  Metal halide perovskites have shown great performance as solar energy
materials, but their outstanding optoelectronic properties are paired with
unusually strong anharmonic effects. It has been proposed that this intriguing
combination of properties derives from the "lone pair" 6$s^2$ electron
configuration of the Pb$^{2+}$ cations, and associated weak pseudo-Jahn-Teller
effect, but the precise impact of this chemical feature remains unclear. Here
we show that in fact an $ns^2$ electron configuration is not a prerequisite for
the strong anharmonicity and low-energy lattice dynamics encountered in this
class of materials. We combine X-ray diffraction, infrared and Raman
spectroscopies, and first-principles molecular dynamics calculations to
directly contrast the lattice dynamics of CsSrBr$_3$ with those of CsPbBr$_3$,
two compounds which bear close structural similarity but with the former
lacking the propensity to form lone pairs on the 5$s^0$ octahedral cation. We
exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden
in the cubic phase, as a fingerprint to detect anharmonicity and reveal that
low-frequency tilting occurs irrespective of octahedral cation electron
configuration. This work highlights the key role of structure in perovskite
lattice dynamics, providing important design rules for the emerging class of
soft perovskite semiconductors for optoelectronic and light-harvesting devices.
