Soft-phonon behavior and transport in single-crystal<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">La</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuO</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Using a flux technique we have grown sizable single crystals of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$. With x rays and neutrons we have studied both the static and dynamic aspects of the tetragonal to orthorhombic structural phase transition; classic soft-phonon behavior is observed at the ((1/2, 1) / 2 , 0) zone boundary involving rotations of ${\mathrm{CuO}}_{6}$ octahedra. The pure and lightly-doped single crystals show hopping conductivity, ln\ensuremath{\rho}\ensuremath{\sim}(${T}_{0}$/T${)}^{1/4}$, indicating that the electronic states at the Fermi energy are localized.