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

Abstract

Abstraer, The results of powder and single-crystal EPR studies on Cu(NH3)sX 2 compounds [X = BF4, CICI4; Br] gire evidence that phase transitions from a eubic c~-phase (g, = 2.12) with ma anti-K2PtCl 6 type structure to low-temperature ~-phases with reduced symmetry occur, which ate indur by orderhag proeesses of the vacaneies 3 of elongated [Cu0qH3)~l~] 2+ pseudo-oetahedra. The b'pe of order is crucially eontrolted by the nature of the anion, la the case of the BF4-and CIOg ~lts the large an[ons ate s'mueture-determining leading to a slightly disturbed anfiferrodistortive order of the elongatr CuJtNs square pyramids at T,-~ 155 K and <130 K, respeet[vely. The resuking pseudo-~etragonal I]-s¨ of the BF 4 eompound is characte¡ by two thombic crystal g tensors (g~allglA = 2.173; gr allg2 A-2.124; gnnllg~^ 2.071). The dibromide sak undergoes a first-ordcr phase transition at Te = 285 K from cubic to tetragonal with a rather Iow cia ratio of about 0.9. The CuUNs square pyramids ha the Low-tcmperature ~-structure are arranged in this case aeeording to a "m[xed ferrodistortive/anª order partero. Interestingly enough the angular dependenee of the g tensor components ]ndicate further structttral ehanges with decreasing temperature, which are of rather local eharaeter, however, not eorrelated with a second phase transit[on. Atomic displaeements lead to reduced Cu~r-Cu" d[stances with[n certain antiferrodistortive pairs, as evideneed by EPR spectroscopy. The molecular g-values at higher temperatures (g~l = 2.243; gi 2.056) transform to those of pairs with eanting aŸ of about 80 ~ (ga = 2.168; g~ = 2.138; g~ = 2.046).

Electron paramagnetic resonance of penta-ammino copper fluoroborate: coexistence of isotropic and anisotropic spectra in tetragonal phase

“…The g-factor is equal to 2.123. The linewidth was not reported [5]. EPR results on [Cu(NH 3 ) 5 ](BF 4 ) 2 •0.1NH 3 , presented in this paper, give g = 2.130 and a linewidth of 31 G. For perchlorates [Cu(NH 3 ) 5 ](ClO 4 ) 2 •xNH 3 the EPR data are available for x = 0 [5], x ≈ 0.1 [6] and x = 0.2 [1].…”

“…Equation ( 2) applied to the analysis of the g-factor change at T c in [Cu(NH 3 ) 5 ](ClO 4 ) 2 •0.2NH 3 (total number of ammonia molecules per Cu(II) ion n = 5.2) showed that the decrease in the volume at the phase transition shifts the equilibrium to the SQP configuration of the Cu(II) ion [4]. The analysis of EPR data of penta-ammino copper(II) perchlorate [Cu(NH 3 ) 5 ](ClO 4 ) 2 • xNH 3 [1,5,6] revealed that there is a significant influence of additional 'lattice ammonia' molecules (not forming [Cu(NH 3 ) 5 ] 2+ complex), hereafter denoted as x, where n = 5 + x, on the g-factor of the EPR line in the high temperature phase [6]. Below the phase transition temperature (T c = 142 K [1] and 153 K [7] for perchlorate and T c = 154 K [5] and 130 K [8] for fluoroborate) the EPR pattern substantially depends on the number of non-coordinated ammonia molecules.…”

“…The analysis of EPR data of penta-ammino copper(II) perchlorate [Cu(NH 3 ) 5 ](ClO 4 ) 2 • xNH 3 [1,5,6] revealed that there is a significant influence of additional 'lattice ammonia' molecules (not forming [Cu(NH 3 ) 5 ] 2+ complex), hereafter denoted as x, where n = 5 + x, on the g-factor of the EPR line in the high temperature phase [6]. Below the phase transition temperature (T c = 142 K [1] and 153 K [7] for perchlorate and T c = 154 K [5] and 130 K [8] for fluoroborate) the EPR pattern substantially depends on the number of non-coordinated ammonia molecules. For perchlorate and fluoroborate single crystals (n = 5), two angular dependent signals are observed [5], whereas for n = 5.2 polycrystalline perchlorate the signal is still a single line [1], only the g-factor and linewidth are changed at the phase transition.…”

“…Below the phase transition temperature (T c = 142 K [1] and 153 K [7] for perchlorate and T c = 154 K [5] and 130 K [8] for fluoroborate) the EPR pattern substantially depends on the number of non-coordinated ammonia molecules. For perchlorate and fluoroborate single crystals (n = 5), two angular dependent signals are observed [5], whereas for n = 5.2 polycrystalline perchlorate the signal is still a single line [1], only the g-factor and linewidth are changed at the phase transition. For n ≈ 5.1 in the low temperature phase of the perchlorate salt the EPR spectrum is a complex one [6].…”

“…Penta-ammino copper(II) fluoroborate is an isomorphic crystal with perchlorate salt, having fcc structure with the lattice constant a = 11.13 Å at room temperature [8]. X-ray temperature dependent studies showed a continuous phase transition starting at T c = 146 K in fluoroborate from cubic to tetragonal structure [5].…”