Walter Sano

EPR of Cu2+ Doped in NH4Br: Orthorhombic centre

Abstract Cu2+-doped ammonium bromide, grown in acidic solution (pH ≈ 0), has been studied by electron paramagnetic resonance in the range 294-77 K. A new Cu2+ centre has been found with spin Hamiltonian parameters almost constant with temperature. At 294K, gx = 2.313 ± 0.005, gy = 2.144 ± 0.005, gz = 2.046 ± 0.005, Ax = 162 ± 3, Ay = 0 ± 5, Az = 13 ± 3, A′x = 0 ± 2 and A′y = 78 ± 3 (A is given in units of 10−4 cm−1). A model with Cu2+ ion at the centre of a Br− square, with two adjacent vacancies at the NH4+ sites, is proposed.

EPR of Mn2+ doped in Zn(BF4)26H2O

Abstract EPR of Mn 2+ doped (∼ 1%) in single crystal of zinc fluoroborate hexahydrate has been studied between 94 and 353 K. Angular variation study of the spectra indicated that the crystal field distortion has a trigonal symmetry along the crystallographic c -axis. The D distortion parameter has two linear regions in its thermal variation. It varies above 200 K as D = (149 + 0.107 T ) G and below 150 K as xz D = (127 + 0.228 T ) G . A “knee”-like curve links these two linear regions confirming an anomaly observed in a previous work at 181 K due to a phase transition. Other spin Hamiltonian parameters are g = 2.000 ± 0.001, a = (8.9 ± 1.5) G , A = (97.9 ± 0.9) G and B = (97.0 ± 0.9) G . They are constant with temperature.

Phase Transition Study of Zn(BF4)26H2O by EPR of Diluted Ni2+ between 98 and 298 K

EPR of Ni 2+ diluted in single crystal of Zn(BF 4 ) 2 6H 2 O was recorded between 98 and 298 K. The thermal variations of the spin Hamiltonian parameters g // , g ⊥ and D , and the linewidth of the spectra showed anomalous behavior when compared with previous work in the isomorphous Ni(BF 4 ) 2 6H 2 O. The anomalies are attributed to a phase transition of second order with \(T_{\text{c}}{\cong}181\) K. D varies linearly with temperature and the linewidth as Δ H = a + b T 2 similar to verified previously in the Ni compound. The phase transition occurs with a change in the coefficients of these laws. From the linewidth, a spin-lattice relaxation time T 1 is determined as 10 -9 to 10 -10 s.

Hyperfine and superhyperfine interactions in the EPR perpendicular spectra of Cu2+ in NH4Br (Acidic)

The study of the perpendicular part of the Electron Paramagnetic Resonance spectra of Cu 2+ , characterized by a variable number of lines when doped in Ammonium Bromide, has been extended examining crystals grown in acidic solution (pH≈1.6). The 19 lines of the spectra that have been observed at 300 K are interpreted like in our quite recent study of Cu 2+ :NH 4 Br (basic). The new hf parameter is A ⊥ =66±2 (the values of A are given in units of 10 -4 cm -1 ) and the shf parameter is A ′ ⊥ =47±3. At 77 K the spectra have ten lines and are interpreted combining seven shf lines and four hf lines. The parameters determined are: g z =2.048±0.005, g x =2.284±0.005, g y =2.189±0.005, A z =108±3, A x =78±3, A y =0±3, A x ′ =0±3 and A y ′ =76±3. The hf and shf interactions are in the same order and are responsible for the characteristic spectra.

Structural phase transition study of NiCl22H2O by EPR

Abstract A single EPR line of isotropic and Lorentzian form was observed in a crystal of NiCl 2 2H 2 O between 113 and 300 K. During a cooling run a sudden line broadening was observed at 200 K associated with a known crystallographic phase transition. On heating, the corresponding transition temperature was 220 K. The EPR spectra are interpreted in terms of a modified Anderson and Weiss model. The crystal field parameter is determined as D = 0.81 K and D = 1.07 K above and below the transition temperature, respectively.