PAPER www.rsc.org/dalton | Dalton Transactions Synthesis and characterisation of the quaternary nitride-fluoride Ce 2 MnN 3 F 2-d David A. Headspith, a Eirin Sullivan, b Colin Greaves b and M. Grazia Francesconi* a Received 30th April 2009, Accepted 19th August 2009 First published as an Advance Article on the web 4th September 2009 DOI: 10.1039/b908591b Ce 2 MnN 3 F 2-d has been synthesised via low-temperature fluorination of the ternary nitride Ce 2 MnN 3 . To the best of our knowledge, Ce 2 MnN 3 F 2-d is the first quaternary nitride-fluoride to be reported. The reaction of Ce 2 MnN 3 with 10% F 2 /N 2 at 95–115 ◦ C yields the fluorinated phase Ce 2 MnN 3 F 2-d , with tetragonal symmetry (P4/nmm a = 3.8554(4) A ˚ and c = 13.088(4) A ˚ based on neutron powder diffraction), accompanied by starting material. Rietveld refinement supports a staged fluorine insertion reaction (into alternate rocksalt layers) to give a product with stoichiometry Ce 2 MnN 3 F 2-d .A comparison with the formation of Sr 2 TiO 3 F 2 and the isostructural LaSrMnO 4 F indicates that two F - anions are inserted but no F - /N 3- substitution takes place. Magnetic susceptibility measurements show a transition from Pauli paramagnetic behaviour, in Ce 2 MnN 3 , to paramagnetic behaviour upon fluorination. The effective magnetic moment in Ce 2 MnN 3 F 2-d , m eff = 5.38 m B , is consistent with an intermediate value between that of Mn 3+ (4.9 m B ) and Mn 2+ (5.9 m B ) supporting the proposed stoichiometry, Ce 2 MnN 3 F 2-d . 1. Introduction An important goal of solid-state chemists is the synthesis and characterisation of new compounds followed by chemical manip- ulation to induce valuable physical properties for technological exploitation. Cationic substitutions and high pressure offer two powerful ways to tune physical properties by manipulating the chemical formula of solids; these methods can provide modified structures, mixed oxidation states and, in general, different elec- tronic configurations of metals. Anionic substitutions provide a lesser utilised alternative via complete or partial substitution of anions. Partial substitution leads to mixed-anion compounds, a category of compounds that has received less attention than its single-anion counterparts. However, chemists are now starting to understand the potential of anionic substitutions for the tuning of physical properties of materials. For example, any colour in the range from light yellow to deep red can be tailored by adjustment of the O 2- /N 3- ratio in the perovskite series Ca (1-x) La x TaO (2-x) N (1+x) . 1 In 1994 the first superconducting oxide- fluoride, Sr 2 CuO 2 F 2+d was initially synthesised by Al-Mamouri et al. from the direct fluorination of Sr 2 CuO 3 with fluorine gas at 210 ◦ C. 2 Sr 2 CuO 3 shows an anion deficient La 2 CuO 4 structure, with Cu 2+ cations coordinated by oxide anions in a square-planar fashion. The Cu-O squares join corners to form one-dimensional chains along the b axis. The fluorination process causes the substitution of one oxide anion with two fluoride anions and the consequent expansion of the copper coordination from square planar to octahedral and filling of the anion vacancies, giving rise to a La 2 CuO 4 -structure. Extra fluoride is also inserted between the Sr-O layers of the structure, causing an imbalance in negative charge, which forces partial oxidation of the copper cation and a The Chemistry Department, The University of Hull, Hull, England. E-mail: M.G.Francesconi@hull.ac.uk b School of Chemistry, University of Birmingham, Birmingham, England induces superconductivity for a range of fluorine contents. This insertion occurs in a similar way to fluorination of La 2 CuO 4 to give La 2 CuO 4 F d . 3 Fluorination of other oxides isostructural with Sr 2 CuO 3 , in par- ticular Ca 2 CuO 3 4 and Ba 2-x Sr x PdO 3 (0 ≤ x ≤ 1), 5–7 produced a simi- lar structural rearrangement: the formation of the oxide-fluorides, Ca 2 CuO 2 F 2+d and Ba 2-x Sr x PdO 2 F 2 , occurs as a consequence of sub- stitution of one oxide with two fluoride anions. However, for these oxides there is no change in the copper/palladium coordination, which remains square planar, because of size effects and cationic preference. Whereas Sr 2 CuO 2 F 2+d has an orthorhombic La 2 CuO 4 - type unit cell, Ca 2 CuO 2 F 2+d and Ba 2-x Sr x PdO 2 F 2 have tetragonal structures derived from a Nd 2 CuO 4 -type unit cell. Only a small number of ternary nitrides are isostructural with Sr 2 CuO 3 . Early examples, from the radioactive metals thorium and uranium, Th 2 MN 3 and U 2 MN 3 (M = Mn, Cr), were prepared over 30 years ago and found to be very similar in structure, with an orthorhombic anion deficient La 2 CuO 4 -type structure. 8 Cerium has been investigated as an alternative to radioactive uranium, and Niewa et al. showed that Ce 2 MnN 3 is isostructural to U 2 MN 3 . 9,10 The structure of Ce 2 MnN 3 comprises corner linked chains of MnN 4 units along [100], with Mn in approximately square planar geometry. 10 These units resemble the CuO 4 squares in Sr 2 CuO 3 . The bonds between nitride anions and manganese differ slightly, altering the square shape. In particular the Mn-N bridging bonds are shorter than the Mn-N terminal bonds. The structure is described as an anion deficient T-type structure, isostructural to Sr 2 CuO 3 , where Ce replaces Sr and Mn substitutes for Cu. The coordination environment of cerium is considered by Landrum et al. to include seven other cerium cations, seven nitride anions and four manganese cations. 11 The two crystallographically different nitrogen anions are coordinated in a distorted octahedral fashion, one by four cerium and two manganese cations, the other by five cerium and one manganese cations. 10 This journal is © The Royal Society of Chemistry 2009 Dalton Trans., 2009, 9273–9279 | 9273 Published on 04 September 2009. Downloaded by University of Hull on 28/08/2014 15:59:11. View Article Online / Journal Homepage / Table of Contents for this issue