Isovalent Substitution of Sr for Ba in Ba 3.75 Nd 9.5 Ti 18 O 54 Microwave Dielectric Ceramics - Experimental and Comput
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Isovalent Substitution of Sr for Ba in Ba3.75Nd9.5Ti18O54 Microwave Dielectric Ceramics – Experimental and Computer Modelling Studies Paisan Setasuwon, Robert Freer, Feridoon Azough and Colin Leach Materials Science Centre, University of Manchester/UMIST Grosvenor Street, Manchester, M1 7HS, UK ABSTRACT Ceramics of Ba3.75Nd9.5Ti18O54 substituted with 2, 5, 10, 20 and 50% Sr have been prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1350-1500°C. Densities were ~95% theoretical. The solubility limit for Sr replacing Ba was found to be ~10%. The relative permittivity (~80) was almost independent of Sr content, but 2% Sr increased the Qxf value to a maximum of 11000. Static simulation of Sr- bearing Ba3.75Nd9.5Ti18O54 successfully reproduced the structure and lattice parameters to within 1-2%. Different site distributions of Ba-Sr ions have been explored. INTRODUCTION Over 20 years ago Kolar et al [1] investigated the TiO2-rich part of the ternary BaO- Nd2O3 TiO2 system. Compositions in the vicinity of 1-1-5 were shown to have relative permittivity (εr) of 70-90, Tεr of ~ -100 ppm/°C and low loss (Q ~ 2000 at 1 MHz). Nenasheva et. al. [2] subsequently reported details of compositions corresponding to 114La (BaO-La2O3-4TiO2). The typical properties are εr = 93-100, Tεr = -800 ppm/°C and Q ~ 5000 at 1 MHz. They substituted Ba with Ca, Sr or Pb which were described by the formula, [AxBa1-x]La2Ti4O12. Secondary phases appeared at x = 0.2 for Ca and 0.4 for Sr and Pb. The solubility of Ca and Sr was comparable because of the similarity in the mean radius of ions (134-144 pm) occupying Ba positions. Using ESCA, they confirmed that Ca, Sr or Pb occupied Ba sites. Razgon [3] found that in 114 compounds the La could be entirely replaced by other lanthanides from Pr to Gd, forming isostructural compounds of 114-type. Gens et. al. [4] reported the crystal-chemical and electrophysical properties of 114Ln (BaLn2Ti4O12, Ln = La, Pr, Nd, Sm, Eu and Gd). They proposed an orthorhombic structure with the space group Pba2 or Pbam. Detailed structural studies have been undertaken by several groups including: Matveeva et. al. [5] who refined refined the crystal structure of Ba3.75Pr9.5Ti18O54 ( or 3.75Pr for short); Ohsato et. al. [6] who confirmed the compositions 114Sm and 3.75Ln (Ln = La, Nd and Sm) to be singlephase, and demonstrated the existence of superlattices in the compounds; Roth et. al. [7] who showed by X-ray diffraction and neutron diffraction that 114RE (RE=Nd,Sm) compounds could be described by the formula Ba2RE4(Bax+RE2/3-2x/3)Ti9O27. By the use of electron diffraction Azough et al [8] demonstrated the space group of 1-1-4Pr to be Pnam with a doubled c-axis. They noted that Pnam is a subgroup of the 2c superlattice of Pbam and the existence of antiphase domains with a displacement vector c/2 suggested that 1-14Pr underwent a phase transition during cooling from Pbam to Pnam in which the c axis doubled. High resolution X-ray diffraction of Ba3.75Nd9.5Ti18O54 enabled the structure to be
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