Coupled spin-lattice transition in U₂Rh₃Si₅: Thermal expansions and magnetization

We have measured the thermal expansion, Δl_i/l_i, and high-field magnetization, M_i, along the three quasiorthorhombic axes of U₂Rh₃Si₅. There are discontinuities in Δl_i(T)/l_i at T₀ = 25.7 K of opposite sign in the a^′ and b directions while the c axis changes steplike, which tracks a large volume change. The magnetization along the b axis, M_b(H), exhibits a dramatic temperature-dependent metamagnetic transition where the induced U moments switch from 0.1 to 1.7μ_B, while the a^′ and c axes remain unperturbed. We relate these anomalies to a first-order phase transition of strongly coupled spin and quadrupole orderings.