“…The energy transfer mechanism was categorized into two types, namely, exchange interaction or multipolar interaction, which depends on the critical distance ( R c ) value. To probe Eu 2+ → Tb 3+ energy transfer mechanism, the value of R c is calculated using the Blasse formula shown in equation 16
where V represents the volume of the unit cell, N is the number of cation sites in the unit cell, and x c is the critical concentration of the doping ions. In the present condition, V = 3461.42 Å 3 , N = 8 × 6 = 48, and x c = 0.185, then the obtained value of R c = 9.06 Å, which is higher than that for the exchange interaction ( R c < 5 Å).…”