Novel Ba(Gd1-xYx)0.78F5: 20 mol% Yb3+, 2 mol% Tm3+ (0 ≤x ≤1.0) solid solution nanocrystals: A facile hydrothermal controlled synthesis, enhanced upconversion luminescent and paramagnetic properties

Highlights: • Novel Ba(Gd_1-xY_x)_0.78F₅:Yb³⁺,Tm³⁺ nanocrystals have been proposed and synthesized. • The UC enhancement mechanism of the co-doped nanocrystals have been proposed. • This work may provide a guidance to design for other fluoride solid solution systems. Ba(Gd_1-xY_x)_0.78F₅: 20 mol% Yb³⁺, 2 mol% Tm³⁺ (0 ≤ x ≤ 1.0) solid solution nanocrystals were successfully synthesized via a facile hydrothermal method, the obtained crystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), the transmission electron microcopy (TEM), energy dispersion spectrometry (EDS), photoluminescence (PL) as well as vibrating sample magnetometer (VSM), respectively. The influences of the concentration of yttrium ion (Y³⁺) on the microstructures, upconversion (UC) luminescent and magnetic properties of the Ba(Gd_1-xY_x)_0.78F₅: 20 mol% Yb³⁺, 2 mol% Tm³⁺ samples have been deeply investigated. XRD results showed that all the Ba(Gd_1-xY_x)_0.78F₅: 20 mol% Yb³⁺, 2 mol% Tm³⁺ crystals are continuous solid solution possess a pure cubic phase structure. SEM and TEM results revealed that all the obtained crystals are quasi-spherical nanocrystals with a diameter ranging from ∼55 nm to ∼240 nm, and the doping of Y³⁺ ion can inhibit the grain growth. PL spectra indicated that all the samples exhibit main blue emission centered at 478 nm ascribed to ¹G₄→³H₆ transition of Tm³⁺ ions, and its UC emission intensity initially increases and then notably decreases with increasing content of Y³⁺ ion, giving the maximum at x = 0.8. VSM results showed that all the samples present benign paramagnetism in the magnetic range of −30 to 30 kOe at 300 K due to no coercivity or no remanence. Our results confirm that the UC luminescence properties of the Ba(Gd_1-xY_x)_0.78F₅: 20 mol% Yb³⁺, 2 mol% Tm³⁺ nanocrystals can be effectively modulated through the replacement of Gd³⁺ ions by Y³⁺ ions, which also can provide a relatively clear understanding of the formation process and the enhancement mechanism of UC emission for other rare earth polyfluoride solid solution nanocrystals.