The novel borate Lu5Ba6B9O27 with a new structure type: synthesis, disordered crystal structure and negative linear thermal expansion.
Acta Crystallogr B Struct Sci Cryst Eng Mater
; 75(Pt 4): 697-703, 2019 Aug 01.
Article
em En
| MEDLINE
| ID: mdl-32830725
ABSTRACT
Single crystals of Lu5Ba6B9O27 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 = 13.0927â
(3), b = 9.9970â
(2) and c = 20.4884â
(4)â
Å, ß = 106.827â
(1)°, V = 2566.86â
(9)â
Å3 and Z = 4. It is described as a framework composed of rings consisting of vertex-sharing [BO3] triangles and [LuO6] 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 Lu5Ba6B9O27 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 [BO3] triangles. The coefficient of negative linear expansion ranges from -1.42 (at 20°C) to -5.57 × 10-6â
°C-1 (at 1000°C). Thermal deformation of the ac plane is described in terms of the theory of shear deformation of monoclinic crystals. The Lu5Ba6B9O27 sample melts at 1170°C.
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Ano de publicação:
2019
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Article