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 Sr2CrII(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 [Pbca (no. 61), Z = 12, a = 10.7064(6) Å, b = 9.2730(5) Å, c = 21.2720(7) Å, R 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 d 4 (S = 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 [CrII 2(PO4)4]8. Topologically, the Cr(1) and Cr(2) cations are arranged in separate alternating layers stacked along the c axis. In their respective layers, Cr(1) shows a gapped 2D topology and only weak interaction with the adjacent Cr(2) layers. However, below T 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 T 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 P2 1 ca.29.99 magnetic space group forces a crystal symmetry lowering, plausibly signed by a change of the magnetostrictive coefficient from positive to negative below T N2. The optical transitions observed for the JT d 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 d orbital splitting by the ligand field for the chromophores Cr(1)­O4+1 and Cr(2)­O4.