New cation-deficient solid solutions of Ca _3–1.5xYb _x□ _0.5xB ₂SiO ₈ (x = 0–0.4) based on Ca ₃B ₂SiO ₈ borosilicate were synthesized. It was found that solid solutions in the range 0 ≤ x < 0.125 are structurally similar to the low-temperature monoclinic β-Ca ₃B ₂SiO ₈, whereas those of the range 0.187 < x < 0.4 are similar to the high-temperature orthorhombic α-Ca ₃B ₂SiO ₈. The crystal structures of solid solutions with x = 0.2 and 0.3 were determined from single crystal X-ray diffraction data and refined in the orthorhombic space group Pnma to R _obs = 0.051 and 0.057, respectively. Thermal expansion of the samples with x = 0.1, 0.3 was investigated using powder high-temperature X-ray diffraction in the temperature range of 20–800 °C. Photoluminescence spectra of Ca _3–1.5xYb _x□ _0.5xB ₂SiO ₈ consist of the intra-configurational ²F _5/2– ²F _7/2 transitions inside of Yb ³⁺ ions with a maximum at 975 nm.