Layer-By-Layer Magnetic Ordering via Idle Spins and the Optical Signature of Jahn-Teller Cr Ions in SrCr(PO).

Blue/turquoise crystals of SrCr(PO) 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 [ (no. 61), = 12, = 10.7064(6) Å, = 9.2730(5) Å, = 21.2720(7) Å, = 0.038]. SrCr(PO) belongs to the small family of inorganic solids containing divalent chromium, where the rare Cr ions are stabilized by the inductive effect of the phosphate groups. As expected from its ( = 2) electronic configuration, the Jahn-Teller effect (JT) is prominent, leading for the two independent Cr ions to square-pyramidal Cr(1)O and square-planar Cr(2)O coordination within a 3D chromium phosphate network [Cr(PO)]. Topologically, the Cr(1) and Cr(2) cations are arranged in separate alternating layers stacked along the axis. In their respective layers, Cr(1) shows a gapped 2D topology and only weak interaction with the adjacent Cr(2) layers. However, below ∼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 ∼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 magnetic space group forces a crystal symmetry lowering, plausibly signed by a change of the magnetostrictive coefficient from positive to negative below . The optical transitions observed for the JT 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 orbital splitting by the ligand field for the chromophores Cr(1)O and Cr(2)O.