Ba ₃NbFe ₃Si ₂O ₁₄: A new multiferroic with a 2D triangular Fe ³⁺ motif

Single crystals of the new multiferroic Ba ₃NbFe ₃Si ₂O ₁₄ have been synthesized and characterized via X-ray diffraction, magnetic susceptibility, NMR, and electric polarization measurements. The underlying topology of the Fe ³⁺ spins is composed of isolated triangular units stacked in 2D layers along the c-direction of the trigonal lattice. Magnetic susceptibility experiments verify the spin state of the iron moments (5 = 5/2), and large antiferromagnetic interactions with θ = -190 K. A cusp in the suscepübility and lambda-like feature in the specific heat indicate a magnetic phase transition at T _N = 26 K, resulting in a frustration index of f= 7. A significant amount of spin entropy is released at higher temperatures, suggestive of short-ranged magnetic correlations developing at 50 K (as evidenced by features in the specific heat and thermal conductivity). The development of a spontaneous electric polarization below 26 K verifies that Ba ₃NbFe ₃Si ₂O ₁₄ is a multiferroic.