Layer-By-Layer Magnetic Ordering via Idle Spins and the Optical Signature of Jahn–Teller Cr²⁺ Ions in Sr₂Cr(PO₄)₂

Blue/turquoise crystals of Sr₂Cr^II(PO₄)₂ with prismatic shape and edge-length of up to 1 mm were obtained by a vapor-phase moderated solid-state reaction at 1273 K in sealed silica tubes. Its crystal structure was solved and refined from a triply twinned ("trilling") crystal [<i>Pbca</i> (no. 61), <i>Z</i> = 12, <i>a</i> = 10.7064(6) Å, <i>b</i> = 9.2730(5) Å, <i>c</i> = 21.2720(7) Å, <i>R</i>₁ = 0.038]. Sr₂Cr(PO₄)₂ belongs to the small family of inorganic solids containing divalent chromium, where the rare Cr²⁺ ions are stabilized by the inductive effect of the phosphate groups. As expected from its <i>d</i>⁴ (<i>S</i> = 2) electronic configuration, the Jahn-Teller effect (JT) is prominent, leading for the two independent Cr²⁺ ions to square-pyramidal Cr(1)O₄₊₁ and square-planar Cr(2)O₄ coordination within a 3D chromium phosphate network [Cr^II₂(PO₄)₄]⁸. Topologically, the Cr(1) and Cr(2) cations are arranged in separate alternating layers stacked along the <i>c</i> axis. In their respective layers, Cr(1) shows a gapped 2D topology and only weak interaction with the adjacent Cr(2) layers. However, below <i>T</i>_N1 ∼11.3 K, Cr(1) orders antiferromagnetically into a noncollinear structure, leaving nearly paramagnetic Cr(2) idle spins, strongly frustrated by the Cr(1) moments of the next layers. On further cooling, below <i>T</i>_N2 ∼3.6 K, the ordering of Cr(2) occurs via an additional magnetic irreducible representation, which splits the Cr(1) into Cr(1)a and Cr(1)b orbits, thus lifting the frustration on Cr(2). The corresponding <i>P2</i>₁<i>ca.29.99</i> magnetic space group forces a crystal symmetry lowering, plausibly signed by a change of the magnetostrictive coefficient from positive to negative below <i>T</i>_N2. The optical transitions observed for the JT <i>d</i>⁴ ions are in good agreement with our crystal picture from the DFT calculations. A detailed analysis within the angular overlap model explains the surprisingly different <i>d</i> orbital splitting by the ligand field for the chromophores Cr(1)O₄₊₁ and Cr(2)O₄.