The effect of ionic composition on acoustic phonon speeds in hybrid perovskites from Brillouin spectroscopy and density functional theory - Kabakova, IV; Azuri, I; Chen, Z; Nayak, PK; Snaith, HJ; Kronik, L; Paterson, C; Bakulin, AA; Egger, DA (JOURNAL OF MATERIALS CHEMISTRY C, 2018)