The oxygen-deficient double perovskite YBaCo 2 O 5 , containing corner-linked CoO 5 square pyramids as principal building units, undergoes a paramagnetic to antiferromagnetic spin ordering at 330 K. This is accompanied by a tetragonal to orthorhombic distortion. Below 220 K orbital ordering and long-range Co 21 ͞Co 31 charge ordering occur as well as a change in the Co 21 spin state from low to high spin. This transition is shown to be very sensitive to the oxygen content of the sample. To our knowledge this is the first observation of a spin-state transition induced by long-range orbital and charge ordering. We report here on structurally related LBaCo 2 O 51x materials, whose structures are derived from perovskites via ordering of the rare earth ͑L͒ and Ba cations into layers along c and removing oxygen exclusively from the L layer [6,7]. This creates an apically connected double layer of corner-sharing CoO 5 pyramids. For x . 0 the extra oxygen ions are incorporated into the L layer of LBaCo 2 O 51x to form disordered octahedra along the c axis. We have synthesized LBaCo 2 O 5.00 and studied the thermal evolution of its structure and properties using synchrotron x-ray [8] (Fig. 1) and the appearance of magnetic superstructure reflections in neutron powder diffraction data. Synchrotron x-ray powder diffraction measurements show that this magnetic transition occurs simultaneously with a tetragonal-to-orthorhombic ͑T -O͒ structural phase transition. Furthermore, as mentioned above, the minority tetragonal phase persists below the T -O transition. The two-phase coexistence was confirmed by ultra-highresolution diffraction measurements using a crystal analyzer. We stress that the observation of two phases in synchrotron x-ray powder diffraction experiments, with an order of magnitude better resolution than available with a standard laboratory x-ray diffractometer, can be traced to minute oxygen-content variations in the order of magnitude around x 0.01 [11]. In the presence of strong lattice coupling these small compositional variations can lead to phase coexistence, which can be observed in high-resolution experiments. This behavior beautifully illustrates how sensitive phase transitions within these systems are to minute variations in the oxygen stoichiometry.At room temperature there is no evidence for any long-range charge ordering. However, upon cooling long-range charge ordering is detected below 200 K, by the appearance of the ͑

Sm, and Y 0.5 Sm 0.5) have been prepared, and structure determinations have been carried out at room temperature using high-resolution synchrotron X-ray powder diffraction data. The octahedral tilting distortion increases as the average ionic radius of the Ln/A cations, 〈r A 〉, decreases. The two crystallographically distinct Mn-O-Mn bonds [Mn-O(eq)-Mn and Mn-O(ax)-Mn] are almost identical for Ln 0.5 Ca 0.5 MnO 3 compounds, with the exception of La 0.5 Ca 0.5 MnO 3. The La 0.5 Ca 0.5 MnO 3 compound and the entire Ln 0.5 Sr 0.5 MnO 3 series adopt structures where the Mn-O(eq)-Mn bond angle is consistently and significantly larger (2-6°) than the Mn-O(ax)-Mn bond angle. All of the Ln 0.5 Ca 0.5 MnO 3 compounds have Pnma symmetry, whereas across the Ln 0.5 Sr 0.5 MnO 3 series with increasing 〈r A 〉, an evolution from Pnma (tilt system ab + a-) over Imma (tilt system ab 0 a-) to I4/mcm (tilt system a 0 a 0 c-) symmetry is observed. It appears that the latter two tilt systems are stabilized with respect to the rhombohedral (R3 hc) aaatilt system, by short-range layered ordering of A-site cations. Changes in the octahedral tilt system at room temperature are linked to changes in the low-temperature magnetic structure. In particular, the simultaneous onset of charge ordering and CE-type antiferromagnetism in the Ln 0.5 Sr 0.5 MnO 3 series appears to be closely associated with the Imma structure. The average Mn-O bond distance is relatively constant across the entire series, but individual Mn-O bond distances show the presence of a cooperative Jahn-Teller effect (orbital ordering) at room temperature in Sm 0.5 Ca 0.5 MnO 3 and Sm 0.25 Y 0.25 Ca 0.5 MnO 3 .

