The Ba2LnFeNb4O15 “tetragonal tungsten bronze”: Towards RT composite multiferroics

Abstract

Several Niobium oxides of formula Ba 2 LnFeNb 4 O 15 (Ln = La, Pr, Nd, Sm, Eu, Gd) with the "Tetragonal Tungsten Bronze" (TTB) structure have been synthesised by conventional solid-state methods. The Neodymium, Samarium and Europium compounds are ferroelectric with Curie temperature ranging from 320 to 440K. The Praseodymium and Gadolinium compounds behave as relaxors below 170 and 300 K respectively. The Praseodymium, Neodymium, Samarium, Europium and Gadolinium compounds exhibit magnetic hysteresis loops at room temperature originating from traces of a barium ferrite secondary phase. The presence of both ferroelectric and magnetic hysteresis loops at room temperature allows considering these materials as composites multiferroic. Based on crystalchemical analysis we propose some relationships between the introduction of Ln 3+ ions in the TTB framework and the chemical, structural and physical properties of these materials.

In-situ formation of barium ferrite in iron-doped “tetragonal tungsten bronze”: Elaboration of room temperature multiferroic composites

“…The diffraction patterns of all composites pellets show that the "pseudo-tetragonal" orthorhombic cell (Pba2 S.G. n°32) previously observed [9] is maintained. For lanthanum composites, the Bragg diffraction peaks related to the fergusonite LaNbO 4 are identified, despite their relative low intensities.…”

“…We recently have shown that a barium ferrite parasitic phase was responsible for RT magnetic hysteresis [9]. A relationship between the chemical stability of samples.…”

Growth and Characterization of Centimeter‐Sized Ba2LaFeNb4O15 Crystals from High‐Temperature Solution under a Controlled Atmosphere

“…[56][57][58][59][60] The above examples demonstrate the intricate relationships between the composition, structure, and properties of TTB systems, which were illustrated in an original way by the study of Ba 2 REFeNb 4 O 15 composite multiferroic ceramics. [61] At room temperature, the crystal structure of Ba 2 REFeNb 4 O 15 was described in a "pseudotetragonal" symmetry, space group Pba2 (no. 32), with a ≈ b = 12.514(1) Å and c = 3.941(1) Å, and there was suspicion of the existence of structural modulations.…”

“…32), with a ≈ b = 12.514(1) Å and c = 3.941(1) Å, and there was suspicion of the existence of structural modulations. [61] The Ba 2 REFeNb 4 O 15 family also shows an interesting correlation between structural and dielectric properties, which depends on the size of rare-earth cations. An uncommon evolution from ferroelectric to relaxor properties was found in the related solid solution Ba 2 Pr x Nd 1-x FeNb 4 O 15 .…”

“…In our early studies of the Ba 2 ReFeNb 4 O 15 (BREFNO) family, [61] we envisaged the existence of such aperiodic features. However, we did not actually expect them in the lanthanum representative, which is paraelectric at room temperature, as we supposed they were related to the ferroelectric state.…”