Structure and magnetism of a new hexagonal polymorph of Ba${}_3$Tb(BO${}_3$)${}_3$ with a quasi-2D triangular lattice

Abstract

This article reports the structural and magnetic characterisation of a new hexagonal ($P{6}_{3}cm$) low-temperature phase of Ba${}_3$Tb(BO${}_3$)${}_3$, isostructural with the heavier lanthanide borates with formula Ba${}_3$Ln(BO${}_3$)${}_3$ (Ln = Dy-Lu). The crystal structure contains a quasi-two-dimensional (2D) triangular lattice of Tb${}^{3+}$ ions and is predicted to display unusual magnetic behaviour as a result of the low dimensionality. Magnetic susceptibility $\ddot{I}‡(T)$ shows antiferromagnetic interactions ($\hat{I}{¸}_{CW}=-7.15$ K) and no sharp ordering transition at $T\ge 2$ K, but analysis of $\ddot{I}{‡}^{\prime }(T)$, isothermal magnetisation and heat capacity suggests the possibility of short-range ordering at $T\approx 10$ K. The material exhibits an inverse magnetocaloric effect at $T<4$ K.