Effect of synthesis conditions on the particle size and luminescence properties of the La3.16Gd10Yb0.7Er0.14Ge2B6O34 germanate borate

La3.16Gd10Yb0.7Er0.14Ge2B6O34 germanate borates have been synthesized using liquid-phase homogenization of starting materials. The smallest crystallite size, 25.3 ± 0.8 nm, has been obtained using a combination of two complexing agents (citric acid and mannitol). We have determined the energy yield of upconversion luminescence in the synthesized borates. Using tartaric acid as a complexing agent, we have obtained a phosphor with an energy yield of upconversion luminescence B en = 0.52% (λex = 974 nm, P = 2.5 W/cm2) and crystallite size of 80.6 ± 0.5 nm.