A new series of rare-earth borate-phosphate family CsNa2Ln2(BO3)(PO4)2 (Ln = Ho, Er, Tm, Yb): tunnel structure, upconversion luminescence and optical thermometry properties

Abstract Developing new oxysalt compounds is always meaningful topic for their low cost and potential applications as laser crystal, piezoelectric crystal, frequency doubling crystal, magnet material and luminescent material. For the first time, this work prepared 1/10 millimeter-level single crystals of borate-phosphates CsNa2Ln2(BO3)(PO4)2 (Ln = Ho, Er, Tm, Yb) by using high temperature flux method with superfluous Cs2O‒NaF‒B2O3‒P2O5 constituents as flux and Ln2O3 as solute. Single crystal X-ray diffraction (SC-XRD) reveals that their structure features a 3D open framework of [Yb2(BO3)(PO4)2]∞ structure that delimiting 1D tunnels filling with [Na]∞ and [Cs]∞ chains. By taking CsNa2Yb2(BO3)(PO4)2 as host matrix and Er3+ as activator, upconversion (UC) phosphor CsNa2Yb2-xErx(BO3)(PO4)2 (x = 0.01−0.10) can be obtained. Excited by 980 nm pumping laser, it can emanate three characteristic bands of Er3+ at 522, 557 and 659 nm. Moreover, optical thermometry by the relative intensity of 522(2H11/2→4I15/2)/557(4S3/2→4I15/2) were investigated at temperatures from 303 to 573 K. The maximum relative sensitivity was measured to be 0.011 K−1 at 303 K, suggestion that CsNa2Yb2-xErx(BO3)(PO4)2 could be a promising candidate for optical thermometry.