The contribution of phonons to the thermal expansion of some simple cubic hexaboride structures: SmB₆, CaB₆, SrB₆ and BaB₆

Abstract

We have performed first-principles calculations of the structure and lattice dynamics in the metal hexaborides SmB6, CaB6, SrB6 and BaB6 using Density Functional Theory in an attempt to un- derstand...

Electronic origin of negative thermal expansion in samarium hexaboride revealed by X-ray diffraction and total scattering

“…There is no evidence from the comparison of PDF and diffraction data for a significant tension effect 25 of the type seen in other systems, particular as recently evaluated in ScF 3 . 28,29 This is consistent with the lattice dynamics calculations 3 and with the effect seen in other systems. [34][35][36] We emphasise the point by comparing, in Fig.…”

“…6, separating the data in the NTE region from the data in the PTE region, with our results giving a wider range of data in the NTE region. For comparison with results from our recent DFT study, 3 Fig. 3 from that paper is reproduced in Fig.…”

“…We recently explored this by calculating the lattice dynamics of SmB 6 and some alkali-earth hexaborides using Density Functional Theory (DFT). 3 We found that the RUMs are actually of relatively high frequency, and that too few phonons will support NTE. Thus we concluded that NTE in SmB 6 does not arise from a phonon-based tension-effect mechanism.…”

“…We had previously anticipated, but ruled out, the possibility of a phonon mechanism for NTE in SmB 6 , based on the existence of RUMs providing a tension effect. 3 Given the lack of a magnetic ordering in SmB 6 , 33 the mostly likely origin of NTE in SmB 6 is an electronic effect. This is clearly possible given the evidence of such an effect leading to NTE in other systems containing samarium.…”

“…Secondly, the weighting of the peaks in the PDF may be too weak for some pairs of atoms, as found in this study. As discussed above with the example of ScF 3 , we would expect to see differences in the distances between average positions of neighbouring atoms and their mean instantaneous separation if the NTE in SmB 6 arises from a tension effect mechanism. Our recent study of the metal hexaborides using DFT methods 3 showed that increasing the size of the metal atom leads to increases in the sizes of the B 6 octahedra and of the length of the inter-octahedral B–B bonds. In this paper, we have extracted relevant interatomic distances from analysis of our diffraction data and from the positions of peaks in the PDF (see comments below regarding the B–B distances in the PDF).…”