P. S. Berdonosov

A reinvestigation of quaternary layered bismuth oxyhalides of the Sillén X1 type

Abstract A family of layered bismuth oxyhalides, L I 0.5 Bi 1.5 O 2 X and L II BiO 2 X has been reinvestigated. Formation of X1-type Sillen compounds has been established for L I =Li, Na, L II =Ca, Sr, Ba, and X =Cl, Br, I, but the details of their crystal structures are different. While all L I 0.5 Bi 1.5 O 2 X , CaBiO 2 Br, and CaBiO 2 I adopt the disordered tetragonal Nd 2 O 2 Te structure, all compounds of L II =Sr and Ba are orthorhombic and isostructural to PbSbO 2 Cl, due to L /Bi cation ordering. Crystal structures have been determined for CaBiO 2 I, SrBiO 2 Br, SrBiO 2 I, and BaBiO 2 I. We discuss the factors which determine the occurrence and type of cation ordering in the quaternary bismuth and antimony X1-type oxyhalides. We also predict that more isostructural compounds can be prepared with antimony.

Crystal structures and variable magnetism of PbCu2(XO3)2Cl2 with X = Se, Te

Novel Cu(2+)-based compounds PbCu2(SeO3)2Cl2 (space group C2/c; a = 13.056(1) Å; b = 9.5567(9) Å; c = 6.9006(6) Å; β = 90.529(7)°; RI = 0.0371) and PbCu2(TeO3)2Cl2 (space group P2(1); a = 7.2401(2) Å; b = 7.2688(2) Å; c = 8.2846(2) Å; β = 96.416(2)°; R(I) = 0.0570) have been obtained by solid-state synthesis. Their crystal structures are remarkably dissimilar and underlie a very different magnetic behavior. While PbCu2(SeO3)2Cl2 can be well described by a spin-chain model with an exchange coupling of J1 ≃ 160 K, PbCu2(TeO3)2Cl2 is a spin-dimer system that, however, features a comparable magnetic nearest-neighbor coupling of J ≃ 213 K. PbCu2(SeO3)2Cl2 orders antiferromagnetically below 12 K, whereas PbCu2(TeO3)2Cl2 lacks long-range magnetic order down to at least 2 K, owing to the strong dimerization of the Cu(2+) spins. Crystal structures of both compounds are rationalized in terms of relevant magnetic exchange pathways, and the implications for a broader range of Cu(2+) compounds are discussed.

The crystal structures of BiTeO3I, NdTeO3X (X=Cl, Br) and Bi5TeO8.5I2: some crystal chemistry peculiarities of layered Bi(Ln)Te oxyhalides

Abstract Four new layered oxyhalides related to the Sillen family have been prepared and characterized by Rietveld refinement of powder X-ray and neutron diffraction data. BiTeO3I and NdTeO3Br both adopt tetragonal symmetry, space group P4/nmm (for BiTeO3I, a=4.10811(8), c=27.988(1) A; NdTeO3Br, a=4.06603(7), c=26.922(1) A, at 25°C). The structures are composed of triple and double fluorite-related mixed metal oxide layers separated by single and double halogen layers, in the sequence MTe2O5XXMTe2O5XM2O2X, which may be represented by the symbol X13X13X22, where the subscript signifies the number of halogen layers and the superscript the number of metal sublayers within the fluorite block, by analogy with Sillen’s notation. The double fluorite layers are occupied exclusively by Bi, whereas there is an ordered arrangement of Bi/Te within the triple fluorite layers, with Te exclusively occupying the outer sublayers of the block. NdTeO3Cl adopts an orthorhombically distorted form of this structure type, space group Pmmn, a=4.08096(8), b=4.03441(8), c=25.7582(7) A at 25°C. Bi5TeO8.5I2 adopts a distorted, non-centrosymmetric version of the simpler X13 structure type, space group Cmm2, a=5.6878(3), b=5.7230(3), c=9.7260(6) A, consisting of single halogen layers sandwiched between triple fluorite layers, in which there is partial ordering of the Bi/Te cations.

Structural characterization of lead (II) oxybromide Pb3O2Br2

Abstract The lead (II) oxybromide Pb 3 O 2 Br 2 has been prepared by solid state reaction between PbO and PbBr 2 and its structure has been characterized by means of X-ray powder diffraction. The compound is crystallized in the orthorhombic system in space group Pnma, the lattice constance are: a = 12.252(3), b = 5.873(1), c = 9.815(3) A. The structure of Pb 3 O 2 Br 2 is built up of the distorted edgeshared [Pb 4 O] tetrahedra forming the doubled infinite chains parallel to [010]. These chains are mutually connected by Br atoms. A comparison with other Pb 3 O 2 X 2 (X = Cl, I) substances is presented.

Crystal structure, physical properties, and electronic and magnetic structure of the spin S = 5/2 zigzag chain compound Bi2Fe(SeO3)2OCl3.

