chromium occurrence in the position of the titanium atom in the KTP:Cr structure. For the KTP:Ba:Cr crystals the temperatures of ferroelectric phase transition drop with the increasing of barium concentration and fall on 40oC approximately for the sample with maximum content of barium. The conductivity of crystals doped with Ba and Cr decreases by about half an order of magnitude compared to pure KTP as shown in Figure 2. The precise X-ray analisys of KТР:Cr(0.05mol%) and KТР:Cr(1mol%) allowed to determine the chemical formula of investigated samples: K0.996Cr0.005Ti0.995ОPO4 and K1.00Cr0.02Ti0.98ОPO4, respectively. X-ray studies confirmed that chromium atoms in these structures are located near Ti1 and Ti2 positions, more likely occupying the position in the octahedron Ti(2)O6 with smaller volume. In the KTP:Cr crystals the titanium octahedra become less distorted as compared with pure KTP, that is consistent with the results of studies [2]. To make clear the scheme of barium and chromium occurrence in the KТiOРO4 crystals precision investigation of their atomic structure will be carry out. [1] С.V. Kannan, S. Ganesamoorthy, G. Bocelli, L. Righi, P. Ramasamy, Growth, structural and optical studies of chromium doped KTiOPO4 single crystals, J.of Cryst. Growth, vol. 252, pp. 328-332, (2003). [2] S. Norberg, V. Streltsov, G. Svensson, J. Albertsson, Dopant positions in strontium/chromium- and barium-doped KTP, determined with synchrotron X-radiation, Acta Cryst, vol.56, pp. 980-987, (2000). [3] P.A. Morris, A. Ferretti, J.D. Bierlein, G.M. Loiacono, Reduction of the ionic conductivity of flux grown KTiOPO4 crystals, J. Crystal Growth, vol.109, pp. 367-375, (1991). [4] T. Hörlin and R. Bolt, Influence of trivalent cation doping on the ionic conductivity of KTiOPO4, Solid State Ionics, vol.78, pp. 55-62, (1995).