Rare Earth Nitrate Hybrid Double Perovskites [Me₄N]₂[MLn(NO₃)₆] (M = Na-Cs; Ln = La-Gd, ex. Pm)

An exploration of the synthetic and structural phase space of rare earth hybrid double perovskites A₂B'BX₆ (A = organocation, B' = M⁺, B = M³⁺, X = molecular bridging anion) that include X = NO₃^- and B' = alkali metal is reported, complementing earlier studies of the [Me₄N]₂[KB(NO₃)₆] (B = Am, Cm, La-Nd, Sm-Lu, Y) (Me₄N = (CH₃)₄N⁺) compounds. In the present efforts, the synthetic phase space of these systems is explored by varying the identity of the alkali metal ion at the B'-site. Herein, we report three new series of the form [Me₄N]₂[B'B(NO₃)₆] (B = La-Nd, Sm-Gd; B' = Na, Rb, Cs). The early members of the Na-series crystallize in the trigonal space group R3̅ from La to Nd where a phase transition occurs in the phase between 273 and 300 K, going from R3̅ to the high-symmetry, cubic space group Fm3̅m. The preceding trigonal members of the Na-series also undergo phase transitions to cubic symmetry at temperatures above 300 K, establishing a decreasing trend in the phase-transition temperature. The remainder of the Na-series, as well as the Rb- and Cs-series, all crystallize in Fm3̅m at 300 K. The temperature-dependent phase behavior of the synthesized phases is studied via variable-temperature spectroscopic methods and high-resolution powder X-ray diffractometry. All phases were characterized via single-crystal and powder X-ray diffraction and Fourier transform infrared (FT-IR) and Raman spectroscopic methods. These results demonstrate the versatility of the perovskite structure type to include rare earth ions, nitrate ions, and a suite of alkali metal ions and serve as a foundation for the design of functional rare earth hybrid double perovskite materials such as those possessing useful multiferroic, optical, and magnetic properties.