Cobalt Centres in Ammonium Chloride: Optical Absorption Studies

Abstract

The optical absorption data for Co2+ ions in NH,Cl single crystals grown under different conditions and measured in the temperature range of 300 to 10 K are presented. The observed absorption spectrum is interpreted by considering the absorption within the crystal field levels determined mainly by the ligands of octahedral coordination. The spectrum a t low temperatures is considered to be due to two different, centres in which Coz+ ion occupies an interstitial position, i.e. the centre of a square of four C1-ions with either two NH, ions or H,O ions on either side of the square. In addition t o these, the stabilization of a centre in which a Co2+ ion occupies an edge centre position at room temperature is also suggested. The crystal field parameters for the cobalt centres stable a t low temperature are evaluated.

Theoretical Investigations of theg-Factors and Hyperfine Structure Constants for a Tetragonal Co2+ Centre in NH4Cl Crystal

“…(2), we obtain V % 511 cm À1 , V H % 456 cm À1 , k % 0:9547, k H % 0:8596, P % 230 Â 10 À4 cm À1 and P H % 231 Â 10 À4 cm À1 . It is noted that because of lattice relaxation, the equatorial Co 2 ±Cl À distance R eq is slightly different from the above value obtained from the lattice constant, as pointed out by Brenosa et al [25] for Cu 2 in NH 4 Cl. However, since the relaxation DR eq is so small (e.g., DR eq % 0:002 AE 0:0006 nm [25] for Cu 2 in NH 4 Cl) that the calculation errors arising from DR eq for S dp t 2g , S dp e g and hence V, V H , k; k H , P, P H are smaller than 0.1%.…”

“…[8]. Substituting all these parameters into the above formulas, we calculate the EPR parameters g k , g c , A k and A c for the tetragonal Co(H 2 O) 2 Cl 4 centre in NH 4 Cl. The results show good agreement with the observed values (see Table 1).…”

“…The electron paramagnetic resonance (EPR) and optical spectra of transition metal (TM) ions in NH 4 Cl crystals have received much interest [1 to 7]. For Co 2 in NH 4 Cl, Zaripov and Chirkin [2,3] studied its EPR spectra at 4.2 K and found that only one tetragonal Co 2 centre is formed.…”

“…For Co 2 in NH 4 Cl, Zaripov and Chirkin [2,3] studied its EPR spectra at 4.2 K and found that only one tetragonal Co 2 centre is formed. By analyzing the EPR spectra of Co 2 and other TM ions in NH 4 Cl at low temperature (below the temperature T C of ferromagnetic phase transition), they concluded that the divalent TM ion (including Co 2 ) is at the face centred interstitial position with two H 2 O molecules replacing two NH 4 ions on either side of TM ion in the h100i direction and so the tetragonal Co(H 2 O) 2 Cl 4 centre is formed. Although the EPR g-factors g k , g c and hyperfine structure constants A k , A c for the above Co(H 2 O) 2 Cl 4 centre were reported by them [2,3] decades ago, no theoretical explanations for these EPR parameters have been made.…”

“…The core polarization constant j in the calculation formulas of hyperfine constants is used as an adjustable parameter. Based on these formulas, the EPR parameters g k , g c , A k , A c for the tetragonal Co(H 2 O) 2 Cl 4 centre in NH 4 Cl crystal are calculated from the above defect model. The results show good agreement with the observed values.…”