Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Anharmonicity and scissoring modes in the negative thermal expansion
  materials ScF$_3$ and CaZrF$_6$   We use a symmetry-motivated approach to analysing X-ray pair distribution
functions to study the mechanism of negative thermal expansion in two
ReO$_3$-like compounds; ScF$_3$ and CaZrF$_6$. Both average and local structure
suggest that it is the flexibility of M-F-M linkages (M = Ca, Zr, Sc) due to
dynamic rigid and semi-rigid "scissoring" modes that facilitates the observed
NTE behaviour. The amplitudes of these dynamic distortions are greater for
CaZrF$_6$ than for ScF$_3$, which corresponds well with the larger magnitude of
the thermal expansion reported in the literature for the former. We show that
this flexbility is enhanced in CaZrF$_6$ due to the rock-salt ordering mixing
the characters of two of these scissoring modes. Additionally, we show that in
ScF$_3$ anharmonic coupling between the modes responsible for the structural
flexibility and the rigid unit modes contributes to the unusually high NTE
behaviour in this material.

32
The categories of this paper are: atom-ph
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
15
The categories of this paper are: nucl-ex
31
The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
36
The categories of this paper are: cmp-lg
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
19
The categories of this paper are: eess
11
The categories of this paper are: hep-lat