Magnetic and vibrational properties and crystal structure of Sr9.2Co1.3(PO4)7 with disordered arrangements of some strontium, cobalt, and phosphate ions

Sr9In(VO4)7 as a model ferroelectric in the structural family of β-Ca3(PO4)2-type phosphates and vanadates

Sr₉In(VO₄)₇ was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca₃(PO₄)₂-type structure and crystallizes in polar space group R3c. The structural parameters of Sr₉In(VO₄)₇ were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) Å and c = 39.6170(3) Å with Z = 6. In³⁺ cations occupy the octahedral M5 site, Sr²⁺ cations occupy the M1, M2, and M3 sites of the β-Ca₃(PO₄)₂-type structure, and the M4 site remains vacant. Sr₉In(VO₄)₇ was characterized by differential thermal analysis (DTA), optical second-harmonic generation (SHG), high-temperature XRD, and dielectric measurements. All these methods prove the existence of a ferroelectric-paraelectric phase transition at T _c = 974 K. This transition is compared with a similar transition in Ca₉In(PO₄)₇ with lower T _c = 902 K. The polar-to-centrosymmetric phase transition in such compounds has a quite unique mechanism of the order-disorder type. The structural transition involves slight shifts of the M1, M2, M3 cations and the E2O₄, E3O₄ tetrahedra, while half of the E1O₄ tetrahedra (E = P or V) statistically reverse their orientation along the three-fold axis, so that the centre of symmetry appears in the structure as a whole. To invert the E1O₄ tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr²⁺ or Ca²⁺) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca₉In(PO₄)₇ corresponds to the smaller rotational vibration amplitude of the P1O₄ tetrahedron required to reverse this tetrahedra at T _c in comparison with V1O₄ in Sr₉In(VO₄)₇.

Optical and dielectric properties of NaCoPO4 in the three phases α, β and γ

In this work, we are interested in the synthesis of monophosphate α-NaCoPO₄, β-NaCoPO₄ and γ-NaCoPO₄ compounds by mechanochemical method and their characterization by X-ray powder diffraction patterns. These compounds are crystallized in the orthorhombic, hexagonal and monoclinic system, in Pnma, P6₅ and P2₁/n space groups, respectively. The optical properties were measured by means of the UV-vis absorption spectrometry in order to deduce the absorption coefficient α and optical band gap E_g. The calculated values of the indirect band gaps (E_gi) for three samples were estimated at 4.71 eV, 4.63 eV and 3.8 for compounds α, β and γ, respectively. The Tauc model was used to determine the optical gap energy of the synthesized compounds. Then, the results of the dielectric proprieties measured by varying the frequency are described.

In this work, we are interested in the synthesis of monophosphate α-NaCoPO₄, β-NaCoPO₄ and γ-NaCoPO₄ compounds by mechanochemical method and their characterization by X-ray powder diffraction patterns.