• Journal of the Korean Ceramic Society
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• v.32 no.5
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• pp.582-586
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• 1995

The crystal structures of (Na0.3Sr0.7)(Ti0.7M0.3)O3 (M=Ta, Nb) compounds were determined using the Rietveld method. Due to the tilting of a oxygen octahedron, (Na0.3Sr0.7)(Ti0.7Nb0.3)O3 had a superlattice of doubled a, b and c of simple perovskite. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was tetragonal with a space group 14/mmm. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was a cubic with space group Pm3m, in which no tilting of oxygen octahedron was observed. The difference in the oxygen tilting of these two materials was due to the larger covalency of Nb-O bond than that of Ta-O bond, which induced a strong $\pi$Nb0 bonding in (Na0.3Sr0.7)(Ti0.7M0.3)O3. Therefore, the higher transition temperature of (Na0.3Sr0.7)(Ti0.7M0.3)O3 could be related to the larger tilting of oxygen octahedron.

• Applied Microscopy
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• v.32 no.1
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• pp.9-15
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• 2002

Microstructural investigations of $(1-x)NdAlO_3-xCaTiO_3$ (NACT) complex perovskite compounds were carried out using X-ray diffractometry, neutron diffraction and transmission electron microscopy. When $0.3{\leq}x{\leq}0.9$, NACT had not only the 1 : 1 chemical ordering of cations but also the antiphase and inphase tilting of oxygen octahedron and the antiparallel shift of cations. Both the antiphase boundaries and the ferroelastic domains were present in the microstructure. The long and straight ferroelastic domains became degenerate as x decreases. When x was smaller than 0.3, the chemical ordering was absent and the antiphase tilting of oxygen octahedron was observed. The defects like tangled dislocations and the second phase were also found in the microstructure.

• Applied Microscopy
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• v.32 no.3
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• pp.257-263
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• 2002

Microstructural investigations of $(Li_{1/2}Pr_{1/2})TiO_3$ (LPT) complex perovskite compounds were carried out using X-ray diffractometry and transmission electron microscopy. LPT has not only the ordering of A-site cation deficiencies but also has the antiphase and inphase tilting of oxygen octahedron and the antiparallel shift of cations. Both the antiphase boundaries and the ferroelastic domains are present in the microstructure. Spinodal decomposition is found in the microstructure. The measured dielectric properties were ${\varepsilon}_r=84.6,\;Q\;{\Large f}_o=776\;GHz,\;{\tau}_{f}=-233.66ppm/^{\circ}C$.

• Applied Microscopy
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• v.32 no.2
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• pp.157-162
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• 2002

Microstructural investigations of $(Na_{1/2}Pr_{1/2})TiO_3$ (NPT) complex perovskite compounds were carried out using X-ray diffractometry and transmission electron microscopy. NPT had not 1:1 chemical ordering of Asite cations but had the antiphase and inphase tilting of oxygen octahedron and the antiparallel shift of cations. Both the antiphase boundaries and the ferroelastic domains were not present in the microstructure. Unidentified second phase was found in the microstructure. The measured dielectric properties were ${\varepsilon}_r=99.6,\;Q\;{\Large f}_o=1124\;GHz,\;{\tau}_{f}=-233.64ppm/^{\circ}C$.

• Applied Microscopy
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• v.34 no.1
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• pp.61-69
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• 2004

