Oliver S. Wenger, Rafael Valiente and Hans U. Güdel
Luminescence Upconversion Under High Pressure in Ni²⁺ Doped CsCdCl₃
High Pressure Research 22, 57-62 (2002)                 

Abstract: The near-infrared to visible upconversion properties of Ni²⁺ doped CsCdCl₃ are studied as a function of hydrostatic pressure. At 15 K and ambient pressure, near-infrared excitation of Ni²⁺ around 810 nm leads to upconversion luminescence centered around 610 nm. Due to the increase of the ligand field strength with increasing pressure, the emission band moves to higher energies with a rate of 19 cm^–1/kbar, and at 46 kbar it is centered around 585 nm. Thus the upconversion luminescence undergoes a color change from red to yellow in the 1 bar-46 kbar pressure range. The 15 K pressure-dependent upconversion excitation spectra reveal a slight red-shift of -4 cm^–1/kbar of the upconversion relevant ³T_2g ¹T_2g excited state absorption (ESA) transition, and this is related to a reduction of the Racah B and C parameters, caused by an increasing degree of covalency in the Ni²⁺-Cl^– interaction towards higher pressures. Thus, with upconversion luminescence excitation spectroscopy we can obtain information about the pressure-dependence of an ESA transition. The pressure-induced intensity redistributions within this excitation spectrum are indicative of an increasing spectral overlap of ³T_2g ¹T_2g ESA with ³A_2g ³T_1g (³F) ground state absorption, potentially leading to more efficient upconversion at higher pressures than at ambient pressure.