The novel borate LuBaBO with a new structure type: synthesis, disordered crystal structure and negative linear thermal expansion.

Single crystals of LuBaBO were obtained by cooling from a melt and polycrystals of the borate were prepared using a multi-step solid-state synthesis. The crystal structure was determined from single-crystal X-ray diffraction data. The borate crystallizes in a new structure type in the monoclinic crystal system in space group C2/c, with cell parameters a = 13.0927 (3), b = 9.9970 (2) and c = 20.4884 (4) Å, β = 106.827 (1)°, V = 2566.86 (9) Å and Z = 4. It is described as a framework composed of rings consisting of vertex-sharing [BO] triangles and [LuO] octahedra. The Ba atoms are in the cavities of the framework. The structure is disordered: one of the B atoms is surrounded by six O atoms with partial occupancies of 0.5. The thermal properties of LuBaBO were investigated by thermal analysis and high-temperature X-ray powder diffraction. Its thermal expansion is highly anisotropic. The negative expansion (contraction) is along the b axis, i.e. parallel to the planes of the largest number of [BO] triangles. The coefficient of negative linear expansion ranges from -1.42 (at 20°C) to -5.57 × 10 °C (at 1000°C). Thermal deformation of the ac plane is described in terms of the theory of shear deformation of monoclinic crystals. The LuBaBO sample melts at 1170°C.