Innovative structural and optical insights into synthesized BaTi(1-2x)SbxCrx(PO4)2 (0 ≤ x ≤ 0.5) yavapaiite phases

Abstract

A series of yavapaiite-type phosphates BaTi_(1-2x)Sb_xCr_x(PO₄)₂ (0≤x ≤ 0.5), was synthesized via solid-state reactions across divers fractions x, and studied by X-ray diffraction, Infrared, Raman UV–visible, and color (CIE-L∗a∗b∗) analyses. These compounds crystallize too in the monoclinic space group C2/m (Z = 2), with yavapaiite structure type and comparable units’ cell parameters. The structure type consists of layers, arranged parallel to (a, b) plane, and formed by corner-connected Ti(Sb/Cr)O₆ octahedra and PO₄³⁻ tetrahedra groups. Rietveld refinements showed that the substitution of Ti⁴⁺ by Sb⁵⁺ and Cr³⁺ cations did not change the yavapaiite crystal structure symmetry, lattice parameters, bond lengths, and angles in studied (0≤x ≤ 0.5) compositions. Specifically, the parameters increase very slightly as x increased from 0 to 0.5 without changing the yavapaiite structure-type. The SEM-EDX high-resolution images showed the formation of divers agglomerated particles with grain boundary and different sizes, and confirmed the presence of expected elements for all studied compositions (0 ≤ x ≤ 0.5). Hirshfeld surface and Fringplots analyses made on BaTi(PO₄)₂ crystal structure (x = 0), highlited that Ba⋯O/O⋯Ba (39.5 %) and O⋯O (32.1 %) are the important intercontacts contributors in the BaTi(PO₄)₂ crystal packing. Infrared and Raman spectra recorded at room temperature revealed characteristic bands of the PO₄³⁻ tetrahedra groups, with slight changes in frequencies positions and intensities with varying the x fractions. The UV–visible absorption spectra showed the effect of Sb/Cr substitution on the optical band gap. Direct band gap values are estimated at 3.62 eV for (x = 0) and around 2.64 eV for (x = 0.1–0.5), indicating a semi-conducting behavior of the substituted materials in the studied composition range. The partial substitution of Ti⁴⁺ with Sb⁵⁺/Cr³⁺ affects the optical properties of BaTi_(1-2x)Sb_xCr_x(PO₄)₂ (0 ≤ x ≤ 0.5) in the way that modifying the Sb/Cr content can remarkably reduce the optical band gap.