A Raman and far‐infrared investigation of phonons in the rhombohedral V<sub>2</sub>–VI<sub>3</sub> compounds Bi<sub>2</sub>Te<sub>3</sub>, Bi<sub>2</sub>Se<sub>3</sub>, Sb<sub>2</sub>Te<sub>3</sub> and Bi<sub>2</sub>(Te<sub>1−<i>x</i></sub>Se<sub><i>x</i></sub>)<sub>3</sub> (0 < <i>x</i> < 1), (Bi<sub>1−<i>y</i></sub>Sb<sub><i>y</i></sub>)<sub>2</sub>Te<sub>3</sub> (0 < <i>y</i> < 1)
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