The phase transition of CrOCl toward a state of antiferromagnetic order below T N = 13.5 K has been identified as a first-order phase transition. The transition is accompanied by a lattice and structural distortion toward a twofold, 2b nuclear superstructure with a-axis unique monoclinic symmetry, as evidenced by temperature-dependent x-ray diffraction experiments. Magnetic-susceptibility and magnetization measurements indicate a transition with strong magnetoelastic coupling to a uniaxial antiferromagnet with ordered moments along c. A second transition is discovered at T c Ϸ 27.2 K that is presumably of purely magnetic origin and might indicate the formation of an incommensurate magnetic superstructure. The different behaviors of TiOCl, VOCl, and CrOCl are the result of the different symmetries of the filled 3d orbitals, which lead to different exchange interactions on the MO double layers of these isostructural compounds.

VOCl develops magnetic order below T N = 80.3͑3͒ K. Employing single-crystal x-ray diffraction and powder neutron diffraction it is shown that the magnetic phase transition is accompanied by a small, temperaturedependent monoclinic lattice distortion with ␥ = 90.212͑2͒°at T = 3.2 K. The phase transition presumably is of second order with a critical exponent ␤ = 0.099͑12͒. The monoclinic distortion lifts the frustration of interchain interactions along 2 b͒ can explain the observed magnetic superstructure with magnetic space group F2Ј / dЈ on the 2a ϫ 2b ϫ 2c supercell ͑standard setting C2Ј / cЈ͒ and with magnetic moments parallel to a. Remarkably, the lattice distortion is not accompanied by a structural distortion so that the driving force for the lattice distortion is completely governed by the magnetic interactions.

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Structure D 2000Low-and High-Temperature Crystal Structures of TiI3. -The title compound is characterized by single crystal XRD at different temperatures. At 326 K TiI 3 crystallizes in the hexagonal space group P63/mcm with Z = 2 (TiI3-type structure). At 100 and 273 K it crystallizes in the orthorhombic space group Pmnm with Z = 4 (RuBr3-type structure). The high-temperature structure contains chains of equidistant Ti atoms. A twofold superstructure develops below 323 K, resulting in the low-temperature structure that is characterized by a dimerization of the metal chains. -(ANGELKORT, J.; SCHOENLEBER, A.; VAN SMAALEN*, S.; J. Solid State Chem. 182 (2009) 3, 525-531; Lab. Crystallogr., Univ. Bayreuth, D-95440 Bayreuth, Germany; Eng.) -W. Pewestorf 25-002

High‐temperature in situ synchrotron X‐ray powder diffraction experiments were carried out to investigate the phase transition mechanism of Zr2P2O9 (2ZrO2·P2O5 or “Z2P”). Linear thermal expansion coefficients were calculated for the low‐temperature phase (α‐Z2P) and the high‐temperature phase (β‐Z2P) from temperature‐dependent changes in lattice parameters. The crystal structures of α‐ and β‐Z2P were determined as a function of temperature by performing Rietveld crystal structure refinements. The structural changes at the phase transition are accompanied by an increase in the average atomic distance between neighboring Zr atoms. The occurrence of shorter metal–metal distances in α‐Z2P is interpreted to result from stronger interactions between partly occupied valence orbitals of the d0 metal atoms. The bond valence method was used to calculate the valence sums of the atoms of α‐ and β‐Z2P, respectively, considering also contributions resulting from covalently bonded atoms. As the bond strength between the metal atoms in Z2P decreases with the transition into the high‐temperature phase, notably, the metal–metal interactions are regarded to constitute a prerequisite for the stabilization of the α‐phase over the energetically favored β‐phase.