Synthesis, Crystal Structure, and Characterization of Ba(Ti_1/2Mn_1/2)O₃:  A High Permittivity 12R-Type Hexagonal Perovskite

Abstract

A 12R-type hexagonal perovskite, Ba(Ti 1/2 Mn 1/2 )O 3 , has been prepared by a mixed oxide route at 1250 °C in air and the crystal structure established by a joint Rietveld refinement of powder X-ray and neutron diffraction data at room temperature [space group R3 hm, a ) 5.69135(2) Å, and c ) 27.91860( 15) Å]. The structure consists of a (hhcc) 3 stacking sequence of BaO 3 close-packed layers along the c-axis with face sharing M 3 O 12 trimers connected by a vertex sharing octahedron. Partial ordering of the Ti +IV and Mn +IV ions occurs in the B-site substructure; Mn +IV ions occupy the central octahedral site in the trimers, Ti +IV ions occupy the octahedral site in the vertex sharing octahedron, and both ions occupy the outer octahedral site of the trimers. Impedance spectroscopy and microwave dielectric resonance measurements on dense pellets (>97% of the theoretical X-ray density) show the title compound to be a modest dielectric insulator at room temperature with high permittivity, r ∼ 45 (at both radio and microwave frequencies); a modest unloaded quality factor, Q u ∼ 2026 (at ∼5.75 GHz); and a temperature coefficient of resonant frequency, TC f ) -4 ppm K -1 .

Magnetic dimers and trimers in the disorderedS=32spin systemBaTi1/2Mn1/

“…A good agreement of the atomic positions and bond distances is found with respect to previously published data [8,21]. Table I: Refined structural parameters and relevant bond lengths for BaTi 1/2 Mn 1/2 O3 at room temperature.…”

“…A 12R-type perovskite structure was employed in the refinement, see Fig 3(a). It consists of face-sharing trimers of octahedra connected by corner-sharing octahedra along the c axis, under the space group R3m [8,21]. All atomic occupancy factors were fixed at the stoichiometric values in our Rietveld refinement of the Neutron diffraction data.…”

“…Few examples of spin gap systems have been exhibited in the case of S = 3/2, among which RCrGeO 5 (R = Y or 154 Sm) have been recently confirmed as such by an investigation using inelastic neutron scattering [12]. Furthermore, in BaTi 1/2 Mn 1/2 O 3 the occupation of one of the transition-metal sites is suggested to be disordered [8], making the system also attractive due to the possibility of investigating the role of disorder in the physics of spin-gap systems [13,14]. In addition, the structure allows the formation of magnetic trimers arXiv:1206.3813v5 [cond-mat.str-el] 5 Jun 2015 which, for an antiferromagnetic exchange, will present a degenarate ground state, connecting the physics of spin gap and frustrated systems.…”

“…In this context, the partially disordered 12R-type perovskite BaTi1/2Mn1/2O3 [8] is noteworthy, mainly due to its low symmetry and high degree of chemical disorder (site occupancy) at the B and B' sites. Indeed, a large Curie-Weiss (θ CW ) constant, with no sign of a magnetic phase transition down to T = 2 K, was reported for this material [8]. Thus, in BaTi 1/2 Mn 1/2 O 3 long range magnetic order, if it occurs, takes place in a temperature T θ CW , the hallmark of a frustrated system.…”

Composite-Hydroxide-Mediated Approach for the Synthesis of Nanostructures of Complex Functional-Oxides

“…28 A mixture of BaCl 2 , MnO 2 , and TiO 2 at 0.422, 0.221, and 0.221 mmol, respectively, was used as the raw material for the synthesis. XRD measurement shows that the crystalline structure of the material is the same as that of BaMnO 3 ( Figure 3E), and EDS shows that the atomic ratio of Ba/Ti/ Mn is about 1:0.5:0.5 (inset of Figure 3F).…”