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Keywords:

  • Mössbauer spectroscopy;
  • Structure elucidation;
  • Borates;
  • Lanthanides;
  • Strontium;
  • Optical properties

Abstract

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 Eu5(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. Eu5(BO2.51(7)N0.49(7))4 adopts a threefold superstructure of the structure of Sr3Ln2(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, 151Eu Mössbauer spectroscopy and MAPLE (Madelung part of lattice energy) calculations. The proposed Eu5(BO3)4 contains nitrogen and has a formula Eu5(BO2.51(7)N0.49(7))4 as confirmed by 151Eu Mössbauer spectroscopy. The optical reflectance spectrum of Eu5(BO2.51(7)N0.49(7))4 is in accordance with the yellow colour of the compound.