Structural and vibrational study of titanium Monophosphates Na0.5M0.25Ti2 (PO4)3 (M = Mn, Ni)

This work is a systematic study of titanium phosphates compounds Na0.5M0.25Ti2(PO4)3 (M = Mn, Ni), which are characterized by X-ray diffraction (XRD), IR spectrometry, Raman and scanning electron microscopy. Indeed, the crystalline structures of the two compounds were determined in the orthorhombic system, with space group Pmmm (Z = 4); the determined unit cell parameters are: a = 14.59Å, b = 13.31 Å, c = 2.6 Å for Na0.5Mn0.25Ti2(PO4)3, and a = 14.60 Å, b = 13.31 Å, c = 2.67Å for Na0.5Ni0.25Ti2(PO4)3. The structures, compared to that of Li0.5M0.25Ti2(PO4)3, are constructed from [TiO6] octahedra and [PO4] tetrahedra connected by sharing angles along the c axis. The cations M = Mn2+, Ni2+, are located in half of the antiprismatic infection sites and are ordered.

Synthesis, structural characterization and ionic conductivity of mixed alkali titanium phosphate glasses

Glasses with formula Na3-xLixCaTi(PO4)3 [10(3-x) mol. % Na2O - 10x mol. % Li2O - 20 mol. % CaO - 20 mol. % TiO2 - 30 mol. % P2O5] (0 ≤ x ≤ 3) were prepared by standard melt-quenching technique, and their structural and physical properties were characterized by thermal analysis, density measurements, Raman, and impedance spectroscopy. When Na+ is gradually replaced by Li+ , molar volume, glass transition temperature (Tg) and ionic conductivity values decrease, pass through a minimum around the composition x = 1.5, then increase, while density values increase, pass through a maximum, then decrease. The non-linear variation of these physical properties is a result of the classical mixed alkali effect. Powder X-ray diffraction shows that crystallization of the glasses leads to the formation of a Nasicon phase for the compositions x = 0 and x = 0.5, and to a mixture of phases for the other compositions. Raman spectroscopy study shows that the glass structure contains P2O7 and PO4 groups, and short -Ti-O-Ti-O-Ti- chains, formed by TiO6 octahedra linked to each other through corners. These chains are linked by phosphate tetrahedra to form -O-Ti-O-P-O- linkages.

Ferroelectric Distortion in Bi4Ti3O12 Studied by Neutron Powder Diffraction

Crystal structures of Bi4Ti3O12 ferroelectrics have been investigated by high-temperature neutron powder diffraction study. The ferroelectricity in Bi4Ti3O12 is shown to originate from the in-plane displacement of TiO6 octahedra with respect to heavy Bi ions, and that the off-center Ti displacement in the octahedra plays a minor role. Bond valence analysis shows that the underbonding of Bi with the adjacent oxide ions at the perovskite A site in high-temperature paraelectric state plays an essential role in the ferroelectric phase transition in the Bi4Ti3O12 system.