The Ga-occupancy factors and the Fe-Fe bond lengths resulting from the Rietveld refinement of X-ray diffraction data show a critical point at a Ga content between x = 5 and 6. The Fe-Fe exchange-interaction constant J(FeFe) derived from the Curie temperature increases linearly with increasing Ga concentration when x less than or equal to 4, remains almost unchanged for x = 4-6, then increases sharply when x > 6. The variation of the Fe-Fe exchange-interaction constant may be qualitatively explained by the variation of the Ga preferential occupancy aad the Fe-Fe bond lengths. (C) 1999 Elsevier Science B.V. All rights reserved.