Low-temperature and high-pressure structural behaviour of NaBi(MoO4)2 -: an X-ray diffraction study

NaBi(MoO4)(2) has been characterized by single-crystal and powder X-ray diffraction in the temperature and pressure ranges 13-297 K and 0-25 GPa, respectively. The domain structure developing below T-c = 241 K proves that NaBi(MoO4)(2) undergoes a ferroelastic phase transition associated with tetragonal I4(1)/ato monoclinic I2/a symmetry change. The character of the unit cell evolution as a function of temperature indicates a continuous transition with the spontaneous strain as an order parameter. The structural distortion, due to small displacements of Bi3+ and Na+ ions, develops slowly. Therefore the overall changes, as measured in single-crystal diffraction at 110 and 13 K, appear to be stable. High-pressure powder X-ray diffraction shows that the elastic behaviour is anisotropic, the linear compressibility along the a- and c-axes of the tetragonal unit cell being beta(4) = 2.75(10) x 10(-3) and beta(c) = 4.30(10) x 10(-3) GPa(-1), respectively. The cell contraction, stronger along the c-axis, causes the distances between the MoO4 layers to be shortened. Consequently, the cation migration in the channels formed by MoO4 tetrahedra becomes hindered, and any symmetry lowering phase transition is not observed up to 25 GPa. The zero-pressure bulk modulus is B-a = 76(5) GPa, and its pressure derivative B-0' = 5.1(5). (c) 2005 Elsevier Inc. All rights reserved.