The compounds β -RE(BO 2 ) 3 [RE = Nd (neodymium meta-borate), Sm (samarium meta-borate) and Gd (gadolinium meta-borate)] were synthesized under high-pressure and high-temperature conditions in a Walker-type multianvil apparatus at 3.5 GPa (Nd), 7.5 GPa (Sm, Gd) and 1050 • C. The crystal structures were determined by single crystal X-ray diffraction data collected at r. t. (Sm, Gd) and at −73 • C (Nd), respectively. The structures are isotypic with the already known ambient-pressure phases β -RE(BO 2 ) 3 (RE = (Tb, Dy) and the high-pressure phases β -RE(BO 2 ) 3 (RE = Ho-Lu).

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The cerium meta-oxoborate δ -Ce(BO2)3 was synthesized under high-pressure / high-temperature conditions of 3.5 GPa and 1050 °C in a Walker-type multianvil apparatus. The crystal structure was determined by single crystal X-ray diffraction data, collected at r. t. The compound crystallizes monoclinicly in the space group P21/c with the lattice parameters a = 422.52(8), b = 1169.7(2), c = 725.2(2) pm, and β = 91.33(3)°. The structure is isotypic to the recently published high-pressure phase δ -La(BO2)3, consisting exclusively of corner sharing [BO4]5− tetrahedra

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Keywords: Erbium fluoride oxide; High-pressure; Crystal structure Er 5 (BO 3 ) 2 F 9 was synthesized under conditions of 3 GPa and 800 °C in a Walker-type multianvil apparatus. The crystal structure was determined on the basis of single-crystal X-ray diffraction data, collected at room temperature. Er 5 (BO 3 ) 2 F 9 is isotypic to the recently synthesized Yb 5 (BO 3 ) 2 F 9 and crystallizes in C2/c with the lattice parameters a = 2031.2(4) pm, b = 609.5(2) pm, c = 824.6(2) pm, and β = 100.29(3)°. The physical properties of RE 5 (BO 3 ) 2 F 9 (RE = Er, Yb) including high temperature behaviour and single crystal IR-/ Raman spectroscopy were investigated.