Polymorphy of monoclinic terbium triborate, TbB₃O₆

Abstract

TbB 3 O 6 , that suffers peritectic decomposition below its melting point, was obtained as single crystals by growth from borate flux. Depending on the borate flux, orthorhombic (space group Pnma) TbB 3 O 6 belonging to the isomorphic series of rare earth triborates with small lanthanides, or monoclinic (space group C2/c) TbB 3 O 6 , belonging the corresponding isomorphic series with large lanthanides, was synthesized. Within a broad range at subambient temperatures monoclinic TbB 3 O 6 shows a structural phase transition with a large thermal hysteresis. The phase transition was characterised by means of differential scanning calorimetry, temperature-dependent polarization microscopy and single-crystal X-ray diffraction. According to the observed features the transition can be classified as a non-ferroic, martensitic phase transformation. [15,16]. Supplementing the isomorphic series of REB 3 O 6 with RE = La -Gd, monoclinic TbB 3 O 6 crystallizes with space group I2/a (i.e. C2/c using standard setting), while crystals of orthorhombic TbB 3 O 6 show a structure that is rather different to that of monoclinic TbB 3 O 6 , with space group symmetry Pnma. In this crystal structure exclusively fourfold coordinated boron occurs. Each six [BO 4 ] tetrahedra are connected to a "super-triangle", where three "inner" [BO 4 ] tetrahedra share a common corner and three further "outer" tetrahedra link each two of the inner tetrahedra by two common corners. All the coplanar "super-triangles" are connected by two apical oxygen atoms to adjacent "super-triangles" along the c-axis and by three oxygen atoms to each neighboring "super-triangle", stacked along the b-axis. Tb is eightfold coordinated by oxygen ligands and is arranged in strongly corrugated layers parallel (100). Triborate compounds of the smaller lanthanides, Dy -Lu, were obtained as orthorhombic crystals with space group symmetry Pnma by high-pressure synthesis at 7.5 GPa and 1000°C [17]. Together with orthorhombic TbB 3 O 6 they form a second isomorphic series among the REB 3 O 6 .

Borates: A Rich Source for Optical Materials

“…These chains are alternately bridged by sharing vertices O(2) atoms of triangular [B(1)­O 3 ] units to final 1 [B 2 O 5 ] ∞ chains along the a axis. We note that REB 3 O 6 (RE = Tb, Sm, Pr, Nd) also exhibit the 1D 1 [B 2 O 4 ] ∞ chains consisting of four-membered [B 4 O 12 ] rings, in which [BO 3 ] and [BO 4 ] units are alternately linked, and there is no linkage between [BO 3 ] and [BO 3 ] or [BO 4 ] and [BO 4 ] units. Besides, ASeB 3 O 7 (A = Na, K) series are found to possess a type D chain that is composed of corner-sharing [B 3 O 7 ] clusters.…”