Crystal structure of apatite type rare-earth silicate (Sr 2RE2)(RE6)(SiO4)6O 2 (RE=La, Pr, Tb, Tm, and Y) Lii‐Cherng Leu, Sherin Thomas, Mailadil Thomas Sebastian, Swavek Zdzieszynski, Scott Misture, Rick Ubic Journal of the American Ceramic Society, 2011 The crystal structures of apatite‐type (Sr 2 RE 2 )(RE 6 )(SiO 4 ) 6 O 2 (RE=La, Pr, Tb, Tm, and Y) ceramics prepared by conventional solid‐state processing has been examined. The phase and structure analysis was carried out using powder X‐ray diffraction (XRD) and transmission electron microscopy. Electron diffraction and Rietveld structure refinement of XRD data indicated that (Sr 2 RE 2 )(RE 6 )(SiO 4 ) 6 O 2 (RE=La, Pr, Tb, and Y) has a typical oxyapatite‐type structure, A I 4 A II 6 (BO 4 ) 6 O 2 in space group P 6 3 / m (No. 176), where the A I site is shared equally and randomly by Sr and RE ions, A II is occupied by RE ions only, and B is occupied by Si. As the metaprism twist angle in this lanthanide series should increase as the size of RE decreases, the unrealistically low metaprsim twist angle for (Sr 2 Tm 2 )(Tm 6 )(SiO 4 ) 6 O 2 suggested that the hexagonal metric of apatite might not be sustained and the symmetry reduced to monoclinic, space group P 2 1 / m (No. 11), in order to compensate for the shorter Tm–O bond length. The P 2 1 / m model for (Sr 2 Tm 2 )(Tm 6 )(SiO 4 ) 6 O 2 also yields a better fit and improvement in bond valence as compared with P 6 3 / m model.
Structure, microstructure, and microwave dielectric properties of (Sr 2-xCax)(MgTe)O6 double perovskites Rick Ubic, Steven Letourneau, Sherin Thomas, G. Subodh, M. T. Sebastian Chemistry of Materials, 2010 Sr2−xCaxMgTeO6 (0 ≤ x ≤ 2) ceramics were prepared by the solid-state ceramic route. Structure and microstructure of the compounds were investigated using XRD, TEM, and SEM methods. The system undergoes a transition from pseudocubic tetragonal I4/m symmetry for x = 0 (Sr2MgTeO6) to pseudotetragonal monoclinic P21/n symmetry for x > 0. The dielectric properties of the ceramics were studied by the resonance method. The temperature coefficient of resonant frequency is negative throughout the series; permittivities were in the range 13.2−14.3, and quality factors varied from 27 500 to 81 000 (5−6 GHz).
Frequency and temperature dependent dielectric properties of GreenTapes Journal of the Australian Ceramic Society, 2010
Microwave dielectric properties of Co2La4Ti 3Si4O22 ceramics Janardhanan Chameswary, Sherin Thomas, Mailadil Thomas Sebastian Journal of the American Ceramic Society, 2010 Co 2 La 4 Ti 3 Si 4 O 22 ceramic was synthesized by the conventional solid‐state ceramic route. The structure and microstructure of the sintered ceramics were characterized by X‐ray diffraction (XRD) and scanning electron microscopic techniques. XRD study showed the presence of SiO 2 as a secondary phase. Attempts were made to obtain phase‐pure material by preparing nonstoichiometric Co 2 La 4 Ti 3 Si (4− x ) O (22−δ) (where x =0.02, 0.05, 0.1, 0.25, and 0.5) ceramics. The secondary phase of SiO 2 could be effectively suppressed for x =0.1 with good microwave dielectric properties. The Co 2 La 4 Ti 3 Si 3.9 O (22−δ) has ɛ r =17.4, Q u × f =48 700 GHz, and τ f =−155 ppm/°C.
Microwave Dielectric Properties of SrRE4Si3O 13 (RE=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, and Y) Ceramics Sherin Thomas, Mailadil Thomas Sebastian Journal of the American Ceramic Society, 2009 The apatite type SrRE 4 Si 3 O 13 (RE=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, and Y) ceramics have been prepared by the conventional solid‐state ceramic route. The phase purity and surface morphology of the sintered ceramics were studied using X‐ray diffraction and scanning electron microscopy methods. These materials showed poor sinterability and was improved by the addition of a small weight percentage of zinc borosilicate glass. The microwave dielectric properties of these materials were studied for the first time. SrRE 4 Si 3 O 13 ceramics have a low relative permittivity (ɛ r ) in the range 9–16, a Q‐factor ( Q u × f ) upto 26 000 GHz and a low temperature coefficient of resonant frequency (τ f ). The SrLa 4 Si 3 O 13 ceramics possessed a high Q u × f of nearly 26 000 GHz but with a high negative τ f of −46 ppm/°C. The τ f of SrLa 4 Si 3 O 13 ceramics was tuned by the addition of suitable amount of TiO 2 .
