${\mathrm{CuGaCr}}_4{\mathrm{S}}_8$, which contains a chromium breathing pyrochlore network, exhibits diverse magnetic phases, including an incommensurate helical state below 31 K and a $\frac{1}{2}$-magnetization plateau above 40 T, owing to the interplay between magnetic frustration and spin-lattice coupling. Here, we perform a single-shot powder x-ray diffraction experiment on ${\mathrm{CuGaCr}}_4{\mathrm{S}}_8$ in a pulsed high magnetic field of 55 T, revealing an orthorhombic-to-cubic (or pseudocubic) structural transition upon entering the $\frac{1}{2}$-magnetization plateau phase at low temperatures. This observation suggests the emergence of a commensurate ferrimagnetic order, where a 3-up–1-down spin configuration is realized in each small tetrahedron, and the all up or all down in each large tetrahedron. We propose two types of 16-sublattice magnetic structures, which are degenerate within exchange interactions between the first-, second-, and third-nearest neighbors.

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• High magnetic fields
• Magnetic phase transitions
• Antiferromagnets
• X-ray diffraction