The Oxonitridoborate Eu5(BO2.51(7) N0.49(7))4 and the Mixed-Valent Borates Sr3Ln2(BO3)4 (Ln = Ho, Er)

The crystal structures of the mixed borates Sr3Er2(BO3)(4) [Pnma, no. 62, Z = 4, a = 738.08(2), b = 1588.94(4), c = 867.81(2) pm, 683 reflections, 70 parameters, R1 = 0.037, wR2 = 0.077] and Sr3Ho2(BO3)(4) [Pnma, no. 62, Z = 4, a = 738.45(7), b = 1591.55(12), c = 871.03(9) pm, 691 reflections, 59 parameters, R1 = 0.069, wR2 = 0.098] and europium oxonitridoborate Eu-5(BO2.51(7)N0.49(7))(4) [Pnma, no. 62, Z = 12, a = 2232.2(5), b = 1603.1(3), c = 879.59(18) pm, 2880 reflections, 313 parameters, R1 = 0.027, wR2 = 0.059] were solved from single-crystal X-ray diffraction data. Eu-5(BO2.51(7)N0.49(7))(4) adopts a threefold superstructure of the structure of Sr(3)Ln(2)(BO3)(4) (Ln = Ho, Er). In both structure types, the different cations are situated on common sites with a pronounced preferential occupation but no complete ordering. These conclusions are based on the crystal structure refinement, Eu-151 Mossbauer spectroscopy and MAPLE (Madelung part of lattice energy) calculations. The proposed Eu-5(BO3)(4) contains nitrogen and has a formula Eu-5(BO2.51(7)N0.49(7))(4) as confirmed by Eu-151 Mossbauer spectroscopy. The optical reflectance spectrum of Eu-5(BO2.51(7)N0.49(7))(4) is in accordance with the yellow colour of the compound.