Upconversion luminescence of t-(Y0.90Yb0.08RE0.02)PO4 uniformly dispersed microspheres (RE=Ho, Er, Tm)

•Ternary t-(Y0.90Yb0.08RE0.02)PO4 (RE=Ho, Er, Tm) spheres have been synthesized.•The sphere diameter shrunk from ˜2.3 to 1.6 μm after 1100 °C calcination.•A 3-photon UC mechanism is responsible for the 5F5→5I8 transition of Ho3+.•2 photons are involved into the 4F9/2→4I15/2 (Er3+) and 3H4→3H6 (Tm3+) transitions.