Synthesis and crystal structures of α- and β-modifications of Cr2IrB2 containing 4-membered B4 chain fragments, the τ-boride Cr7.9Ir14.1B6 and orthorhombic Cr2B

Single crystals of two forms of Cr2IrB2, the τ-boride Cr7.9Ir14.1B6 and orthorhombic Cr2B were synthesised from the elements at temperatures between 1450 and 1650 °C. α-Cr2IrB2 was obtained at 1650 °C and crystallizes in the Mo2IrB2 crystal structure type (oP20, Pnnm, Z = 4, a = 9.210(1) Å, b = 7.112(1) Å, c = 2.9833(5) Å, 493 refl., 27 param. R1(F) = 0.0220, wR2(F2) = 0.0521) containing 4-membered B4 chain fragments with the terminating B atoms in trans position. β-Cr2IrB2 (1450 °C) forms a low temperature form in a new crystal structure type (oC40, Cmcm, Z = 8, a = 3.0024(7) Å, b = 9.945(2) Å, c = 13.236(2) Å, 424 refl., 28 param., R1(F) = 0.0279, wR2(I) = 0.0532) containing B4 chain fragments of trigonal prisms BM6 with the terminating B atoms in cis position. These B4 units were observed for the first time. A structure refinement with single crystal data was done for the τ-boride Cr7.9Ir14.1B6 (Fm3¯m, a = 11.2024(19) Å, 151 refl., 17 param., R1(F) = 0.0275, wR2(F2) = 0.0297). Refinement of single crystal data shows that orthorhombic Cr2B (oF48, Fddd, Z = 16, a = 4.275(2) Å, b = 7.452(3) Å, c = 14.795(5) Å, 214 refl., 17 param., R1(F) = 0.0251, wR2(F2) = 0.0625) belongs to the CuMg2 type and not to the Mn4B type with partial boron occupation. The compositions were confirmed by EDX and WDX measurements.

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