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This letter discusses the optical properties of Cr3+ ions in mixed borate crystals with the formula RX3(BO3)4, where R3+=Y3+ or Gd3+ and X3+=Al3+ or Sc3+. Measurements of the optical absorption spectra have been used to estimate the crystal field strength Dq and the Racah parameters B and C. These quantities confirm that in YAl3(BO3)4(YAB) and GdAl3(BO3)4(GAB)Cr3+ ions occupy strong crystal field sites 2E)<E(4T2)], whereas in YSc3(BO3)4(YSB) and GdSc3(BO3)4(GSB) the Cr3+ ions occupy weak field sites 4T2)<E(2E)]. The consequence is that the fluorescence spectrum of Cr3+:YAB and GAB reveals the sharp R lines at low temperature and R lines plus broadband at 300 K. Both Cr3+:YSB and GSB emit into the broadband 4T2→4A2 transition at all temperatures. The potential of these materials as laser gain media is discussed.

We report on the electron paramagnetic resonance (EPR) and optical absorption and fluorescence spectroscopy of YAl3(BO3)4 single crystals doped with 0.2 mol% of trivalent chromium. From EPR we determine that the Cr3+ ions reside in sites of essentially octahedral symmetry with an orthorhombic distortion. The ground state 4A 2 splitting is determined to be 2√D2 + 3E2 ≈ 1.05 ± 0.04 cm−1, where D and E are fine-structure parameters, and we can attribute this splitting to the combined effect of a low-symmetry distortion and spin–orbit coupling. The g-values and fine-structure parameters D and E of the ground state 4A2 are measured to be gx ≈ gy ≈ gz = 1.978 ± 0.005, |D| = 0.52 ± 0.02 cm−1 and |E| = 0.010 ± 0.005 cm−1 respectively. From 10 K optical absorption we have measured the position and crystal-field splittings of the 2E, 2T 1, 4T 2, 2T 2 and 4T 1 states with the 4T 2 and 4T 1 levels appearing as vibronically broadened bands.

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