Oliver S. Wenger, Rafael Valiente and Hans U. Güdel
Optical spectroscopy of the Ni²⁺-doped layer perovskites Rb₂MCl₄ (M = Cd, Mn):
Effects of Ni²⁺-Mn²⁺ exchange interactions on the Ni²⁺ absorption, luminescence, and upconversion properties
Phys. Rev. B 64, 235116/1-8 (2001)                 

Abstract: Single crystals of Ni²⁺-doped Rb₂CdCl₄ and Rb₂MnCl₄ are studied by polarized optical absorption and luminescence spectroscopy at 15 K. In Ni²⁺:Rb₂MnCl₄, Ni²⁺-Mn²⁺ exchange interactions, which occur exclusively within the perovskite layers of this material, dramatically affect the absorption, luminescence, and up-conversion properties of Ni²⁺ for light polarized within the layers (s): The Ni^{2+ 3}A_2g ¹E_g and ¹A_1g ground-state-absorption (GSA) transitions are each enhanced by about an order of magnitude when compared to Ni²⁺:Rb₂CdCl₄. Similarly, in Ni²⁺:Rb₂MnCl₄ the ³T_2g ¹T_2g excited-state absorption (ESA) as well as its reverse luminescence transition experience an intensity enhancement in their electronic origin region in s polarization. One- and two-color up-conversion-excitation experiments show that, due to the in-plane exchange enhancement of the up-conversion-relevant GSA and ESA transitions in Ni²⁺:Rb₂MnCl₄, the near-infrared-(NIR) to-visible (VIS) up-conversion process at 15 K in this material is up to a factor of 55 more efficient for s than for p-polarized excitation, and also one to two orders of magnitude more efficient than in Ni²⁺:Rb₂CdCl₄ at the same temperature. The structural conditions for efficient Ni²⁺-Mn²⁺ exchange interactions are discussed. The NIR-to-VIS up-conversion efficiency can be further enhanced by an energy-transfer step from Ni^{2+ 1}T_2g to Mn^{2+ 4}T_1g, however, this occurs very inefficiently in Ni²⁺:Rb₂MnCl₄.