AN EPR INVESTIGATION OF THE PHASE-TRANSITIONS AND COOPERATIVE ORDERING PHENOMENA OF PENTAAMMINECOPPER(II) POLYHEDRA IN CU(NH3)(5)X(2) [X=BF4, CLO4, BR]

The results of powder and single-crystal EPR studies on Cu(NH3)(5)X(2) compounds [X = BF4, ClO4; Br] give evidence that phase transitions fr om a cubic alpha-phase (g(i) similar or equal to 2.12) with an anti-K2 PtCl6 type structure to low-temperature beta-phases with reduced symme try occur, which are induced by ordering processes of the vacancies sq uare of elongated [Cu(NH3)(5) square](2+) pseudo-octahedra. The type o f order is crucially controlled by the nature of the anion. In the cas e of the BF4- and ClO4-, salts the large anions are structure-determin ing leading to a slightly disturbed antiferrodistortive order of the e longated (CuN5)-N-II square pyramids at T-c similar or equal to 155 K and <130 K, respectively. The resulting pseudo-tetragonal beta p-struc ture of the BF4- compound is characterized by two rhombic crystal g te nsors (g(2)(B) parallel to g(1)(A) = 2.173; g(1)(B) parallel to g(2)(A ) = 2.124; g(3)(B) parallel to g(3)(A) = 2.071). The dibromide salt un dergoes a first-order phase transition at T-c = 285 K from cubic to te tragonal with a rather low c/a ratio of about 0.9. The (CuN5)-N-II squ are pyramids in the low-temperature beta-structure are arranged in thi s case according to a ''mixed ferrodistortive/antiferrodistortive'' or der pattern. Interestingly enough the angular dependence of the g tens or components indicate further structural changes with decreasing temp erature, which are of rather local character, however, not correlated with a second phase transition. Atomic displacements lead to reduced C u-II-Cu-II distances within certain antiferrodistortive pairs, as evid enced by EPR spectroscopy. The molecular g-values at higher temperatur es (g(parallel to) = 2.243; g(perpendicular to) = 2.056) transform to those of pairs with canting angles of about 80 degrees (g(1) = 2.168; g(2) = 2.138; g(3) = 2.046).