The crystal chemistry of the Sm³⁺ to Sm²⁺ reduction in tetraborate lattices was investigated. In crystalline SrB₄O₇ in air it is mainly Sm²⁺ that is incorporated from a melt or glass containing predominantly Sm³⁺. For the process in air, a reduction and pick-up mechanism is assumed to take place at the crystal/nutrient interface. Stabilization of Sm²⁺ in SrB₄O₇ at high temperature and in an oxidizing atmosphere seems to be a particular property of the system, because no Sm²⁺ inclusion could be observed along the series MB₄O₇ (M = Ca, Ba, Cd, Pb), if similar reaction conditions were applied. So far, there is only one other oxide lattice (BaB₈O₁₃) known where at high temperatures significant amounts of Sm²⁺ are obtained for reactions in the air.Single crystals of SrB₄O₇ : Sm²⁺ were grown by the Czochralski method (k_eff for Sm is 0.5). Optical hole burning experiments for the transition ⁵D₁–⁷F₀ were performed at 80 K. A hole with a width of 0.21 cm^–1 and a depth of 5.25% was formed for the first time for Sm²⁺ in a borate crystal excited by the beam of a single frequency dye laser. A rather small inhomogeneous linewidth of 0.28 cm^–1 allowed the burning of a single hole only.