2. Methods
Crystalline samples of KPb2Cl5 doped with Er3+ and Yb3+ were prepared in a high frequency furnace. KCl, PbCl2, ErCl3 and YbCl3 were mixed in stoichiometric amounts. The blend was put into a quartz ampoule, which was then evacuated and sealed to keep out air and moisture during synthesis. The ampoules were put in a high frequency furnace and first heated to 200 °C for 2 h, and then kept at 900 °C for 1 h to melt the starting materials. Subsequently the sample was cooled to 380 °C over 96 h and then kept at that temperature for 10 h. The samples were left to slowly cool to room temperature over a period of 36 h. During the slow cooling steps the samples were annealed to reduce the number of lattice defects and increase long range order. The samples were crushed with a pestle and mortar, and X-ray diffraction measurements were performed to check for phase purity.
Diffuse reflectance spectra were measured with a Perkin-Elmer Lambda 950 UV/vis/IR absorption spectrometer. Emission and excitation spectra were measured with a SPEX DM3000F spectrofluorometer with a 450 W Xe lamp as the excitation source. Excitation and emission wavelengths were selected with double-grating 0.220 m SPEX 1680 monochromators (1200 l/mm) blazed at 300 and 500 nm, respectively. Emission spectra were also recorded by focusing the emitted light on a fiber guiding the light to a 0.3 m monochromator (Acton Research, Spectra Pro) where emission light is dispersed by a 150 l/mm grating blazed at 500 nm. Dispersed light was detected with Princeton Instruments 300i charge coupled device (CCD). Emission and excitation measurements, particularly in the infrared region, were performed using an Edinburgh instruments FLS920 fluorescence spectrometer. In this spectrofluorometer, NIR emission (800–1700 nm) is detected with a liquid nitrogen-cooled Hamamatsu R5509-72 PMT. Both the SPEX spectrofluorometer and Edinburgh fluorescence spectrometer are equipped with an Oxford helium flow cryostat for low temperature measurements. The spectra were not corrected for the instrumental response. Luminescence lifetime measurements with excitation wavelengths between 487 and 489 nm were performed with the use of a Lambda Physik LPX100 excimer (XeCl)/LPD3000 tunable dye laser system filled with a Coumarin 102 dye solution and a Tektronix 2430 digital oscilloscope.