Development of the BaO-Mg(Zn) O-Ta_2O_5 ternary phase diagram for single crystal growth of the Ba(Mg_<1/3> Ta_<2/3>) O_3 and Ba(Zn_<1/3> Ta_<2/3>) O_3 using off-stoichiometric composition method

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We completed investigation of the phase relation in the BaO-MgO(ZnO)-Ta_2O_5 system equilibrated at 1450℃. In the BaO-MgO-Ta2O5 system we found three ternary phases, namely 3:1:1 perovskite Ba_3MgTa_2O_9, 9:1:7 TTB(tetragonal tungsten bronze)-type Ba_9MgTa_<14> O_<45> and 4:1:5 Ba_4MgTa_<10> O_<30> with close relation to the tungsten bronze structure. Another ternary phase, Ba10Mg0.25Ta7.9O30, with 10 layer hexagonal perovskite structure can be only obtained after equilibration at 1600℃. The Ba_9MgTa_<14> O_<45> phase shows quantum ferroelectric properties.We have also clarified several contradictory reports related to the ternary and binary phases in the BaO-MgO-Ta_2O_5 composition field. For example, we found no evidence of the hexagonal Ba_9Mg_4Ta_<20> O_<63> phase that was claimed by R. S. Roth[` Phase Equilibria of Experimental Ceramic Systems', NIST Project Report, Report No.1, Contract No.SB1341-02-W-0787, National Institute of Standards and Technology, Ceramics Division ; Gaithersburg, Maryland ; 2 pp.(2003)], nor we can confirm the existence of the Ba_7Ta_6O_<22> phase reported by several authors[J. S. Kim, J.-W. Kim, C. I. Cheon, Y.-S. Kim, S. Nahm, J. D. Byun, J. Eur. Ceram. Soc., 2001, 21, 2599 ; C.-M. Cheng, Y.-T. Hseih, C.-F. Yang, Ceram. Int., 2002, 28, 255 ; X. M. Chen, Y. Suzuki, N. Sato, J. Mater. Sci.: Materials in Electronics, 1994, 5, 244]. The TTB type Ba_9MgTa_<14> O_<45> forms along the BaTa_2O_6. Ba3MgTa2O9 join. We found that only hexagonal polymorph of the BaTa_2O_6 phase exists along the Mg_4Ta_2O_9-BaTa_2O_6 tie line.In the BaO-ZnO-Ta_2O_5 system we could not confirm the existence of the Ba_4ZnTa_<10> O_<30> compound. Instead, a Ba_3ZnTa_8O_<24> phase has been found along the ZnTa_2O_6-BaTa_2O_6 tie line. Using single crystal structural analysis we have demonstrated that the Ba_3ZnTa_8O_<24> phase is isotypical with the Ba_4MgTa_<10> O_<30> phase and discussed the reasons for the difference in the chemical composition of these phases.