The microwave dielectric properties and microstructures of the Complex Perovskite (1-x)$(Li_{1/2}Sm_{1/2})TiO_3-x (Na_{1/2}Sm_{1/2})TiO_3$(LNST) system were investigated using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). LNST had not only the antiphase tilting of oxygen octahedron but also the inphase tilting of oxygen octahedron and the antiparallel shift of cations. Also, when $0.0{\leq}x{\leq}0.4$, LNST had the vacancy ordering of A-sites because of the evaporation of Li ions. From the observation of the microstructure, abnormal grain growth phenomena were observed over the whole range of x. The temperature coefficient of resonant frequency ($T_{cf}$) of the $({Li_{1/2}}^{+1}{Sm_{1/2}}^{+3})TiO_3$(LST) system has a large negative value ($-220ppm/^{\circ}C$) but the $({Na_{1/2}}^{+1}{Sm_{1/2}}^{+3})TiO_3$(NST) system which substituted $Na^{+1}$ has a large positive value ($+173ppm/^{\circ}C$). The dielectric properties of ${\varepsilon}_r=103,\;Q*f_{0}=3,700GHz$ and $T_{cf}=+50ppm/^{\circ}C$ at 4GHz were obtained when x =0.4.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.5 no.4
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• pp.394-399
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• 1995

Dielectric properties and crystal structure changes with temperature were observed on $(Na_{0.5}Sr_{0.5})(Ti_{0.5}Nb_{0.5})O_{3}$ which had a superstructure due to oxygen octahedron tilting. Dielectric loss peak observed at 380 K was found to have a relation with a primitive cell change from tetragonal to cubic, however, in this case, dielectric constant variation was not observed. Therefore it was found that the dielectric loss was more senstive than the dielectric constant for detecting the structure change. After the structure change of primitive cell from tetragonal to cubic, X-ray diffraction peaks of superstructure, which completely disappear above 500 K, were still observed. And no dielectric property variations were found with the disappearence of superstucture.

• Journal of the Korean Ceramic Society
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• v.36 no.2
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• pp.136-144
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• 1999

Crystal structures of tungsten-bronze type microwave dielectric ceramics, $BaOLa_2O_34TiO_2$ (BLT) and $BaO(Nd_{0.77}Y_{0.23})_4TiO_2$ (B(NY)T), were analysed using the Rietveld method. The most relibale refinement was obtained by refining the cation and anion positions from the x-ray and neutron diffraction data, respectively. The ambiguites inherent in the refined crystal structure by Mateeva et al. were resolved. The $BaORe_2O_34TiO_2$ structure consiste of $3\times2$ perovskite blocks and 4 pentagon-channels. The Ti-O6 octahedrons are distroted and tilted, which, consequently, induces the displacements of Ba and Re ions producing the superlattics (c$\approx$ 7.6 $\AA$). The B(NY)T showed more severely tilted Ti-O6 octahedrons. The relative dielectric constant $\varepsilon_{\gamma}$ and temperature coefficient $\tau_\varepsilon$ are 109.5 and-$180 ppm/^{\circ}C$ in BLT, 76 and $+40 ppm/^{\circ}C$ in B(NY)T, respectively. The small Re ions produced a positive $\tau_\varepsilon$. The relation between $\tau_\varepsilon$ and the octahedron tilting in complex perovskite is discussed for the tungsten bronze type structure.

• Applied Microscopy
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• v.34 no.2
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• pp.113-120
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• 2004

Microstructural characteristics of the Complex Perovskite (1-x) $(Li_{1/2}Sm_{1/2})TiO_3-x (Na_{1/2}Sm_{1/2})TiO_3$ (LNST) system have been investigated using the transmission electron microscopy (TEM). When $0.0{\leq}x{\leq}0.6$, the vacancy ordering forming the 1/2 (001) superlattice reflections due to the A-site cation deficiencies has apperaed. It could be confirmed by presence of ABPs. But it was difficult to form the vacancy ordering since vacancy concentration gradually lowered as the amount of the substituted Li ions decrease. Antiphase boundaries (APBs) were presented in microstructures of LNST when $0.8{\leq}x{\leq}1.0$. It was considered that these boundaries were caused by the 1:1 chemical ordering of A-site cations, Na and Sm ions. LNST had not only the antiphase tilting of oxygen octahedron but also the inphase tilting of oxygen octahedron and the antiparallel shift of cations all of them. It could be confirmed by presence of ferroelastic domains