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The novel borate Lu 5 Ba 6 B 9 O 27 with a new structure type: synthesis, disordered crystal structure and negative linear thermal expansion
Author(s) -
Filatov Stanislav K.,
Biryukov Yaroslav P.,
Bubnova Rimma S.,
Shablinskii Andrey P.
Publication year - 2019
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520619007443
Subject(s) - monoclinic crystal system , crystallography , thermal expansion , crystal structure , boron , octahedron , materials science , single crystal , negative thermal expansion , anisotropy , x ray crystallography , diffraction , chemistry , physics , optics , organic chemistry , metallurgy
Single crystals of Lu 5 Ba 6 B 9 O 27 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 C 2/ 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 3 and Z = 4. It is described as a framework composed of rings consisting of vertex‐sharing [BO 3 ] triangles and [LuO 6 ] 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 Lu 5 Ba 6 B 9 O 27 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 3 ] triangles. The coefficient of negative linear expansion ranges from −1.42 (at 20°C) to −5.57 × 10 –6 °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 Lu 5 Ba 6 B 9 O 27 sample melts at 1170°C.