  Several Niobium oxides of formula Ba2LnFeNb4O15 (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
crystal-chemical analysis we propose some relationships between the
introduction of Ln3+ ions in the TTB framework and the chemical, structural and
physical properties of these materials.
