Elastic and thermal properties of W7Re13B and synthesis of a new ternary phase W1.3Re2.7B2

Abstract

We synthesized phase pure samples of W7Re13B via arc-melting and subsequent annealing and a new compound W1.3(2)Re2.7(2)B2, which crystallizes in the Mg2Cu structure type. Based on pressure-dependent synchrotron X-ray diffraction (XRD), the bulk modulus of W7Re13B is B0,X−ray = 333(6) GPa. Resonant ultrasound spectroscopy was used to determine the elastic stiffness coefficients c11 = 507(3) GPa and c12 = 283(2) GPa and to derive B0,RUS = 358(2) GPa, c44 = 112(2) GPa, Young's modulus E = 672(7) GPa, shear modulus G = 112(2) GPa, a Poisson ration ν = 0.19 and a Debye temperature θD,elastic = 182(2) K. The linear coefficient of thermal expansion is α93K−773K = 5.5(2) 10−6 K−1 and was determined by dilatometry. Microhardness measurements at varying loads were employed to obtain the hardness, which is HV (W7Re13B) ≈ 19 GPa at high loads. Microcalorimetry gave Cp,298 (W7Re13B) = 517(10) J/mol, ΔH2980 (W7Re13B) = 109(2) kJ/mol, S2980 (W7Re13B) = 766(8) J/mol K and θD,298 (W7Re13B) = 259(3) K. The structure of W1.3Re2.7B2 was solved and refined by single-crystal X-ray diffraction. W1.3Re2.7B2 crystallizes in the orthorhombic space group Fddd with a = 4.5286(3) Å, b = 7.9660(5) Å, c = 15.632(2) Å and Z = 8. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were employed to constrain the chemical composition. Microcalorimetry gave Cp,298 (W1.3Re2.7B2) = 132(1) J/mol, ΔH2980 (W1.3Re2.7B2) = 24.74(20) kJ/mol, S2980 (W1.3Re2.7B2) = 160(2) J/mol K and θD,298 (W1.3Re2.7B2) = 317(3) K. The bulk modulus of this phase from pressure-dependent diffraction studies up to 42 GPa is B0,X−ray (W1.3Re2.7B2) = 335(5) GPa. The data obtained here were used to evaluate models for predicting the hardness of materials, and we show that, for the compounds studied here, often used approaches are unsatisfactory.