OPTICAL AND EPR STUDY OF PRECIPITATED PHASES IN NABR-MN2+

As-grown NaBr:Mn2+ crystals have been investigated using photostimulated luminescence, EPR and optical absorption techniques in the 6 K-300 K temperature range. Through them the formation of a precipitated phase involving Mn2+, whose magnetic phase transition temperature is well below 6 K, is established. This conclusion then supports the presence of the Suzuki phase previously inferred from Raman data. The value Omega * (A1g)=130+or-7 cm-1 derived at 14 K from the vibrational progression in the 4T1(G) band concurs with the corresponding Raman value. It is shown that, only through the 4A 1(G), 4E(G) peak, lying at 419.6 nm at 14 K for the present case, the presence of MnBr64- cannot be definitely established. This situation is however improved by looking at the Stokes shifts, the 10 Dq values and also the optical bandwidths. The analysis of 10 Dq and Omega (A1g) suggests that the Mn2+-Br- distance R is higher than for MnBr2 or Cd0.85Mn 0.15Br2. However, an increase Delta R=4.8 pm with respect to MnBr2 does not fully account for the 'anomalous' 4A1(G), 4E(G) peak position in the present case where a value J=0.014 cm-1 has been derived for the exchange constant from the EPR bandwidth Delta Hpp=117 G. Attention has been paid to the influence of temperature changes on the optical peak positions as well as to the effects of a quenching at 600 degrees C. Although Delta H pp becomes 2.5 times higher, no evidence is obtained on the formation of isolated Mn2+ ions after quenching.