Abstract
Below \ensuremath{\sim}60 K ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}\mathrm{Cu}{\mathrm{O}}_{4} (0.10\ensuremath{\le}x\ensuremath{\le}0.15)$ reveals superstructure reflections consistent with the tetragonal space group $\frac{P{4}_{2}}{\mathrm{ncm}}$ [low temperature tetragonal (LTT)]. However, twins, characteristic of the related orthorhombic phase [low temperature orthorhombic (LTO)], persist in this material. Electron microscopy shows that the LTT superstructure reflections originate from the twin-boundary region. This suggests a phase mixture of orthorhombic and tetragonal with a gradual change from LTO (space group $\mathrm{Bmab}$) in the interior of the twin domains, via the $\mathrm{Pccn}$, to LTT at the twin boundary. This is modeled as a spatial modulation, the tipping angle of Cu${\mathrm{O}}_{6}$ octahedra being an order parameter.
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