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RESEARCH PRODUCT

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

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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 CuIIN5 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 CuIIN5 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 CuII−CuII 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).