Mechanism of Crystal Structure Transformation and Abnormal Reduction in Ca5-y(BO3)(3-x)(PO4)(x)F (CBPxF):yBi(3+)

Tricoordinated planar triangle (PO4)(3-) may be formed due to the structural differences between planar triangular (BO3)(3-) and tetrahedral (PO4)(3-) when (BO3)(3-) is gradually substituted by (PO4)(3-). This transformation of structure may affect the luminescence properties of phosphor. Therefore, a series of Ca5-y(BO3)(3-x)(PO4)(x)F (CBPxF):yBi(3+) (y = 0.05, 0.15; x = 0-3), Ca5-y(PO4)(3-X)(BO3)(X)F (CPBXF):yBi(3+) (y = 0.05, 0.15; X = 0-1), Ca-4.9(PO4)(3)F (CPF):0.1Eu(3+), Ca-4.95(PO4)(3)F (CPF):0.05Bi(3+), and nCaF(2)/CaCl2 (n = 0-0.1) are synthesized to explore transformation of the crystal structure on luminescence properties. In CBPxF:0.15Bi(3+) (x = 0-3), (PO4)(3-) is doped to substitute for (BO3)(3-), the position of emission spectra remains unchanged and the emission intensity decreases rapidly with increasing x. The underlying main reason for that is formation of the triangular plane (PO4)(3-), which has been verified by performing a series of verification experiments of CPBXF:yBi(3+) (y = 0.5, 0.15; X = 0-1). In CPBXF:yBi(3+) (y = 0.5, 0.15; X = 0-1), (BO3)(3-) is doped to substitute for (PO4)(3-), P-O2 bond breaks and the coordination of (PO4)(3-) varies from four to three when 0.5 < X < 1; meanwhile, the crystal structure transforms from Ca-5(PO4)(3)F (ICSD-9444) to Ca-5(PO4)(3)F (ISCD-30261), which impedes abnormal reduction from Bi3+ to Bi2+. Furthermore, Bi3+ should non-luminance in the plane triangular (PO4)(3-), but luminescence in (BO3)(3-). Therefore, the emission intensity starts to increase and the emission position suddenly changes from 553 to 474 nm in CPBXF:yBi(3+) (y = 0.05, 0.15; 0.5 < X < 1). From this, the crystal structures of CBU:yBi(3+) (y = 0.05, 0.15; x = 0-3) has been inferred to transform from Ca-5(BO3)(3)F (ISCD-65763) to Ca-5(PO4)(3)F (ISCD-30261), and then to Ca-5(PO4)(3)F (ISCD-9444) with x increasing. Emission position remains unchanged and the emission intensity decreases rapidly in CBPxF:yBi(3+) (y = 0.05, 0.15; x = 0-3) do to formation of the triangular plane (PO4)(3-). In addition, the rate of abnormal reduction from Bi3+ to Bi2+ can be improved by reducing the electronegativity of the environment around the activator or increasing the ionization energy of the activator, which has been confirmed by verification experiments of CPF:0.05Bi(3+), nCaF(2)/CaCl2 (n = 0-0.1), and CPF:0.1Eu(3+).