An EPR investigation of the phase transitions and cooperative ordering phenomena of pentaamminecopper(II) polyhedra in Cu(NH3)5X2 [X=BF4, CIO4; Br]

The results of powder and single-crystal EPR studies on Cu(NH₃)₅X₂ compounds X=BF₄, CIO₄; Br] give evidence that phase transitions from a cubic α-phase (g _i ?2.12) with an anti-K₂PtCl₆ type structure to low-temperature β-phases with reduced symmetry occur, which are induced by ordering processes of the vacancies □ of elongated Cu(NH₃)₅□]²⁺ pseudo-octahedra. The type of order is crucially controlled, by the nature of the anion. In the case of the BF ₄ ^? and ClO ₄ ^? salts the large anions are structure-determining leading to a slightly disturbed antiferrodistortive order of the elongated Cu^IIN₅ square pyramids atT _c?155 K and <130 K, respectively. The resulting pseudo-tetragonal β-structure of the BF ₄ ^? compound is characterized by two rhombic crystalg tensors (g ₂ ^B∥g ₁ ^A=2.173;g ₁ ^B∥g ₂ ^A=2.124;g ₃ ^B∥g ₃ ^A=2.071). The dibromide salt undergoes a first-order phase transition atT _c=285 K from cubic to tetragonal with a rather lowc/a ratio of about 0.9. The Cu^IIN₅ square pyramids in the low-temperature β-structure are arranged in this case according to a “mixed ferrodistortive/antiferrodistortive” order pattern. Interestingly enough the angular dependence of theg tensor components indicate further structural changes with decreasing temperature, which are of rather local character, however, not correlated with a second phase transition. Atomic displacements lead to reduced Cu^II?Cu^II distances within certain antiferrodistortive pairs, as evidenced by EPR spectroscopy. The molecularg-values at higher temperatures (g _∥=2.243;g _⊥=2.056) transform to those of pairs with canting angles of about 80° (g ₁=2.168;g ₂=2.138;g ₃=2.046).