Synthesis, structural, morphology, spectroscopic and optical study of new metal orthophosphate MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba) compounds

Abstract

This paper reports the synthesis, structure, BFDH crystal morphology, Raman and Infrared spectroscopic studies of three novel metal orthophosphate salts MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba), abbreviated [Sr], [Pb], [Ba]. The structures of these phases are refined from X-ray powder diffraction data by the Rietveld method. The yavapaiite [Ba] phase is monoclinic of C2/m space group [Z=2; a= 8.106(4) Å; b= 5.178(3) Å c= 7.806(1) Å, β=94.79(4)°, V= 327(1) Å3]. Its structure consists of layers, running along the (a, b) plane, and built up of corner-connected Ga(Sb)O6 octahedra and PO43− tetrahedra. The two isotypic phases [Sr]/[Pb] of distorted yavapaiite structure crystallize in the monoclinic C2/c space group (Z=4) with parameters: a=16.455(4)/16.622(7) Å; b=5.158(1)/5.163(2) Å c= 8.005(1)/8.067(3) Å; β=115.49(1)/114.85(2)°, V= 613(1)/628(5) Å3. Their structures consist of MO10 (M= Sr, Pb), PO43− tetrahedra and Sb(Ga)O6 octahedra. In [Pb] phase, the Pb is surrounded by eight oxygen atoms (Pb-O: 2.65-2.77 Å) and two oxygen atoms forming abnormal bond distances Pb-O(4) [3.447(3)Å]. The BFDH crystal morphology predicts hight (h k l) facets for each compound, the center-to-plane distances dhkl are slightly strong in the distorted Yavapaiite [Pb/Sr] rather than in [Ba]. The IR and Raman study is performed based on theoretical group analysis considering Cs site symmetry of PO43− groups in [Ba], and Ci symmetry in [Pb]/[Sr] phases. The assignment and discussion of the observed bands were done by comparison with other yavapaiite materials. The UV–visible investigation reveals direct optical gap energies of values 4.16 eV for [Ba] and 4.24 eV for [Sr], indicating a semi-conducting nature of the prepared materials.