A generic definition of oxidation state (OS) is formulated: "The OS of a bonded atom equals its charge after ionic approximation". In the ionic approximation, the atom that contributes more to the bonding molecular orbital (MO) becomes negative. This sign can also be estimated by comparing Allen electronegativities of the two bonded atoms, but this simplification carries an exception when the more electronegative atom is bonded as a Lewis acid. Two principal algorithms are outlined for OS determination of an atom in a compound; one based on composition, the other on topology. Both provide the same generic OS because both the ionic approximation and structural formula obey rules of stable electron configurations. A sufficiently simple empirical formula yields OS via the algorithm of direct ionic approximation (DIA) by these rules. The topological algorithm works on a Lewis formula (for a molecule) or a bond graph (for an extended solid) and has two variants. One assigns bonding electrons to more electronegative bond partners, the other sums an atom's formal charge with bond orders (or bond valences) of sign defined by the ionic approximation of each particular bond at the atom. A glossary of terms and auxiliary rules needed for determination of OS are provided, illustrated with examples, and the origins of ambiguous OS values are pointed out. An electrochemical OS is suggested with a nominal value equal to the average OS for atoms of the same element in a moiety that is charged or otherwise electrochemically relevant.

YBaFe(2)O(5) has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe(2)O(5) adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO(5) square pyramids separated by Y(3+) ions. At T(N) congruent with 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe(2)O(5) orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions. The magnetic structure is characterized by AFM Fe-O-Fe superexchange coupling within the double layers and a ferromagnetic Fe-Fe direct-exchange coupling between neighboring double layers. Upon cooling below approximately 335 K, a premonitory charge ordering (2Fe(2.5+) --> Fe(2.5+delta) + Fe(2.5)(-delta)) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At approximately 308 K, a complete charge ordering (2Fe(2.5+) --> Fe(2+) + Fe(3+)) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe(2)O(5). The magnetic structure rearranges to a G-type AFM structure, where both the Fe-O-Fe superexchange and the Fe-Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe(2+) and Fe(3+) running along b and an unexpectedly large cooperative Jahn-Teller distortion about the high-spin Fe(2+) ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe(2+) d(xz) orbitals. Comparisons with YBaMn(2)O(5) and YBaCo(2)O(5) are made to highlight the impact of changing the d-electron count.

Electronic, magnetic, and structural phase transitions in nearly stoichiometric TbBaFe 2 O 5ϩw (0.00Ͻw Ͻ0.05) have been investigated. At high temperatures this compound is a paramagnetic, mixed-valence (Fe 2.5ϩ ) conductor with identical square-pyramidal coordinations at all iron atoms. Upon cooling below T N ϭ450 K, an antiferromagnetic ͑AFM͒ spin order appears, accompanied by a magnetostrictive orthorhombic distortion. At lower temperatures the increasing distortion sets the frame for a first attempt to order charges. Mössbauer spectroscopy shows that one squeezed and one expanded square pyramid appear with different orientations of their magnetic and electric field tensors, each centered by its own mixed-valence iron state, one Fe 2.5ϩ⑀ , the other Fe 2.5Ϫ⑀ . The lattice retains its distortion, but a small, structurally homogeneous, and continuous increase in volume is experienced. At somewhat lower temperature (T V ) a discontinuous increase of the orthorhombic distortion occurs, marking the second attempt to order charges, now with the classical symptoms of the Verwey transition: a large change in volume, entropy, and electrical conductivity. Below T V , a normal Fe 3ϩ high-spin state in a symmetrical square-pyramidal coordination appears, whereas Fe 2ϩ is distorted. The long-range order of this arrangement is solved from high-resolution powder neutron diffraction data. Rietveld refinements show that the charge-ordered spins have AFM interactions in all three directions (G type͒ whereas in the mixed-valence state a ferromagnetic ͑FM͒ interaction appears between the iron atoms facing each other across the Tb layer. This FM interaction is suggested to be essential for the appearance of the mixed-valence state via the double-exchange sharing of the Fe 2ϩ -originated electron. This also allows for the total ordered spin moment being unchanged at the Verwey transition, following one single Brillouin curve. Analogous cases are pointed out where the Verwey transition proceeds in a similar manner, also at the molecular level.

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Oxidation state (OS) is defined using ionic approximation of bonds. Two principal algorithms are outlined for OS determination in a chemical compound described by a Lewis formula or bond graph. Typical origins of ambiguous OS values are pointed out, and the relationship between OS and the d n electron configuration of transition metals is commented on.

A mixed-valence state, formally denoted as Fe 2.5ϩ , is observed in the 300 K Mössbauer spectra of the most reduced samples of SmBaFe 2 O 5ϩw . Upon cooling below the Verwey-type transition temperature (T V Ϸ200 K), the component assigned to Fe 2.5ϩ separates into a high-spin Fe 3ϩ state and an Fe 2ϩ state with an unusually low internal field. The separation of the mixed-valence state at T V is also confirmed by magnetic susceptibility measurements and differential scanning calorimetry. A model is proposed which accounts for the variation of the amount of the mixed-valence state with the oxygen content parameter w.