Crystal structures of (Li0.925Eu3+0.025)TaO3, (Li0.968Eu3+0.032)(Ta0.81Ti0.19)O2.937, (Li0.967Sm3+0.033)(Ta0.89Ti0.11)O2.978, and (Li0.950Sm3+0.033Mg0.017)(Ta0.89Ti0.11)O2.987 were investigated by X-ray powder diffraction. The initial structural parameters, taken from those of the isomorphous compound (Li0.977Eu3+0.023)(Ta0.89Ti0.11)O2.968 (space group R3c and Z = 6), were refined by the Rietveld method. A pattern-fitting method based on the maximum-entropy method was subsequently used to determine the three-dimensional electron-density distributions (EDDs) that are free from the structural bias. We confirmed that the EDDs are in accord with the resulting structural models, each of which was composed of the [(Ta, Ti)O6] octahedron and [(Li, Eu, Sm, Mg)O12] polyhedron. We compared these polyhedra and found that the prominent difference among these compounds was the centroid-to-(Li, Eu, Sm, Mg) distance (eccentricity) of [(Li, Eu, Sm, Mg)O12]. The high correlation was demonstrated between the magnitude of eccentricity and photoluminescence intensity under near ultraviolet excitation.