Phase diagram and Eu valence state in EuPtP_1-xAs_x

EuPtP is known to exhibit two valence ordering transitions, one at T ₁ = 235 K and the other at T ₂ = 190 K. For T > T ₁, the Eu ions are in the valence fluctuating state. For T ₂ < T < T ₁, the Eu valence state is ordered along the c-axis with the sequence of Eu²⁺-Eu²⁺-Eu³⁺-Eu²⁺-Eu²⁺-Eu³⁺. Moreover, the stacking is replaced by another sequence of Eu²⁺-Eu³⁺-Eu²⁺-Eu³⁺ at temperatures below T ₂. The substitution of As for P expands the lattice and decreases both T ₁ and T ₂. In this paper, we report the magnetic susceptibility and electrical resistivity of EuPtP_1-xAs_x single crystals. We observed two valence ordering transitions for 0 ≤ x ≤ 0.4, while a single valence ordering transition was detected for 0.55 ≤ x ≤ 0.6. No valence ordering transitions were observed for x = 0.7 down to the lowest temperature. Based on the experimental results, we present the phase diagram of EuPtP_1-xAs_x and discuss the Eu valence state in this system.