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We describe the synthesis and the crystallographic and magnetic properties of a novel NaCoCr2(PO4)3 phosphate. A conventional solid-state reaction was used to obtain single-phase powders. A Rietveld analysis of powder X-ray diffraction data proposes an orthorhombic symmetry similar to α-CrPO4-type structure in space group Imma with the following unit cell parameters: a = 10.413(1) Å; b = 13.027(1) Å; c = 6.372(1) Å. The framework consists of PO4 tetrahedra, M(1)O6 (M(1) = Cr) octahedra, and M(2)2O10 (M(2) = 0.5Cr+0.5Co) binuclear unit of edge-sharing MO6 octahedra. It can be described in terms of two building blocks: sheets consisting of corner-sharing M(2)2O10 units with PO4 tetrahedra found parallel to the (b,c) plane, and chains made by corner-sharing CrO6 octahedra and PO4 tetrahedra running along the b axis. From the interconnection of the sheets and chains, a 3D rigid skeleton is formed, exhibiting two kinds of intersecting tunnel channels containing the Na(+) ions. The proposed structure derives from the α-CrPO4-type structure considering a positive charge balance according to the equation Cr(3+) → Co(2+) + Na(+), resulting in sodium countercation introduction within the unoccupied channels shown in the α-CrPO4 framework. Temperature-dependent DC and AC magnetic susceptibility is indicative of a long-range magnetic ordering occurring at 32 K. Further, spin-flop transition sheds light on a chromium-based phosphate for the first time.

An electron and joint neutron and X-ray diffraction study of the synthetic copper/chromium phosphate NaCuCr2(PO4)3 (NaCuP) is reported. A noncentrosymmetric Imm2 space group belonging to the well-known α-CrPO4 type is observed contrary to what is reported in NaMCr2(PO4)3 (M = Co and Ni) phosphates. The structural model is validated by bond valence sum analysis and charge-distribution (CHARDI) calculations and supported by complementary infrared and Raman spectroscopy investigations. Both Raman spectroscopy and theoretical study by deformation density approach further suggest the presence of Cu2+ (3d9) and Cr2+ (3d4) Jahn–Teller polaron effects as a key factor to the centro Imma to noncentrosymmetric Imm2 phase change.

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Synthesis and Characterization of the Phosphates Na 1+xMg1+xCr2-x(PO4)3 (x = 0; 0.2) and NaZnCr 2(PO4)3 with the -CrPO4 Structure. -Single crystals of Na 1.2Mg1.2Cr1.8(PO4)3 are prepared by drying (353 K, 24 h) a 6N HNO3 solution of Na2CO3, Mg(NO3)2, Cr(NO3)3, (NH4)H2PO4, and MoO3 followed by a complex heating schedule (the formed Na 2Mo2O7 is used as a flux in 3-fold molar excess; Pt crucible, 473-873 K, 24 h/stage; 1523 K, 1 h). Polycrystalline NaMgCr 2(PO4)3 and Na 1.2Mg1.2Cr1.8(PO4)3 are prepared by calcination in air of stoichiometric amounts of Na2CO3, metal nitrates, and (NH4)H2PO4 (several stages  1473 K, 24 h/stage). All compounds crystallize in the orthorhombic system (space group Imma, Z = 4, powder and single crystal XRD). The structures are closely related to the -CrPO 4 type with disordered Cr 3+ and M 2+ (M: Mg, Zn) cations. The 3D framework is built up by PO4 tetrahedra, M(1)O 6 octahedra and M(2)2O10 edge-shared double-octahedra linked through common corners and edges with Na located within tunnels. Results of EPR and magnetic susceptibility measurements indicate antiferromagnetic coupling between nearest neighbor Cr ions. At high temperature, a deviation from the Curie-Weiss law is observed which is attributed to magnetic frustration. -(SOUIWA, K.; HIDOURI*, M.; TOULEMONDE, O.; DUTTINE, M.; AMARA, M. B.; J. Alloys Compd. 627 (2015) 153-160, http://dx.

The phosphate KCoCr(PO4)2 and iron-substituted variants KCoCr1–x Fe x (PO4)2 (x = 0.25, 0.5, and 0.75) were synthesized by a solid-state reaction route, while a high substitution level of Fe was achieved. Their structures were refined using powder X-ray diffraction and indexed in a monoclinic system with a P21/n space group. A 3D framework with six-sided tunnels parallel to the [101] direction was formed in which the K atoms are located. Mössbauer spectroscopy confirms the exclusive presence of octahedral paramagnetic Fe3+ ions, with isomer shifts increasing slightly with x substitution. Electron paramagnetic resonance spectroscopy confirmed the presence of paramagnetic Cr3+ ions. The activation energy, determined by dielectric measurements, shows that the iron-containing samples present higher ionic activity. Relative to the electrochemical activity of K, these materials could be good candidates for positive and/or negative electrode materials for energy storage applications.

Transition. -The new title compound is prepared by solid state reaction of a stoichiometric mixture of Na 2CO3, Co(NO3)2, Cr(NO3)3, and (NH4)H2PO4 (Pt crucible, 1423 K, 48 h). NaCoCr2(PO4)3 is characterized by powder XRD, and magnetic and specific heat measurements. It crystallizes in the orthorhombic space group Imma with Z = 4. The framework consists of PO 4 tetrahedra, CrO6 octahedra, and (0.5Cr + 0.5Co)2O10 binuclear units of edge-sharing (0.5Cr + 0.5Co)O 6 octahedra. Na + ions are located within intersecting tunnels delimited by the framework. The compound shows a magnetic phase transition from para-to antiferromagnetic behavior at about 32 K. A spin-flop transition is likely due to the competition within the (0.5Cr + 0.5Co)2O10 bioctahedral units between Co 2+ -O(2p)-Cr 3+ ferromagnetic 90 superexchange interactions and Cr 3+ -O(2p)-Cr 3+ antiferromagnetic 90 superexchange interactions, and mediated by direct exchange. -(SOUIWA, K.; CHENNABASAPPA, M.; DECOURT, R.; BEN AMARA, M.; HIDOURI*, M.; TOULEMONDE, O.; Inorg. Chem. 54 (2015) 15, 7345-7352, http://dx.doi.org/10.1021/acs.inorgchem.5b00776 ; Fac. Sci., Univ. Monastir, 5019 Monastir, Tunisia; Eng.) -W. Pewestorf 41-012