Stability of Br-substituted CuCr2Se4spinels

Cu-rich CuyCr2Se4-zBrxspinels react with air according to CuyCr2Se4-zBrx+ (n/2) O2→Cuy-nCr2Se4-zBrx+ n CuO. By this reaction, the lattice constant aoof the spinels decreases and their Curie temperature Tcincreases. For example, a Cu1.03Cr2.00Se2.96Br0.96spinel with ao= 1.0444 nm and Tc= 216 K reacted with air to Cu0.87Cr2.00Se2.96Br0.96with ao= 1.0416 nm and Tc= 262 K. On the other hand, a Cu-deficient Cu0.78Cr2.00Se2.68Br1.15spinel with ao= 1.0405 nm and Tc= 212 K did not react with air. By this, contradictions in the literature concerning the stability of CuyCr2Se4-zBrxspinels in air are resolved.