Discovery of Biaxial Negative Thermal Expansion in KNbSi2O7 from Computations and Experiments

High Resolution Image Download MS PowerPoint Slide Framework-type negative-thermal expansion (NTE) materials are often composed of corner-sharing octahedra and tetrahedra. Considering the insertion of K into a framework structure, aiming at increasing the average atomic volume (AAV), we have focused on KNbSi 2 O 7 (KNSO). The thermal expansion behavior of KNSO has been predicted by molecular-dynamics (MD) calculations using machine-learning force field (MLFF). We have discovered that the a and b axes of KNSO show NTE behavior, while the c axis possesses positive thermal expansion (PTE) behavior through its synthesis and temperature-dependent X-ray diffraction (XRD) patterns, which are consistent with the MD calculation results. We present that the thermal expansion behavior of KNSO cannot be explained only by the anisotropic negative Grüneisen parameters based on the quasi-harmonic approximation (QHA). Our results imply that (i) anisotropic thermal expansion behaviors can be reproduced by the MD calculations and (ii) the second-order Jahn–Teller (SOJT) effect elongates the c -axis lattice constant, leading to the increase of lattice flexibility that enhances NTE behavior. The present results stimulate the exploration of NTE materials from first-principles calculations combined with MLFF.