The novel phosphors Ba₆(Lu_1–xEu_x)₅B₉O₂₇ (x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.14, 0.2) were obtained by cooling from a melt. Single-crystal X-ray diffraction experiments were provided for all the samples. No structural transitions caused by the Eu³⁺ → Lu³⁺ isomorphic substitution were revealed in this series of solid solutions. The data showed that an expected increase of cell parameters is caused by an increase in the Eu³⁺ content. First, the Eu³⁺ ions occupy large [Lu2O₆] and [Lu3O₆] polyhedra that are the most suitable ones for them, then (at x = 0.14) they start to occupy the smallest [Lu1O₆] polyhedra. Luminescent properties of the Ba₆(Lu_1–xEu_x)₅B₉O₂₇ (x = 0.02–0.2) phosphors were investigated. The highest emission intensity with quantum yield of 28% was demonstrated by Ba₆(Lu_0.92Eu_0.08)₅B₉O₂₇. The concentration quenching effect was revealed for the 0.1 ≤ x ≤ 0.2 samples that could be caused by a beginning of the substitution of the smallest [Lu1O₆] polyhedra. It was shown that the successive substitution occurs at several cation positions of different sizes affects the quenching of luminescence.