Local and Cooperative Jahn–Teller Effect and Resultant Magnetic Properties of M₂AgF₄ (M = Na–Cs) Phases

The crystal structure, magnetic properties, heat capacity, and Raman spectra of double-perovskite M₂AgF₄ (M = K, K_3/4Rb_1/4, K_1/2Rb_1/2, K_1/4Rb_3/4, and Rb) phases have been examined, adding to the body of previous results for the M = Na, Cs derivatives. The results suggest that double-perovskite K₂AgF₄ adopts a disordered orthorhombic Bmab structure with an antiferrodistortive arrangement of the elongated and tilted [AgF₆] octahedra rather than the structure with the ferrodistortive arrangement of compressed octahedra, as suggested previously (Mazej, Z.; Goreshnik, E.; Jagličić, Z.; Gaweł, B.; Łasocha, W.; Grzybowska, D.; Jaroń, T.; Kurzydłowski, D.; Malinowski, P. J.; Koźmiński, W.; Szydłowska, J.; Leszczyński, P. J.; Grochala, W. KAgF₃, K₂AgF₄ and K₃Ag₂F₇: important steps towards a layered antiferromagnetic fluoroargentate­(II). CrystEngComm 2009, 11, 1702–1710). A re-examination of the previously collected single-crystal X-ray diffraction data confirms the current structure assignment, and it is also in agreement with recent theoretical calculations. High-field electron paramagnetic resonance spectra reaffirm the presence of elongated [AgF₆] octahedra in the crystal structure of all M₂AgF₄ phases studied. The local structure of the M = K derivative is most complex, with regions of the sample that are quite orthorhombically distorted, whereas other regions more closely resemble the tetragonal phase. The mixed-cation K/Rb phases are also inhomogeneous, containing regions of the pure K compound and regions of another high-symmetry phase (likely tetragonal) of a mixed (Rb-richer) compound with unknown composition. The temperature-resolved phase diagram of all K/Rb phases has been established and positioned within the entire M = Na, K, Rb, Cs series.