Giant negative thermal expansion in bonded MnCoGe-based compounds with Ni2In-type hexagonal structure.

MnCoGe-based compounds undergo a giant negative thermal expansion (NTE) during the martensitic structural transition from Ni2In-type hexagonal to TiNiSi-type orthorhombic structure. High-resolution neutron diffraction experiments revealed that the expansion of unit cell volume can be as large as ΔV/V ∼ 3.9%. The optimized compositions with concurrent magnetic and structural transitions have been studied for magnetocaloric effect. However, these materials have not been considered as NTE materials partially due to the limited temperature window of phase transition. The as-prepared MnCoGe-based compounds are quite brittle and naturally collapse into powders. By using a few percents (3-4%) of epoxy to bond the powders, we introduced residual stress in the bonded samples and thus realized the broadening of structural transition by utilizing the specific characteristics of lattice softening enforced by the stress. As a result, giant NTE (not only the linear NTE coefficient α but also the operation-temperature window) has been achieved. For example, the average α̅ as much as -51.5 × 10(-6)/K with an operating temperature window as wide as 210 K from 122 to 332 K has been observed in a bonded MnCo0.98Cr0.02Ge compound. Moreover, in the region between 250 and 305 K near room temperature, the α value (-119 × 10(-6)/K) remains nearly independent of temperature. Such an excellent performance exceeds that of most other materials reported previously, suggesting it can potentially be used as a NTE material, particularly for compensating the materials with large positive thermal expansions.