Multistage symmetry breaking in the breathing pyrochlore latticeLi(Ga,In)Cr4O8

Abstract

We present magnetic susceptibility, dielectric constant, high-frequency electron spin resonance, $^{7}\mathrm{Li}$ nuclear magnetic resonance, and zero-field muon spin relaxation measurements of ${\mathrm{LiACr}}_{4}{\mathrm{O}}_{8}$ ($\mathrm{A}=\mathrm{Ga}$, In), towards realizing a breathing pyrochlore lattice. Unlike the uniform pyrochlore ${\mathrm{ZnCr}}_{2}{\mathrm{O}}_{4}$ lattice, both the In and the Ga compounds feature two-stage symmetry breaking: a magnetostructural phase transition with subsequent antiferromagnetic ordering. We find a disparate symmetry breaking process between the In and the Ga compounds, having different degrees of bond alternation. Our data reveal that the Ga compound with moderate bond alternation shows the concomitant structural and magnetic transition at ${T}_{S}=15.2$ K, followed by the magnetic ordering at ${T}_{m}=12.9$ K. In contrast, the In compound with strong bond alternation undergoes a thermal crossover at ${T}^{*}\ensuremath{\approx}20.1$ K from a tetramer singlet to a dimer singlet or a correlated paramagnet with a separate weak magnetostructural transition at ${T}_{S}=17.6$ K and the second antiferromagnetic ordering at ${T}_{m}=13.7$ K. This suggests that the magnetic phases and correlations of the breathing pyrochlore lattice can be determined from the competition between bond alternation and spin-lattice coupling, thus stabilizing long-range magnetic ordering against a nonmagnetic singlet.