  Single-crystal elastic moduli, Cij, and the B1-B2 phase transition of NaCl
were investigated experimentally, using time-domain Brillouin scattering
(TDBS), and theoretically, via density-functional-theory (DFT), to 41 GPa.
Thus, we largely extended pressure range where Cij and elastic anisotropy of
the solid are measured, including the first experimental data for the
high-pressure B2 phase, NaCl-B2. NaCl-B1 exhibits a strong and growing with
pressure anisotropy, in contrast to NaCl-B2. Theoretical values obtained using
different advanced DFT functionals were compared with our measurements but no
one could satisfactorily reproduce our experimental data for NaCl-B1 and
NaCl-B2 simultaneously. For all available DFT results on the principal shear
moduli and anisotropy, the deviation became pronounced when the degree of
compression increased significantly. Similar deviations could be also
recognized for other cubic solids having the same B1-type structure and similar
bonding, such as CaO, MgO, or (Mg1-x,Fex)O. Furthermore, the available
experimental data suggest that the B1-B2 phase transition of NaCl and the above
mentioned compounds are governed by the Born stability criterion C44(P) - P >
0.
