As one of the compositional designs for enhancing the unconversion luminescence efficiency of Er[sup 3+] in ZrF[sub 4]-based glasses, the anion substitution effect has been examined in glasses 58ZrF[sub 4][center dot](39-x)BaF[sub 2][center dot]xBaX[sub 2][center dot]3Er(or Eu)F[sub 3] (x = 0-29 for X = Cl and x = 0-10 for X = Br in batch compositions). Prominent enhancement of upconversion emission intensity was observed on the substitution of Cl for F. The Cl substitution brought about two effects, of which one was an increase in the deviation from inversion symmetry in the F[sup [minus]] coordination around Er[sup 3+] and the other was an increase in both phonon energy and electron-phonon coupling strength. The former raises the transition probability in the upconversion process, leading to an enhancement of the emission intensity, while the latter increases the multiphonon relaxation in the upconversion process, leading to a reduction of the emission intensity. Due to these antithetical effects on the upconversion process, the upconversion emission intensity of the Cl substituted glasses exhibits a maximum value around the 58ZrF[sub 4][center dot]20BaF[sub 2][center dot]19BaCl[sub 2][center dot]3ErF[sub 3] composition. The substitution of small amounts of Cl for F is concluded to be quite effective for enhancing the upconversionmore » efficiency of ZrF[sub 4]-based glasses. 11 refs., 7 figs., 2 tabs.« less