We report the synthesis and characterization of the new bismuth iron selenite oxochloride Bi2Fe(SeO3)2OCl3. The main feature of its crystal structure is the presence of a reasonably isolated set of spin S = 5/2 zigzag chains of corner-sharing FeO6 octahedra decorated with BiO4Cl3, BiO3Cl3, and SeO3 groups. When the temperature is lowered, the magnetization passes through a broad maximum at Tmax ≈ 130 K, which indicates the formation of a magnetic short-range correlation regime. The same behavior is demonstrated by the integral electron spin resonance intensity. The absorption is characterized by the isotropic effective factor g ≈ 2 typical for high-spin Fe(3+) ions. The broadening of ESR absorption lines at low temperatures with the critical exponent β = 7/4 is consistent with the divergence of the temperature-dependent correlation length expected for the quasi-one-dimensional antiferromagnetic spin chain upon approaching the long-range ordering transition from above. At TN = 13 K, Bi2Fe(SeO3)2OCl3 exhibits a transition into an antiferromagnetically ordered state, evidenced in the magnetization, specific heat, and Mössbauer spectra. At T < TN, the (57)Fe Mössbauer spectra reveal a low saturated value of the hyperfine field Hhf ≈ 44 T, which indicates a quantum spin reduction of spin-only magnetic moment ΔS/S ≈ 20%. The determination of exchange interaction parameters using first-principles calculations validates the quasi-one-dimensional nature of magnetism in this compound.

Comparative study of helimagnets MnSi and Cu

The heat capacity of helical magnets Cu2OSeO3 and MnSi has been investigated at high pressures by the ac-calorimetric technique. Despite the differing nature of their magnetic moments, Cu2OSeO3 and MnSi demonstrate a surprising similarity in behavior of their magnetic and thermodynamic properties at the phase transition. Two characteristic features of the heat capacity at the phase transitions of both substances (peak and shoulder) behave also in a similar way at high pressures if analyzed as a function of temperature. This probably implies that the longitudinal spin fluctuations typical of weak itinerant magnets like MnSi contribute little to the phase transition. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. In case of MnSi the sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first order phase transition in MnSi as well as in Cu2OSeO3.

_2

Cs7Nd11(SeO3)12Cl16, the complex selenite chloride of cesium and neodymium, was synthesized in the NdOCl-SeO2-CsCl system. The compound has been characterized using single-crystal X-ray diffraction, electron diffraction, transmission electron microscopy, luminescence spectroscopy, and second-harmonic-generation techniques. Cs7Nd11(SeO3)12Cl16 crystallizes in an orthorhombic unit cell with a = 15.911(1) Å, b = 15.951(1) Å, and c = 25.860(1) Å and a noncentrosymmetric space group Pna2(1) (No. 33). The crystal structure of Cs7Nd11(SeO3)12Cl16 can be represented as a stacking of Nd11(SeO3)12 lamellas and CsCl-like layers. Because of the layered nature of the Cs7Nd11(SeO3)12Cl16 structure, it features numerous planar defects originating from occasionally missing the CsCl-like layer and violating the perfect stacking of the Nd11(SeO3)12 lamellas. Cs7Nd11(SeO3)12Cl16 represents the first example of a noncentrosymmetric structure among alkaline-metal lanthanide selenite halides. Cs7Nd11(SeO3)12Cl16 demonstrates luminescence emission in the near-IR region with reduced efficiency due to a high concentration of Nd(3+) ions causing nonradiative cross-relaxation.

OSeO

The reaction of lanthanide oxohalides with CuO and SeO2 gave the products Cu3Ln(SeO3)2O2X (Ln = lanthanide, X = Cl, Br). The oxochlorides are formed with all lanthanides, while oxobromides are formed only for Ln = La-Gd. The structures of the Cu3Ln(SeO3)2O2Cl phases for Ln = Nd (I) and Y (II) were determined by the Rietveld method. It was found that Cu3Ln(SeO3)2O2X has the structure of francisite (orthorhombic system, space group Pmmn), which does not change substantially upon the variation of the Ln3+ radius. No similar compounds with tellurium(IV) were found.

_3

Abstract Five new equiatomic lanthanoid–cadmium oxide pnictides LnOCdPn (Ln=rare-earth, Pn=pnictogen) have been prepared. All of them crystallize in the tetragonal LaOAgS structure type. The phosphide series is represented by La compound only; arsenides were found for Ln=La–Nd. The structure of CeOCdAs ( P 4/ nmm , a =4.1852(1)A, c =9.1595(3)A, R p =0.054, R wp =0.071) has been solved from X-ray powder data. Similarities between structures of CeOCdAs, isoelectronic CeOZnSb, and related LaOCd 0.5 Se are discussed.

at high pressures

We report the crystal structure, magnetization measurements, and band-structure calculations for the spin-1/2 quantum magnet CaCu2(SeO3)2Cl2. The magnetic behavior of this compound is well reproduced by a uniform spin-1/2 chain model with the nearest-neighbor exchange of about 133 K. Due to the peculiar crystal structure, spin chains run in the direction almost perpendicular to the structural chains. We find an exotic regime of frustrated interchain couplings owing to two inequivalent exchanges of 10 K each. Peculiar superexchange paths grant an opportunity to investigate bond-randomness effects under partial Cl-Br substitution.